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Shifting dominance among Scarid species on reefs representing a gradient of fishing pressure
Abstract
The present paper analyzes data collected between 2001 and 2002 on 81 reef fish species targeted by fishers at 5 sites in the Kingdom of Tonga (South Pacific). We first ranked the sites with respect to fishing pressure using two independent methods: (i) Tongan demography and reef surfaces available for fishing, and (ii) the differential effects of fishing on the whole set of 81 species grouped by their life history traits (LHT). We then focused on Parrotfish (Scaridae), which are heavily targeted in coral reef fisheries. We used the identified gradient of fishing pressure to study the effect of fishing on the community structure and test the hypothesis of "shifting dominance" amongst the 20 Scarid species present in the surveys. In addition to the classical effect of decreasing fish size in a family strongly targeted by fishers, the shifting dominance phenomenon includes a decrease in the abundance of the large-bodied and highly targeted species, favouring their replacement by smaller-bodied species from the same family, which are less impacted by fishing. In a context of interspecific competition amongst Scarids, the stress of fishing appears as a factor favouring the replacement of species with large maximum size, and LHT promoting low resilience, by smaller species with the opposite attributes. The discussion focuses on the various processes that can explain the shifting dominance phenomenon. The total density of resilient species, which increased along the gradient of increasing fishing pressure, can be used as an indicator of the over-exploitation of fish communities for reef fisheries management.
... Directly, through overfishing, with a selective effect on targeted species, and indirectly, through habitat degradation, with a more widespread effect on both targeted and non-targeted species (Pauly, 1990;Williams et al., 2008;Stewart et al., 2010). Studies in insular territories around the world have shown a negative relationship between human population density and biomass and abundance of fish populations in coral reefs adjacent to inhabited territories (Clua & Legendre, 2008;Williams et al., 2008;Bellwood, Hoey & Hughes, 2012). Related information such as access to local markets and urbanization are also reliable indicators to approach the fishing pressure level of a particular locality (Aswani & Sabetian, 2009;Brewer et al., 2012Brewer et al., , 2013Cinner et al., 2013). ...
... However, at the SBR, no standardized information precludes us to quantify fishing pressure. Therefore, we considered additional drivers based on the literature of the topic (Clua & Legendre, 2008;Williams et al., 2008;Aswani & Sabetian, 2009;Bellwood, Hoey & Hughes, 2012;Brewer et al., 2012Brewer et al., , 2013Cinner et al., 2013). To include species with different levels of interest for the fisheries, we evaluated eleven species of parrotfish across a wide range of body sizes along the established human pressure gradient. ...
... Dulvy et al. (2004) suggested that indirect effects of fishing pressure may be due to reduced predation or interspecific competition. In theory, once predators are removed from the ecosystem, their preys should increase as there is no control over them, yet, there is no clear evidence of this relation (Clua & Legendre, 2008). Some studies have found no changes in preys after the demise of predators (Jennings & Polunin, 1997;Taylor et al., 2018;Roff et al., 2019). ...
Fish populations that bear considerable pressure levels tend to show a decline in the average size of individuals, with the small and unexploited species replacing the large and exploited ones. It is important to carry on with their characterization in areas where they are becoming an important source of food for local human populations. An example of such species are parrotfishes, whose responses to external factors such as fishing need to be understood and predicted. In this study, we used a diver-operated stereo-video to examine individual body size, sex ratios and proportion of species of the parrotfish assemblage and analyze them on a qualitative fishing pressure gradient at four oceanic islands in the Colombian Caribbean. We reported over 10,000 occurrences of eleven parrotfish species, of which we estimated the total length of over 90%, grouping them into three size categories (large, medium, and small). Our data showed a spatial variation of parrotfishes’ abundances, biomass, and individual body size. Observed differences are size-category-dependent throughout the qualitative fishing pressure. In general, the medium-bodied species had smaller sizes, lower abundances, and thus lower contribution to the total parrotfish biomass at the most heavily fished island. Unexpectedly, we found evidence of possible indirect effects over the small-bodied species Scarus iseri and Scarus taeniopterus with significantly greater abundances, and larger sizes of males of S. iseri , at the higher fishing pressure sites. Overall, our data highlights the extent of the spatial variation in the parrotfish communities at relatively short distances, and present new insights into the responses of parrotfish species on a spectrum of body sizes along a gradient of human pressure.
... Ces résultats contradictoires mettent en exergue les problèmes majeurs qui apparaissent dans la littérature quant à l'identification du régime alimentaire « réel » des poissons dits herbivores, qui présentent bien souvent un régime alimentaire mixte, et, dans la plupart des cas, s'avèrent être plutôt détritivores ou omnivores, comme démontré par Wilson et al. (Russ, 1991 ;Jennings & Lock, 1996 ;Jennings & Polunin, 1996 ;, in Clua & Legendre, 2008 ; ■ des effets indirects, qui restent de manière générale encore mal connus en milieu corallien par rapport aux milieux tempérés (e.g. Jackson et al., 2001). ...
... Jackson et al., 2001). Ces effets indirects incluent : 1) une diminution du spectre de taille des espèces ciblées via le prélèvement préférentiel des individus de plus grande taille (Roberts, 1995 ;Dulvy et al., 2004 ;Russ & Alcala, 1998 ;Floeter et al., 2006 ;Graham et al., 2005) ; 2) une modification de la structure taxonomique des communautés, c'est-à-dire le remplacement des espèces dominantes de grande taille ciblées par la pêche, par des petites espèces non ou moins ciblées par la pêche, comme observé chez les populations de Scaridae des Tonga dans le Pacifique Sud (Clua & Legendre, 2008) ; 3) un changement du réseau trophique des communautés suite au prélèvement de certaines catégories trophiques comme les piscivores ou les carnivores (Pinnegar et al., 2000). ...
... En outre, des modifications de la structure en taille et de la structure taxonomique des populations exploitées pourraient avoir des effets sur les communautés benthiques via une modification de la fonction de ces populations de poissons sur les récifs. La fonction des Scaridae par exemple, c'est-à-dire les caractéristiques de leur activité de broutage (intensité, fréquence d'alimentation, types d'algues consommées, bioérosion des récifs, etc.), est en effet non seulement dépendante de la taille des individus mais aussi des espèces (Bonaldo et al., 2006;Bonaldo & Bellwood, 2008 ;Hoey & Bellwood, 2008 ;Lokrantz et al., 2008 ;Jayewardene, 2009 ;Clua & Legendre, 2008). La fonction de broutage des poissons herbivores pourrait ainsi être doublement modifiée sous l'effet de la pêche. ...
... In the Indo-Pacific, there is considerable consistent evidence of the effects of fishing on parrotfishes at both the population and assemblage level; such effects include reductions in the abundance, biomass and/or average size of the larger parrotfish species as well as shifts in the size-structure of entire parrotfish assemblages through the decline of large individuals [49][50][51]. In line with this evidence, Clua and Legendre [52] formerly highlighted the potential of parrotfishes as a family to reveal gradients of fishing pressure in the South Pacific through a combination of both a reduction in fish size of individual species and shifts in species dominance driven by size-dependent vulnerability to fishing. There is evidence that parrotfishes might also help reveal gradients of fishing pressure in the Caribbean. ...
... They found evidence that the absolute density of the larger and smaller species decreased and increased, respectively, along islands constituting an increasing gradient in fishing pressure [53]. Our results are also consistent with those of Clua and Legendre [52] in the South Pacific, who monitored densities of 20 parrotfish species across 5 sites separated by tens to hundreds of km and exhibiting markedly different levels of fishing pressure. They found that the relative density of the larger and smaller parrotfish species decreased and increased, respectively, as fishing pressure increased, with only moderate changes for those species in the intermediate size classes [52]. ...
... Our results are also consistent with those of Clua and Legendre [52] in the South Pacific, who monitored densities of 20 parrotfish species across 5 sites separated by tens to hundreds of km and exhibiting markedly different levels of fishing pressure. They found that the relative density of the larger and smaller parrotfish species decreased and increased, respectively, as fishing pressure increased, with only moderate changes for those species in the intermediate size classes [52]. They concluded that fishing was likely leading to shifts in the dominance of scarid species in their study system [52]. ...
There is great need to identify simple yet reliable indicators of fishing effects within the multi-species, multi-gear, data-poor fisheries of the Caribbean. Here, we investigate links between fishing pressure and three simple fish metrics, i.e. average fish weight (an estimate of average individual fish size), fish density and fish biomass, derived from (1) the parrotfish family, a ubiquitous herbivore family across the Caribbean, and (2) three fish groups of "commercial" carnivores including snappers and groupers, which are widely-used as indicators of fishing effects. We hypothesize that, because most Caribbean reefs are being heavily fished, fish metrics derived from the less vulnerable parrotfish group would exhibit stronger relationships with fishing pressure on today's Caribbean reefs than those derived from the highly vulnerable commercial fish groups. We used data from 348 Atlantic and Gulf Rapid Reef Assessment (AGRRA) reef-surveys across the Caribbean to assess relationships between two independent indices of fishing pressure (one derived from human population density data, the other from open to fishing versus protected status) and the three fish metrics derived from the four aforementioned fish groups. We found that, although two fish metrics, average parrotfish weight and combined biomass of selected commercial species, were consistently negatively linked to the indices of fishing pressure across the Caribbean, the parrotfish metric consistently outranked the latter in the strength of the relationship, thus supporting our hypothesis. Overall, our study highlights that (assemblage-level) average parrotfish size might be a useful alternative indicator of fishing effects over the typical conditions of most Caribbean shallow reefs: moderate-to-heavy levels of fishing and low abundance of highly valued commercial species.
... They are known for their brightly coloured scales and beak-like teeth, which they use to scrape algae and other organisms off coral and rocks. Parrotfish are herbivores and play an important role in maintaining the health of coral reefs by controlling the growth of algae (Clua and Legendre, 2008). They also have a unique digestive system that allows them to extract nutrients from the coral they consume, which helps to recycle nutrients within the reef ecosystem. ...
... The mucus helps to bind the sediment and prevent it from damaging their delicate digestive system. Overall, parrotfish are an important and fascinating part of the marine ecosystem contributing to the formation and maintenance of the sandy substrate on coral reefs (Clua and Legendre, 2008). ...
... There have been few detailed investigations of parrotfish vulnerability to overexploitation. While some early studies suggest a weak response to fishing pressure Alcala 1998, Jennings et al. 1998), recent investigations have revealed that some species of parrotfishes are highly vulnerable to fisheries exploitation (Bejarano et al. 2013, Taylor andChoat 2014), with changes to community structure (Hawkins andRoberts 2003, Clua andLegendre 2008) resulting from extractive activities. Furthermore, several studies have revealed strong responses of some parrotfishes to protection from no-take marine reserves (e.g. ...
... Comeros-Raynal et al. 2012). There is however, a growing literature documenting clear responses to exploitation and protection, which can cause changes in parrotfish communities impacting on ecological functions (Russ and Alcala 1989, Polunin and Roberts 1993, Hawkins and Roberts 2003, Clua and Legendre 2008, 2012a, Edwards et al, 2014, Taylor et al. 2014. Most notably, fishing exploitation reduces the abundance and biomass of target species, and is commonly manifest in greater abundance/biomass inside areas closed to fishing, such as no take marine reserves, compared to areas where fishing occurs (Russ and Alcala 1989, Polunin and Roberts 1993, Rakitin and Kramer 1996, Friedlander and DeMartini 2002, Hawkins and Roberts 2003, Floeter et al. 2006, Heenan and Williams 2013, Rasher et al. 2013. ...
... Sensitivity, in turn, is primarily driven by life-history traits, especially those influencing population growth rate and turnover time. Parrotfish fisheries target a large number of species (Rhodes et al. 2008), yet the magnitude of demographic responses to fishing pressure are often highly species-specific (Dulvy and Polunin 2004, Clua and Legendre 2008, Bellwood et al. 2012, Valles and Oxenford 2014. These responses are predicted by inter-specific variability in life-history trait values, whereby large-bodied and late-maturing species are most sensitive . ...
... These responses are predicted by inter-specific variability in life-history trait values, whereby large-bodied and late-maturing species are most sensitive . Length-based traits such as mean maximum length are reliable predictors (Clua and Legendre 2008), and maximum length metrics are easily derived from fishery surveys or underwater visual census. However, age at female maturation has been found to be the optimal predictor of vulnerability to overexploitation among parrotfish species , thus encouraging the collection of age-based information for harvested species. ...
... mean maximum size was reported in only five of the publications incorporated in the initial review; Table S4), but the findings of these publications suggest further work is warranted in exploring the response of these indicators to fishing pressure. For example, whereas the abundance of fish is not a consistent indicator of fishing effects on reefs, the abundance of large individuals and mean maximum fish size are potentially more sensitive and/or specific to fishing on coral reefs, showing declines in response to increased exploitation (Dulvy et al. 2004b;Clua and Legendre 2008;Guillemot et al. 2014). Where sequential hermaphrodites, such as parrotfishes, are important fishery targets, mean length at sex change has been found to be lower at intensively fished sites compared with areas subject to less exploitation (Taylor 2014). ...
... Methods for unravelling the relative impacts of different factors should be essential components of any fishery assessment. Structural equation modelling, redundancy analysis and BIO-ENV are examples of techniques that allow the variance in indicator values to be separated among different explanatory variables (Clarke and Ainsworth 1993;Clua and Legendre 2008;Wilson et al. 2008;Link et al. 2010). ...
Coral reef fisheries are of great importance both economically and for food security, but many reefs are showing evidence of overfishing, with significant ecosystem-level consequences for reef condition. In response, ecological indicators have been developed to assess the state of reef fisheries and their broader ecosystem-level impacts. To date, use of fisheries indicators for coral reefs has been rather piecemeal, with no overarching understanding of their performance with respect to highlighting fishing effects. Here, we provide a review of multispecies fishery-independent indicators used to evaluate fishing impacts on coral reefs. We investigate the consistency with which indicators highlight fishing effects on coral reefs. We then address questions of statistical power and uncertainty, type of fishing gradient, scale of analysis, the influence of other variables and the need for more work to set reference points for empirical, fisheries-independent indicators on coral reefs. Our review provides knowledge that will help underpin the assessment of the ecological effects of fishing, offering essential support for the development and implementation of coral reef fisheries management plans.
... In recent decades, parrotfishes have emerged as a major fisheries resource throughout the tropics, escalating concerns for their sustainability and their functional role as grazers and bioeroders of reef substrate (Bellwood et al. 2012(Bellwood et al. , D'agata et al. 2014. Parrotfishes typically demonstrate a weak response to fishing in areas where higher trophic-level species dominate the fishery (Russ andAlcala 1998, Jennings et al. 1999), but strong species-level responses to fishing pressure (Clua and Legendre 2008) driven by differing life histories among species suggest a high capacity for fishinginduced changes in parrotfish assemblages (Taylor et al. in press). The magnitude of this effect at different spatial scales is poorly known. ...
... An increased focus on the identifiable effects of fishing on parrotfish communities has emerged recently and studies have spanned several spatial scales, from within islands to across entire ocean basins as well as circumtropical metaanalyses (Hawkins and Roberts 2003, Clua and Legendre 2008, Stockwell et al. 2009, Bellwood et al. 2012, Comeros-Raynal et al. 2012, Pinca et al. 2012, D'agata et al. 2014, Edwards et al. 2014, Lindfield et al. 2014, Taylor 2014, Vallès and Oxenford 2014. Many of these studies either ignore or statistically control for environmental variability and biogeographic species distributions in order to isolate the effects of fishing, and have surprising. ...
Parrotfishes are considered to have a major influence on coral reef ecosystems through grazing the benthic biota and are also primary fishery targets in the Indo-Pacific. Consequently, the impact of human exploitation on parrotfish communities is of prime interest. As anthropogenic and environmental factors interact across spatial scales, sampling programs designed to disentangle these are required by both ecologists and resource managers. We present a multi-scale examination of patterns in parrotfish assemblage structure, size distribution and diversity across eight oceanic islands of Micronesia. Results indicate that correlates of assemblage structure are scale-dependent; biogeographic distributions of species and island geomorphology hierarchically influenced community patterns across islands whereas biophysical features and anthropogenic pressure influenced community assemblage structure at the within-island scale. Species richness and phylogenetic diversity increased with greater broad-scale habitat diversity associated with different island geomorphologies. However, within-island patterns of abundance and biomass varied in response to biophysical factors and levels of human influence unique to particular islands. While the effect of fishing activities on community composition and phylogenetic diversity was obscured across island types, fishing pressure was the primary correlate of mean parrotfish length at all spatial scales. Despite widespread fishery-induced pressure on Pacific coral reefs, the structuring of parrotfish communities at broad spatial scales remains a story largely dependent on habitat. Thus, we propose better incorporation of scale-dependent habitat effects in future assessments of overexploitation on reef fish assemblages. However, strong community-level responses within islands necessitate an improved understanding of the phylogenetic and functional consequences of altering community structure.
... The reduced dominance of large-bodied parrotfishes and their declining average size is indicative of fisheries exploitation [53][54][55]. This can also affect the functional role of parrotfishes on coral reefs, since larger individuals can scrape and excavate much greater volumes of algal material than smaller fish [56,57]. ...
... The micro-excavator C. sordidus was the only parrotfish to increase in the catch composition in recent years and was the most abundant parrotfish during our fishery-independent surveys. This supports claims that smaller parrotfish species are more resilient to fishing pressure than larger species and may support sustainable catches by spearfishers [53,58,59]. However, continued fishing for smaller species will also result in larger species being captured when encountered, thereby further increasing the risk of local extinctions of vulnerable species [58]. ...
In recent decades, spearfishing with SCUBA has emerged as an efficient method for targeting reef fish in deeper waters. However, deeper waters are increasingly recognised as a potential source of refuge that may help sustain fishery resources. We used a combination of historical catch data over a 20-year time period and fishery-independent surveys to investigate the effects of SCUBA spearfishing on coral reef fish populations in the southern Mariana Islands. Two jurisdictions were studied; Guam, where SCUBA spearfishing is practiced, and the nearby Commonwealth of Northern Mariana Islands (CNMI), where SCUBA spearfishing has been banned since 2003. Fishery-independent data were collected using baited remote underwater stereo-video systems (stereo-BRUVs) stratified by depth, marine protected area status and jurisdiction. Herbivores (primary consumers) dominated spearfishing catches, with parrotfish (scarines) and surgeonfish/unicornfish (acanthurids) the main groups harvested. However, the large, endangered humphead wrasse (Cheilinus undulatus) was the main species by weight landed by SCUBA spearfishers. SCUBA spearfishing was associated with declining size of scarines over time and catches shifting from a dominance of large parrotfishes to a mixed assemblage with increasing proportions of acanthurids. Comparisons between Guam and the nearby CNMI revealed differences in the assemblage of fished species and also greater size of scarines and acanthurids in deep water where SCUBA fishing is banned. These results suggest that SCUBA spearfishing impacts reef fish populations and that the restriction of this fishing method will ensure refuge for fish populations in deeper waters. We recommend a ban on SCUBA spearfishing to preserve or aid the recovery of large, functionally important coral reef species and to improve the sustainability of coral reef fisheries.
... Parrotfishes are typically targeted only after more valuable species, such as grouper, are extirpated, but they are increasingly found in catches from reefs across the wider Caribbean . Consequently, large-bodied parrotfishes are often rare on heavily fished reefs, with assemblages shifting toward smallerbodied species (Clua & Legendre, 2008). These changes in species structure and decreasing mean size have been shown to significantly impact reef benthic dynamics Shantz et al., 2020). ...
Parrotfishes are a functionally critical component of Caribbean reef fish assemblages, with large‐bodied parrotfish species exerting particularly important top‐down control on macroalgae. Despite their importance, low biomasses of large‐bodied parrotfishes on many reefs hamper our ability to study and understand their ecology. Florida reefs, where most parrotfish fishing has been illegal since 1992, present a unique opportunity to explore covariates of their distribution. Using boosted regression tree models and 23 covariates, this study identified the major predictors of four species of Atlantic large‐bodied parrotfishes. Maximum hard substrate relief, the area of the surrounding reef, and the availability of seagrass habitat were each positively related to parrotfish presence. Strong positive relationships between parrotfish presence and biomass and the biomass of other parrotfishes on a reef suggest that all four species responded to a similar subset of environmental conditions. However, relationships between parrotfish presence and biomass and depth, habitat type, coral cover, and the proximity of a reef to deepwater habitats differed among species, highlighting distinct habitat preferences. These results can improve managers’ ability to target important biophysical correlates of large‐bodied parrotfishes with appropriate management interventions and identify areas for protection.
... Actively managing reef fisheries via strategies to reduce fishing effort, restrict some gear types, restrict catches, change targets, enforce size limits, or implement no-take areas (Edgar et al., 2014) embedded in traditional management systems are therefore essential to improve overall coral reef health and support reef resilience (Clua and Legendre, 2008;Jackson et al., 2001). This can be enhanced through the combination of early detection of changes in fisheries to prepare industry (Hobday et al., 2016), effective natural resource management and aquaculture techniques to reduce wild fish catches, increasing the supply of fish to the wild (seeding), and supporting sustainable food production systems in Pacific island nations . ...
Coral reefs in the tropical Pacific region are exposed to a range of anthropogenic local pressures. Climate change is exacerbating local impacts, causing unprecedented declines in coral reef habitats and bringing negative socio-economic consequences to Pacific communities who depend heavily on coral reefs for food, income and livelihoods. Continued increases in greenhouse gas emissions will drive future climate change, which will accelerate coral reef degradation. Traditional systems of resource governance in Pacific island nations provide a foundation to address local pressures and build reef resilience to climate change. Management and adaptation options should build on the regional diversity of governance systems and traditional knowledge to support community-based initiatives and cross-sectoral cooperation to address local pressures and minimize climate change impacts. Such an inclusive approach will offer enhanced opportunities to develop and implement transformative adaptation solutions, particularly in remote and regional areas where centralized management does not extend.
... Surgeonfish and parrotfish constitute the dominant herbivorous fish on Philippine coral reefs, but are also among the common target fish in Philippine coral reef fisheries (Muallil et al., 2019(Muallil et al., , 2020, especially with the decline of the more expensive reef-associated species such as groupers, snappers, and jacks among others (Clua and Legendre, 2008;Lavides et al., 2010;Nañola Jr et al., 2011). Overfishing of herbivores may lead to algal overgrowth resulting in phase shifts from coral-to algaldominated coral reef ecosystems even in the absence of eutrophication (McManus et al., 2000). ...
The importance of herbivory in preventing phase shifts to algal dominance on coral reefs is widely acknowl- edged. However, there is a paucity of information on which and how much herbivores are needed to effectively prevent an overgrowth of different algal types in natural environments. Our study showed the overall negative relationship between herbivorous fish biomass and benthic algal cover on Philippine coral reefs. The strongest negative relationships were between parrotfish biomass and both total algal and algal turf covers, and between total herbivore biomass and macroalgal cover. We further identified 4.0 mt/km2 and 2.0 mt/km2 for parrotfish, and 1.0 mt/km2 for all herbivores, as the potentially critical biomass levels needed to keep total algal, algal turf, and macroalgal covers, respectively, at low levels. The mean ( SE) cover of all algae was about 37.0 ± 0.7% in areas with 4.0 mt/km2 parrotfish biomass compared with 46.2 ± 0.8% and 44.0 ± 1.1% in areas with 2.0–3.9 mt/km2 and <2.0 mt/km2 parrotfish biomass, respectively. Algal turf cover of 10.1 ± 0.7% in areas with <2.0 mt/km2 parrotfish biomass was almost twice as high as those with 2.0 mt/km2 parrotfish biomass. Macroalgal covers were only 4.7 ± 0.5% and 10.6 ± 1.1% in areas with 1.0–2.0 and >2.0 mt/km2 total herbivore biomass, respectively, compared with 39.8 ± 1.6% in those with <1.0 mt/km2 total herbivore biomass. Our findings sup- port the hypothesis that phase shifts to algal dominance on coral reefs would not happen as long as there are no major losses of important herbivore species and herbivore biomass levels do not fall below critical levels.
... For example, average individual Scarid weight has been shown to act as a simple indicator of the influence of fishing effects on moderate-to-heavily fished Caribbean reefs, where fish groups of high commercial value might be too rare to be used as reliable indicators(Vallès and Oxenford 2014). Furthermore, monitoring the Scaridae assemblage, particularly the dominance of resilient species, can give an insight into fishing pressure(Clua and Legendre 2008). Such data collection could enable comparisons of catch and effort on a finer scale (e.g. ...
Coral reef ecosystems around the world are declining as a result of human impacts including overfishing, pollution and climate change. The coral reefs of South Sinai are significant because of their unique biodiversity, the income that they generate for Egypt through diving tourism and their importance as traditional fishing grounds for the Mzeina Bedu. This PhD takes a multidisciplinary approach to evaluate the status of coral reefs in South Sinai, with particular emphasis on Bedouin fishers and their fisheries. In Chapter 1, I examine the importance of culture and traditional knowledge in the successful management of coral reefs and natural resources in general. In Chapter 2, I focus on the Bedouin tribes of South Sinai to understand their history and socioeconomic status; investigating their fishing techniques and sites as well as their traditional understanding of coral reef biodiversity and ecology. Chapter 3 focusses on the ecological status of finfish populations at sites along the South Sinai coastline, evaluating the status of stocks and potential Bedouin fishery impacts. Chapter 4 examines the invertebrate fishery, which is mainly practised by Bedouin women and targets Tridacna clams. Chapter 5 takes a fishery-dependent approach to assess the catch of Bedouin fishers, to understand important biological and socioeconomic parameters that influence the fishery. In Chapter 6, I use statistical modelling techniques on long-term coral reef ecosystem data sets from the study area to analyse longer-term trends in the ecological status of reefs in South Sinai and likely causes for these trends. Coral reefs provide income, tourism, food and coastal protection to local communities and indigenous people throughout the tropics. The socio-cultural facets of the Mzeina Bedu have been inextricably connected to the reefs and associated fisheries of South Sinai for generations. However, exploited finfish and invertebrate communities have declined in both size and abundance with increased fishing pressure, resulting in ecosystem-wide impacts. The Mzeina themselves should be integral to any proposed fisheries monitoring or management initiatives, and technological approaches may provide useful cost-effective tools. Fisheries ecosystem-level declines have been apparent over at least the last decade and sustained monitoring is essential to ensure that the impact of management initiatives may be measured. If urgent collaborative management and enforcement actions are implemented alongside a programme to develop livelihood opportunities for the Mzeina, the reefs of South Sinai could return to a state that supports both the socioeconomic needs of the Bedu and continues to generate substantial tourism income.
... Not only do nominally herbivorous fish vary in their intrinsic vulnerability to exploitation (Clua and Legendre 2008), but they are also functionally multifaceted. Nominally herbivorous species have been traditionally classified into a limited number of distinct functional feeding groups (e.g., grazers-detritivores, scraperssmall-excavators, and bioeroders-large-excavators, macroalgal browsers, and algal farmers; Green and Bellwood 2009). ...
Securing ecosystem functions is challenging, yet common priority in conservation efforts. While marine parks aim to meet this challenge by regulating fishing through zoning plans, their effectiveness hinges on compliance levels and may respond to changes in fishing practices. Here we use a speciose assemblage of nominally herbivorous reef fish in Karimunjawa National Park (zoned since 1989) to investigate whether areas subject to a restrictive management regime sustained higher biomass over seven years compared to areas where moderate and permissive regulations apply. Using a trait‐based approach we characterize the functional space of the entire species pool and ask whether changes in biomass translate into changes in functional structure. We track changes in predator biomass, benthic community structure, and fishing practices that could influence herbivore trajectories. Overall herbivore biomass doubled in 2012 compared to 2006–2009 and remained high in 2013 across all management regimes. We found no evidence that this biomass build‐up resulted from predator depletion or increased food availability but suggest it emerged in response to a park‐wide cessation of fishing with large drive nets known as muroami. The biomass increase was accompanied by a modest increase in taxonomic richness and a slight decrease in community‐scale rarity that did not alter functional redundancy levels. Subtle changes in both functional specialization and identity of assemblages emerged as generalist species with low intrinsic vulnerability to fishing recovered sooner than more vulnerable specialists. While this implies a recovery of mechanisms responsible for the grazing of algal turfs and detritus, restoring other facets of herbivory (e.g., macroalgal consumption) may require more time. An increase in the cost‐benefit ratio per journey of muroami fishing facilitated a ban on muroami nets that met minimal resistance. Similar windows of opportunity may emerge elsewhere in which gear‐based regulations can supplement zoning plans, especially when compliance is low. This does not advocate for implementing such regulations once a fishery has become unprofitable. Rather, it underlines their importance for breaking the cycle of resource depletion and low compliance to zoning, thus alleviating the resulting threats to food security and ecosystem integrity.
... Among these metrics, the derivation of fishing from the sizes of herbivorous species, especially parrotfishes, seems particularly promising. Although parrotfishes are typically secondary targets compared to more valuable species such as grouper , large-bodied parrotfishes are often rare on heavily fished reefs, with assemblages shifting towards smallerbodied species (Clua & Legendre, 2008;Taylor, Houk, Russ, & Choat, 2014). Consequently, mean parrotfish size, but not density or total biomass, appears to be a better metric of Caribbean fishing than the biomass of some more commercially important species (Vallès, Gill, & Oxenford, 2015;Vallès & Oxenford, 2014). ...
Aim
Use a fishery‐independent metric to model and map regional‐scale fishing impact, and demonstrate how this metric assists with modelling current and potential fish biomass to support coral‐reef management. We also examine the relative importance of anthropogenic and natural factors on fishes at biogeographical scales.
Location
Reefs of five jurisdictions in Micronesia.
Methods
A subset of 1,127 fish surveys (470 surveys) was used to calculate site‐specific mean parrotfish lengths (a proxy for cumulative fishing impact), which were modelled against 20 biophysical and anthropogenic variables. The resulting model was extrapolated to each 1 ha reef cell in the region to generate a fishing impact map. The remaining data (657 surveys) were then used to model fish biomass using 15 response variables, including fishing impact. This model was used to map estimated current regional fish standing stocks and, by setting fishing impact to 0, potential standing stocks.
Results
Human population pressure and distance to port were key anthropogenic variables predicting fishing impact. Total fish biomass was negatively correlated with fishing, but the influence of natural gradients of primary productivity, sea surface temperature, habitat quality and larval supply was regionally more important.
Main conclusions
Mean parrotfish length appears to be a useful fishery‐independent metric for modelling Pacific fishing impact, but considering environmental covariates is critical. Explicitly modelling fishing impact has multiple benefits, including generation of the first large‐scale map of tropical fishing impacts that can inform conservation planning. Using fishing impact data to map current and potential fish stocks provides further benefits, including highlighting the relative importance of fishing on fish biomass and identifying key biophysical variables that cause maximum potential biomass to vary significantly across the region. Regional‐scale maps of fishing, fish standing stocks and the potential benefits of protection are likely to lead to improved conservation outcomes during reserve network planning.
... At the same time, sustainable reef fisheries depend on healthy coral reef habitats to support fish and invertebrates. Actively managing reef fisheries via reducing fishing effort and improving enforcement on gear, catch restrictions and quantity, and enforcement of non-take areas (Edgar et al., 2014) are therefore essential to improve overall coral health and support reef resilience (Clua and Legendre, 2008;Jackson et al., 2001). This can be enhanced through the combination of Natural Resource Management and aquaculture techniques to reduce wild fish catches, increase the supply of fish to the wild (seeding), and support food production to PICT . ...
In the Pacific Island region, anthropogenic-induced ocean warming is impacting coral reefs through thermal coral bleaching (Adjeroud et al., 2009; Cumming et al., 2000; Davies et al., 1997; Kleypas et al., 2015; Lovell et al., 2004; Mangubhai, 2016; Obura and Mangubhai, 2011; Rotmann, 2001) and by reducing coral calcification rates (high confidence) (De'ath et al., 2009; Nurse et al., 2014). Ocean acidification is also affecting calcification rates (low confidence) (Barros and Field, 2014; IPCC, 2014; Johnson et al., 2016b), while tropical cyclones (TC) are becoming more intense (low confidence) (Elsner et al., 2008; Nott and Walsh, 2015), causing widespread coral damage (Johnson et al., 2016a; Mangubhai, 2016). Accelerating climate change is currently the strongest driver affecting coral dynamics (Aronson and Precht, 2016) exacerbating other non-climate pressures, such as pollution from land-based activities and coastal development, eutrophication, overfishing, crown-of-thorns starfish outbreaks, direct physical impacts (e.g. tourism-related impacts, trampling and anchor damage), and coral diseases, which together are causing changes in coral communities not previously recorded (high confidence) (Brown et al., 2017a; Hoffmann, 2002; Johnson, 2017; Maata and Singh, 2008; Maynard et al., 2015b; Mayor, 1924; Morris et al., 2008; Morrison, 1999; Movono et al., 2018; Singh et al., 2009; Sykes and Morris, 2009; Zann, 1994). Predicted increase in atmospheric carbon dioxide (CO2) concentrations will continue to increase ocean temperatures and acidity (high confidence) (Gattuso et al., 2015; Newman et al., 2018) as well as increase the intensity of TC (low confidence) (Elsner et al., 2008). Climate pressures will exacerbate non-climate pressures (high confidence) (Aronson and Precht, 2016; Sheridan et al., 2014; Wild et al., 2011), thus increasing the frequency and severity of coral bleaching, disease incidence, and mortality (high confidence) (IPCC, 2014; Langlais et al., 2017; Reisinger et al., 2014; Ruiz-Moreno et al., 2012; Sheridan et al., 2014). Declines in coral reef habitat will cause negative social-economic and ecological effects (high confidence) (IPCC, 2014; Morrison, 1999). Traditional systems of resource governance in the Pacific are often associated with healthy reefs (Hoffmann, 2002) and coastal communities depend heavily on coral reefs for food, income and livelihoods. Therefore, management and adaptation options must consider and build on the regional diversity of governance systems to enable community-based initiatives and cross-sectoral cooperation, taking into account traditional knowledge that can inform sustainable solutions to problems (Aswani et al., 2017; Morrison, 1999; Remling and Veitayaki, 2016; Veitayaki, 2014) and allow the involvement of a broader section of the community. Such an inclusive approach will offer enhanced opportunities to develop and implement measures to reduce non-climate pressures and develop early warning systems, to identify potential refuges for coral reef communities (Karnauskas and Cohen, 2012) and also to test coral farming, assisted colonisation and shading or other climate mitigation techniques (Barros and Field, 2014; Coelho et al., 2017).
... At the same time, sustainable reef fisheries depend on healthy coral reef habitats to support fish and invertebrates. Actively managing reef fisheries via reducing fishing effort and improving enforcement on gear, catch restrictions and quantity, and enforcement of non-take areas (Edgar et al., 2014) are therefore essential to improve overall coral health and support reef resilience (Clua and Legendre, 2008;Jackson et al., 2001). This can be enhanced through the combination of Natural Resource Management and aquaculture techniques to reduce wild fish catches, increase the supply of fish to the wild (seeding), and support food production to PICT . ...
... There are now numerous examples of the effects of fishing on parrotfish assemblages and simple demographic traits such as body size consistently explain much of the differences among species in vulnerability to overexploitation (Hawkins and Roberts 2003, Clua and Legendre 2008, Taylor et al. 2014, Vallès and Oxenford 2014. Overall, the effects of fishery management strategies on parrotfishes and the types of management strategies available are variable (Harborne and Mumby Chapter 16). ...
... They are also highly prevalent components of reef-associated commercial, artisanal and subsistence fisheries throughout the tropics, with the majority of species being actively targeted. Larger-bodied parrotfish species are particularly vulnerable to overexploitation (Jennings et al., 1999;Hawkins & Roberts, 2003;Clua & Legendre, 2008;Mumby et al., 2013;, but a dearth of location-specific life-history information often hinders stock assessment of this group. ...
... A historical catch reconstruction suggests that total catches peaked in the 1970 s and 1980 s and have declined since (Sun et al., 2011). Reef fish and invertebrates are currently considered moderately to seriously overexploited throughout Tonga (Anon., 2010;Bell et al., 1994;Clua and Legendre, 2008;Friedman et al., 2009). ...
Community based and co-management approaches are increasingly used strategies for marine conservation and sustainable management in the tropical Pacific. However, our understanding of the effectiveness of co-management on marine resources and socio economic conditions is relatively limited, often due to insufficient resources to support monitoring based on ecological condition or catch landings data. Monitoring programmes based on the perceptions of resource users are often presented as a cost effective alternative to understanding the status and changes in resource and socio economic conditions. Ecological, catch landings and perception-based data, and their collection methods, have different benefits and limitations for community-based programmes. Here we present a study of the first community-based, co-managed area in the Kingdom of Tonga − the small island of ‘O’ua. We examine both perception-based data from interviews and catch landings data to describe fishing activities, catches and changes in resource status and socio economic conditions since the inception of co-management. Landings data were collected by the community over a five year period; perceptions of change and management performance were collected through structured interviews with fishers based on the same time period. The majority of fishing within the ‘O’ua co-managed area was by men, using hand spears in fibreglass vessels <5 m in length powered by an outboard engine. We found that catch per unit effort was high (especially the estimates generated from perception data) compared to other parts of the Pacific. Since the inception of co-management fishers reported improved socio economic conditions, a greater sense of stewardship over resources, active involvement in management and the effective exclusion of ‘outside’ fishers. We compared catch and catch-per-unit-effort estimates generated from landings and perception data. While fishers perceived that catches had improved since the inception of co-management, landings data suggested that catches were either stable or declining. These differences are important as they would suggest very different management responses. We discuss the trade-offs between the catch landings and perception data in terms of accuracy, precision, participation and cost for the purpose of guiding adjustments to co-management.
... In Fiji, bumphead parrots filled fish markets on some islands when night spearfishing was introduced, and now the species is locally extinct on some islands [74]. It is well known in fisheries generally that fishing removes the largest fish first (e.g., [68,[114][115][116][117][118][119][120], Figure 1). "Some of the larger slower growing species can become very vulnerable to modern gears and be fished to extinction" [75]. ...
Widespread coral reef decline has included the decline of reef fish populations, and the subsistence and artisanal fisheries that depend on them. Overfishing and destructive fishing have been identified as the greatest local threats to coral reefs, but the greatest future threats are acidification and increases in mass coral bleaching caused by global warming. Some reefs have shifted from dominance by corals to macroalgae, in what are called "phase shifts". Depletion of herbivores including fishes has been identified as a contributor to such phase shifts, though nutrients are also involved in complex interactions with herbivory and competition. The depletion of herbivorous fishes implies a reduction of the resilience of coral reefs to the looming threat of mass coral mortality from bleaching, since mass coral deaths are likely to be followed by mass macroalgal blooms on the newly exposed dead substrates. Conventional stock assessment of each fish species would be the preferred option for understanding the status of the reef fishes, but this is far too expensive to be practical because of the high diversity of the fishery and poverty where most reefs are located. In addition, stock assessment models and fisheries in general assume density dependent populations, but a key prediction that stocks recover from fishing is not always confirmed. Catch Per Unit Effort (CPUE) has far too many weaknesses to be a useful method. The ratio of catch to stock and the proportion of catch that is mature depend on fish catch data, and are heavily biased toward stocks that are in good condition and incapable of finding species that are in the worst condition. Near-pristine reefs give us a reality check about just how much we have lost. Common fisheries management tools that control effort or catch are often prohibitively difficult to enforce for most coral reefs except in developed countries. Ecosystem-based management requires management of impacts of fishing on the ecosystem, but also vice versa. Marine Protected Areas (MPAs) have been a favorite management tool, since they require little information. MPAs are excellent conservation and precautionary tools, but address only fishing threats, and may be modest fisheries management tools, which are often chosen because they appear to be the only feasible alternative. "Dataless management" is based on qualitative information from traditional ecological knowledge and/or science, is sufficient for successful reef fisheries management, and is very inexpensive and practical, but requires either customary marine tenure or strong governmental leadership. Customary marine tenure has high social acceptance and compliance and may work fairly well for fisheries management and conservation where it is still strong.
... While some herbivore species are likely limited by fishing throughout large parts of the Caribbean, it is less clear how their loss impacts the total level of herbivory on these reefs. For example, smaller-bodied species may increase in abundance as they are released from competition or predation, potentially compensating for the loss of larger species (Hay 1984, Dulvy et al. 2004, Clua & Legendre 2008. Nonetheless, different herbivore species and size classes have distinct diet preferences and feeding modes, and thus they are unlikely to perform the same ecological functions (see above). ...
Herbivory is a key process on coral reefs that can facilitate reef-building corals by excluding algae that otherwise negatively impact coral settlement, growth, and survivorship. Over the last several decades, coral cover on Caribbean reefs has declined precipitously. On many reefs, large structurally complex corals have been replaced by algae and other non-reef-building organisms, resulting in the collapse of physical structure and the loss of critical ecosystem services. The drivers of coral decline on Caribbean reefs are complex and vary among locations. On many reefs, populations of key herbivores have been greatly reduced by disease and over-fishing, and this has resulted in the proliferation of algae that hinder coral recovery following major disturbances. Yet, evidence that increases in herbivory can promote coral recovery on Caribbean reefs has been mixed. Here, we discuss key contingencies that will modify the relationships between herbivores, algae, and corals and identify critical knowledge gaps that limit our ability to predict when and where herbivores are most likely to facilitate coral persistence and recovery. Impacts of herbivores on coral reef ecosystems will vary greatly in space and time and will depend on herbivore diversity and species identity. While there are still a large number of knowledge gaps, we make several management recommendations based on our current understanding of the processes that structure reef ecosystems. Reversing the fate of Caribbean coral reefs will require the development of integrated management strategies that simultaneously address multiple stressors in addition to the impacts of fisheries on herbivore assemblages.
... Des données collectées à Tonga, entre 2001 et 2002 ont permis de tester l'hypothèse d'inversion de dominance dans la famille des Scaridae (poissons-perroquets)(Clua & Legendre, 2008). Dans un contexte de compétition interspécifique au sein de la famille des Scaridae, le stress lié à la pêche apparait comme un facteur favorisant le remplacement des espèces de plus grande taille, qui possèdent des traits de vie favorisant une faible résilience, par des espèces présentant les attributs inverses. ...
... Both studies dealt with parrotfishes. Clua and Legendre (2008), using the life history-based categories proposed by Kulbicki (1992), demonstrated a clear pattern of decreasing relative abundance of the larger and more vulnerable species of parrotfish along a gradient of increasing fishing pressure among five fishing areas in Tonga (South Pacific). In contrast, the smaller and less vulnerable parrotfishes increased in relative abundance (dominance) along the same gradient. ...
Coral reef fishes differ in their intrinsic vulnerability to fishing and rates of population recovery after cessation of fishing. We reviewed life history-based predictions about the vulnerability of different groups of coral reef fish and examined the empirical evidence for different rates of population recovery inside no-take marine reserves to (1) determine if the empirical data agree with predictions about vulnerability and (2) show plausible scenarios of recovery within fully protected reserves and periodically-harvested fishery closures. In general, larger-bodied carnivorous reef fishes are predicted to be more vulnerable to fishing while smaller-bodied species lower in the food web (e.g., some herbivores) are predicted to be less vulnerable. However, this prediction does not always hold true because of the considerable diversity of life history strategies in reef fishes. Long-term trends in reef fish population recovery inside no-take reserves are consistent with broad predictions about vulnerability, suggesting that moderately to highly vulnerable species will require a significantly longer time (decades) to attain local carrying capacity than less vulnerable species. We recommend: (1) expanding age-based demographic studies of economically and ecologically important reef fishes to improve estimates of vulnerability; (2) long term (20–40 years), if not permanent, protection of no-take reserves to allow full population recovery and maximum biomass export; (3) strict compliance to no-take reserves to avoid considerable delays in recovery; (4) carefully controlling the timing and intensity of harvesting periodic closures to ensure long-term fishery benefits; (5) the use of periodically-harvested closures together with, rather than instead of, permanent no-take reserves.
... In many tropical areas, parrotfishes are targeted by artisanal fishers and their populations have been affected greatly (Hawkins & Roberts 2004b;Clua & Legendre 2008;). Since fishing typically selects large individuals, males suffer higher mortality than females. ...
Tropical seas under the influence of the Asian monsoon show significant seasonality in rainfall, temperature and irradiance. The influence of these changes on coral reef fish communities is generally considered small but has rarely been studied. The extreme summer temperatures in this region are the main cause of seasonal changes in the composition and biomass of benthic algae, inducing changes in the vertical distribution of grazing fishes. More specifically, this thesis investigates the effects of seasonality on the population ecology of a common grazer, the rusty parrotfish (Scarus ferrugineus), on a southern Red Sea reef. Seasonal changes in the availability of turf algae, its preferred food type, lead to significant changes in intake rates. Higher food intake in spring results in peak body condition, liver mass and growth rate in early summer. Unexpectedly, reproduction is more intense during the cool winter season. In this regard S. ferrugineus behaves like a capital breeder: it utilizes energy stored during the warmer parts of the year to fuel reproduction in the cool season. Feeding rate in large males slows down in late summer during the hottest time of year. Males are larger in body size and experience higher mortality than females. This observations suggests that the high summer temperatures (34°C) are close to the critical temperatures of the largest individuals of S. ferrugineus. High male mortality sets an upper age limit for sex change and as a result, females reaching 4 years and beyond do not gain in fitness by changing to male.
The thesis is available at: http://irs.ub.rug.nl/ppn/374900914
... Evidence suggests that, as a group, they have historically been exposed to lower fishery exploitation compared with more heavily targeted families including epinephelids, lutjanids, lethrinids, and carangids (Russ and Alcala 1998;Jennings et al. 1999) and that the vast majority of species are designated as having a small likelihood of local extinction (Comeros-Raynal et al. 2012). However, recent studies increasingly demonstrate strong species-specific reactions to both exploitation and protection, potentially causing considerable changes to community structure as well as ecological function (Hawkins and Roberts 2003;Clua and Legendre 2008;Bellwood et al. 2012;Houk and Musberger 2013;Mumby et al. 2013;Edwards et al. 2014). In support, parrotfishes may be highly susceptible to harvest for several reasons: They are very conspicuous, have shallow depth distributions and small home ranges, and are easily targeted at night. ...
A scarcity of life-history data currently exists for many exploited coral reef fishes, hindering our ability to interpret fishery dynamics and develop sound conservation policies. In particular, parrotfishes (Family Labridae) represent a ubiquitous and ecologically important group that is increasingly prevalent in commercial and artisanal fisheries worldwide. We used both fishery-dependent and fishery-independent data to examine the effect of life histories on vulnerability to overexploitation in parrotfishes. Vulnerability for each species was derived from independent measures associated with both temporal (20-year catch records) and spatial datasets. Most life-history traits examined were significant predictors of vulnerability across species, but their relative utility differed considerably. Length-based traits (e.g., lengths at maturity and sex change, maximum length) were generally superior to age-based traits (e.g., life span), but one age-based trait, age at female maturation, was the best predictor. The results suggest that easily derived metrics such as maximum length can be effective measures of sensitivity to exploitation when applied to phylogenetically related multispecies assemblages, but more holistic and comprehensive age-based demographic data should be sought, especially in data-deficient and heavily impacted regions. Given the increasing prevalence of parrotfishes in the global coral reef harvest, species-specific responses demonstrate the capacity for heavy fishing pressure to alter parrotfish assemblages considerably.
... In Fiji, bumphead parrots filled fish markets on some islands when night spearfishing was introduced, and now the species is locally extinct on some islands [74]. It is well known in fisheries generally that fishing removes the largest fish first (e.g., [68,[114][115][116][117][118][119][120], Figure 1). "Some of the larger slower growing species can become very vulnerable to modern gears and be fished to extinction" [75]. ...
Studies on remote, uninhabited, near-pristine reefs have revealed surprisingly large populations of large reef fish. Locations such as the northwestern Hawaiian Islands, northern Marianas Islands, Line Islands, U.S. remote Pacific Islands, Cocos-Keeling Atoll and Chagos archipelago have much higher reef fish biomass than islands and reefs near people. Much of the high biomass of most remote reef fish communities lies in the largest species, such as sharks, bumphead parrots, giant trevally, and humphead wrasse. Some, such as sharks and giant trevally, are apex predators, but others such as bumphead parrots and humphead wrasse, are not. At many locations, decreases in large reef fish species have been attributed to fishing. Fishing is well known to remove the largest fish first, and a quantitative measure of vulnerability to fishing indicates that large reef fish species are much more vulnerable to fishing than small fish. The removal of large reef fish by fishing parallels the extinction of terrestrial megafauna by early humans. However large reef fish have great value for various ecological roles and for reef tourism.
... Few studies have focused heavily on vulnerability of parrotfishes to overexploitation but several studies suggest that they display a 'weak' response to fishing pressure (Russ & Alcala, 1998;Jennings et al., 1999b). Fishery-induced changes in community structure (Hawkins & Roberts, 2003;Clua & Legendre, 2008) and responses to protection (McClanahan et al., 2007;Stockwell et al., 2009), however, imply strong species-specific reactions to exploitation. Maximum body size has been used as a life-history proxy to assess parrotfish vulnerabilities (Jennings et al., 1999b), but variable growth rates and maturation schedules make this assumption tenuous. ...
Fishery-independent sampling was used to determine growth patterns, life span, mortality rates and timing of maturation and sex change in 12 common parrotfishes (Labridae: tribe Scarinae) from five genera (Calotomus, Cetoscarus, Chlorurus, Hipposcarus and Scarus) in Micronesia. Interspecific variation in life-history traits was explored using multivariate analysis. All species displayed strong sex-specific patterns of length-at-age among which males reached larger asymptotic lengths. There was a high level of correlation among life-history traits across species. Relationships between length-based and age-based variables were weakest, with a tenuous link between maximum body size and life span. Cluster analysis based on similarities among life-history traits demonstrated that species were significantly grouped at two major levels. The first grouping was driven by length-based variables (lengths at maturity and sex change and maximum length) and separated the small- and large-bodied species. Within these, species were grouped by age-based variables (age at maturity, mortality and life span). Groupings based on demographic and life-history features were independent of phylogenetic relationships at the given taxonomic level. The results reiterate that body size is an important characteristic differentiating species, but interspecific variation in age-based traits complicates its use as a life-history proxy. Detailed life-history metrics should facilitate future quantitative assessments of vulnerability to overexploitation in multispecies fisheries.
Au-delà de la perte de richesse spécifique, les activités humaines entraînent probablement la diminution de la diversité phylogénétique et fonctionnelle portée par les espèces dans les communautés. Comprendre les effets des activités humaines sur l’ensemble des facettes de la biodiversité liées au fonctionnement des écosystèmes et évaluer les outils de conservation de ces facettes restent des enjeux majeurs de l’écologie notamment en milieu marin.Les objectifs de la thèse sont donc i) d’évaluer les impacts humaines sur la diversité des lignées phylogénétiques et des fonctions au sein des communautés de poissons coralliens indépendamment des facteurs environnementaux naturels, ii) d’évaluer la capacité des aires marines protégées à conserver efficacement ces facettes et à produire des valeurs de références pour la gestion des écosystèmes et iii) de mesurer la vulnérabilité fonctionnelle intrinsèque, sans pression humaine, des communautés de poissons.. Pour cela nous avons dû échantillonner des sites très isolés des populations humaines à travers l’Indopacifique et nous avons utilisé des modèles permettant de prendre en compte les effets de seuil et les interactions pour extraire les effets ‘purs’ liés à l’homme.Dans un premier temps, nous montrons, à l’échelle du Pacifique sud-ouest, que le nombre d’espèces des poissons perroquets diminue linéairement mais uniquement de 12% le long d’un gradient d’impact humain alors que les diversités phylogénétique et fonctionnelle diminuent de 36% et de 47%, respectivement, avec de forts effets de seuil.Dans un deuxième temps, en considérant un gradient d’impact humain et une large gamme d’aires marines protégées (AMPs) en Nouvelle-Calédonie, nous démontrons que les sites très isolés des activités humaines (>20 heures de temps de trajet depuis Nouméa la capitale régionale) possèdent des communautés de poissons avec une plus forte diversité fonctionnelle et biomasses de prédateurs apicaux que la plus grande et plus ancienne AMP intégrale.Finalement, en considérant quatre sites isolés des activités humaines à travers l’Indo-Pacifique, nous avons révélé que la diversité des fonctions portées par les poissons est très vulnérable, 60% n’étant portées que par une espèce, même sans impact humain.Nos travaux montrent la très forte vulnérabilité aux activités humaines des facettes fonctionnelles et phylogénétiques de la biodiversité, avec un manque de capacité des AMPs à restaurer l’ensemble des rôles fonctionnels des poissons et une redondance très limitée pour ces fonctions même dans les sites les plus isolés.
Nearshore marine fisheries provide the main source of protein for nearly 9 million people in coastal villages of Tanzania, yet for decades the fisheries have shown signs of overexploitation. These fisheries are small-scale and co-managed by local coastal communities and governmental authorities in groups known as Beach Management Units (BMUs). BMUs record individual fishing trip data (e.g. gear, vessel, taxa); however, the catch data have only been analyzed in nationally aggregated statistics. The objective of this study was to determine if BMU catch-assessment surveys can provide information on the fishing characteristics of small-scale fishing communities in Tanzania. We collected all available landings data from 2014 to 2017 from BMUs in fourteen villages in two spatially, socially, and ecologically distinct districts (Pangani and Rufiji) of the country. Our results show that each village had unique patterns for vessel-use, gear-use, and taxa landed, and that every village was specialized in some measure. Specifically, two villages in Pangani district landed octopus or parrotfish almost exclusively, suggesting potential trophic cascades after years of overexploitation. Furthermore, village fisheries had shared characteristics within their district, thus describing how fishing patterns vary at multiple spatial scales along the coast. Although imperfect, the catch data collected by the community organizations have generated the first descriptions of how village-based fisheries in Tanzania function. Using these findings, we suggest implementing local monitoring data and analysis into the fisheries management plans at the village and district scale. Continuing to collect and analyze community collected data is necessary to gain insights into the range of characteristics of small-scale fisheries to improve current management programs.
There is an urgent need to develop simple indicators of fishing effects for the implementation of ecosystem-based fisheries management in the Caribbean. In this study, we compare the ability of three simple metrics (average individual fish weight, fish density, and fish biomass) derived from the parrotfish assemblage and from an assemblage of highly valued commercial fish species to track changes in fishing pressure at spatial scales relevant to small Caribbean islands. Between June and August 2011, we conducted five consecutive visual fish surveys at six reefs ≤10 km apart along the west coast of Barbados, representing a spatial gradient in fishing pressure. We used these data to identify the fish metrics most strongly correlated with fishing pressure and describe their functional relationship with fishing pressure. Overall, average individual parrotfish weight and biomass and density of commercial fish species were the metrics most strongly correlated with fishing pressure, although for the latter two, such correlations depended on the range of fish body sizes analyzed. Fishing pressure accounted for most of the variability in all correlated fish metrics (adjR
2 ≥ 0.75). However, functional relationships with fishing pressure differed qualitatively between metrics. In particular, average individual parrotfish weight was the metric most sensitive to incremental changes in fishing pressure. Overall, our study highlights that assemblage-level average individual parrotfish weight deserves a place in the toolbox of Caribbean reef managers as a simple indicator of both fishing effects on parrotfish assemblages and overall fishing pressure on the reef fish community.
Two fishing grounds were selected in each of the three archipelagos of Tonga, according to fishing pressure (high and low). In each ground, socioeconomic surveys provided an evaluation of fishing pressure. Reef fish stocks were assessed through underwater visual censuses along 241 transects and their habitat was described with a novative method (Medium Scale A p p r o a c h ) , better adapted to their life territories. The analysis of ecological and fishing data showed a variation between global fishing pressure at the archipelago level, which was responsible, in combination with ecological factors, for more differences than between the pair of sites within each archipelago. The fishing factor explains globally less variance of fish populations (1.6 to 5.7%) than factors acting at micro-scale (depth, hard substrate and live coral coverages, heterogeneity and topographic complexity) and at meso-scale (oceanic influence), that explain 23.3 to 34.3% of variance. The study confirmed fishing effects already known (such as reduction of fish populations average size, compensatory increase of density in the small size classes) and showed, at least for scarids, a “shifting dominance” phenomenon, based on the decrease of large-size target species, which benefits to small size species, less vulnerable to fishing. Clustering of species according to diet or life history traits revealed gradual changes of density and biomass in specific groups according to fishing pressure gradient. This result provides potential for setting up indicators of stock status for better management of reef fish resources.
This paper is the first of its kind in Indonesia where effects of artisanal fishing pressure from particular gear types shows an influence on the biomass of reef fish families measured by visual census. Surveys of reef fish communities and benthic variables at 36 fishing grounds, of varying fishing pressure, showed that Scaridae abundance was the factor most influencing fish community structure. The positive relationships between algal turf and reef fish variables and the negative relationships between hard coral cover and three reef fish variables (Scaridae biomass, herbivore biomass and fish species richness) suggests that substrate variables may be used to predict reef fish community variables and that herbivorous fish promote algal turfs at the expense of other benthos. Relationships between reef fish guilds and fishing pressure showed a strong negative impact of muro-ami and gill netting on seven families of reef fish. The strong influence of netting on Caesonidae and Lethrinidae biomass was consistent with the targeting of these families by these fishing methods in Karimunjawa. Both gillnets and muro-ami netting were also negatively correlated with invertivores, a trophic group consisting of a range of reef fish families in Karimunjawa. Although the low biomass of fish populations is likely associated with past and existing fishing practices, negative relationships between fishing pressure indices and reef fish biomass were consistent with the selectivity of families by different fishing gears in Karimunjawa. The implication for management is that both non-selective gears (netting) and selective fishing practices (e.g., handspear and speargun) are capable of modifying the structure of reef fish populations and require active management controls.
This book is available for purchase, The 1993 version is available on the page at http://distancesampling.org/downloads/distancebook1993/index.html
Despite the widely accepted importance of herbivory as a determinant of reef benthic community structure, few studies have examined the relative contributions of individual species to ecosystem processes at the whole reef scale. This study quantifies the grazing impact of individual species of roving herbivorous fishes across an inner shelf fringing reef at Orpheus Island, Great Barrier Reef, Australia. Estimates of roving herbivore impact based on dawn to dusk observations of feeding rates, measurement of bite sizes and relative abundance revealed that the Orpheus Island system was dominated by 3 species: Scarus rivulatus, Chlorurus microrhinos and Siganus doliatus. The estimated impact of all 3 species varied significantly across the reef depth gradient, with the rate of disturbance peaking at the crest and decreasing significantly down the slope and across the reef flat. The estimated species-specific disturbance levels suggested that during the course of a single month 104 % of a square metre area of the reef crest is grazed by S. rivulatus while 40 % is subject to grazing by C. microrhinos. A total of 26 cm3 of algal material is removed from the same area by S. doliatus. Overall, there was a 240-fold decrease in grazing activity across the reef flat from that at the crest. The pattern of grazing impact of the numerically dominant siganid and scarid fishes was negatively correlated with the distribution of macroalgae across the same reef gradient. The results of the current study provide support for the hypothesis that algal community structure is shaped by levels of herbivory.
Overfishing is considered one of the three most significant threats to coral reef ecosystems. Exponentially increasing human populations in the tropics have placed enormous demands upon reefs as a food source. At high intensities, termed ecosystem or Malthusian overfishing, fishing causes major direct and indirect effects on the community structure of fishes and other organisms. It reduces species diversity and leads to local extinctions not only of target species but also of other species not fished directly. Conceivably it could also lead to global extinctions. Loss of keystone species, such as predators of echinoderms, through fishing, can lead to major effects on reef processes, such as accretion of calcium carbonate. Ultimately, sustained heavy fishing may lead to loss of entire functional groups of species, resulting in impairment of the potentially important ecosystem functions provided by those groups. Overfishing has been shown to interact with other agents of disturbance to reduce the ability of reefs to recover from natural occurrences such as hurricanes. Effective management of fishing will require a deeper understanding of the effects of exploitation than we now possess. Research initiatives are underway to examine the responses of fish populations to fishing, generally responses to protection from fishing. There is, however, an urgent need to look beyond fish communities and to consider the entire reef ecosystem. Studies that integrate population and community biology with ecosystem processes will provide a much better understanding of the effects of biodiversity loss on reef function and will improve our ability to manage these complex systems.
Efecto de la pesca sobre la estructura ecosistémica de los arrecifes de coral
In an alpine area of the Sierra Nevada of California, naturally acidic waters and introduced fishes both strongly affect the distributions of native amphibians, zooplankton, and macroinvertebrates. The study area in Kings Canyon National Park contains 104 lakes with pH values between 5.0 and 9.3, including 10 lakes with pH < 6.0 (defined here as acidic lakes) and 18 lakes with introduced trout. We surveyed 33 of these lakes (8 acidic, 7 non-acidic with trout, 18 non-acidic without trout) for water chemistry and faunal assemblages. Yellow-legged frog tadpoles (Rana muscosa), common microcrustaceans (Daphnia, Hesperodiaptomus, Diaptomus), and larvae of a caddisfly (Hesperophylax) were rare or absent in acidic lakes but common in non-acidic lakes, and microcrustacean and macroinvertebrate species richness decreased with decreasing pH. Large and (or) mobile, conspicuous taxa, including tadpoles, large-bodied microcrustaceans (Hesperodiaptomus, Daphnia middendorffiana), and many epibenthic or limnetic macroinvertebrates (baetid and siphlonurid mayfly nymphs, notonectids, corixids, limnephilid caddis larvae, and dytiscid beetles), were rare or absent in trout lakes but were relatively common in lakes lacking trout, and the taxon richness of macroinvertebrates was reduced by trout.
This study used underwater visual census techniques to quantify the distribution, abundance,
fork lengths and biomass of scarid fishes on multiple reefs across the continental shelf of the
northern Great Barrier Reef. Spatial patterns in fish distribution were examined over a cross shelf
environmental gradient using a hierarchical sampling design that covered a spectrum of scales ranging
from metres to tens of kilometres. The design included replicate reefs within mid and outer continental
shelf positions and replicate sheltered and exposed sites within each reef. Most of the 21 species
surveyed were found to be widely distributed across mid and outer shelf reefs and the number of
species did not change significantly between reefs, exposures or across the shelf. Despite a similar
complement of species, scarid assemblages differed markedly in the relative and absolute abundance
of taxa between exposed and sheltered habitats on mid and outer shelf reefs. Principal components
and cluster analyses indicated that changes in the numerical abundance of taxa resulted in consistently
different assemblages from 9 sheltered mid shelf sites and 9 exposed outer shelf sites at opposite
ends of the surveyed environmental gradient. Variance components calculated for scarid abundance
and biomass across 4 spatial scales revealed differences in the scales at which significant
variability occurred between sheltered and exposed reef habitats. On exposed reef crests significant
variation occurred at both scales of tens of kilometres (shelf position) and hundreds of metres (sites),
while on sheltered back reefs significant variation was attributable only to the site scale. Mean scarid
abundance (±SE) for exposed outer shelf sites (3060 ± 460 ha–1) was on average 4 times higher than in
all other habitat zones surveyed, while mean biomass (±SE) was only 3 times higher (920 ± 190 kg ha–1).
The discrepancy between abundance and biomass estimates is explained by a 30% average decline
in parrotfish fork lengths on exposed outer shelf reef crests relative to other habitats. Log-linear
analysis indicated that at least 8 species of scarids displayed significant changes in body size according
to their location across the continental shelf and/or the prevailing exposure regime. The observed
patterns of variation in density and length frequency suggest density-dependent processes and that
changes in mortality or growth rates may exist between habitats across the continental shelf.
The relationships between 6 measures of substratum topography or vertical relief and a tropical fish assemblage were evaluated and the merits of the various measures were assessed. A method of quantifying surface topography in the field using a profile gauge is introduced and found to be rapid and to generate concise summary statistics. Of the 6 measures of substratum topography compared, 1 was novel, 2 were derivations of geomorphological techniques and 3 had been previously used in reef fish studies. Measures included the sum of consecutive substratum height differences, vector standard deviation (SD), substratum angle SD, substratum height SD, substratum height coefficient of variation (CV) and the ratio of contoured surface distance to linear distance ('chain-and-tape'). All but the CV were found to grossly differentiate among surfaces that varied in corrugation height and frequency. Consecutive substratum height difference, substratum angle SD and vector SD were best able to differentiate between 9 schematic profiles of varying topography. A field test of the methods was carried out on 100 quadrats (3 x 3 m) from the backreef of Lizard Island, northern Great Barrier Reef, Australia, in which the surface topography, fish and benthic communities had been sampled. The consecutive height difference method performed consistently well, scoring the highest average correlations with total fish abundance, diversity and the abundance of individual species. In an examination of the relationship between the backreef fish assemblage, substratum topography and the cover of benthic organisms, topography was only secondary in importance to depth in describing the patterns of damsel fish abundance at the scale sampled.
Exploitation influences community structure directly through preferential removal of larger-bodied fishes and indirectly because larger-bodied fishes may exert top-down control upon other community members. We consider the direct and indirect effects of subsistence exploitation upon the size structure of coral reef fish communities along an island-scale spatial gradient of fishing intensity. We explored the effect of fishing intensity and sample date (three dates over a year) at six islands and the overall effect of fishing intensity averaged over sample dates at 13 islands. Fishing intensity consistently explained more of the variation in the size metrics than sample date. In response to exploitation, the mean weight of individuals declined by 52%, the weighted average maximum size (Lmax) declined by 25%, and slopes of community size spectra steepened. The larger size classes (>26 cm) declined in relative numbers by 63% and relative biomass by 61% in response to exploitation. However, the numbers and biomass of the three smallest size classes (<25 cm) increased by 31% and 9%, respectively, in response to exploitation. This increased abundance is consistent with a weak compensatory response presumably from a reduction in predation upon smaller size classes as an indirect effect of exploiting larger size classes.
Abstract Biodiversity is frequently associated with functional redundancy. Indo-Pacific coral reefs incorporate some of the most diverse ecosystems on the globe with over 3000 species of fishes recorded from the region. Despite this diversity, we document changes in ecosystem function on coral reefs at regional biogeographical scales as a result of overfishing of just one species, the giant humphead parrotfish (Bolbometopon muricatum). Each parrotfish ingests over 5 tonnes of structural reef carbonates per year, almost half being living corals. On relatively unexploited oceanic reefs, total ingestion rates per m2 balance estimated rates of reef growth. However, human activity and ecosystem disruption are strongly correlated, regardless of local fish biodiversity. The results emphasize the need to consider the functional role of species when formulating management strategies and the potential weakness of the link between biodiversity and ecosystem resilience.
The fish assemblages of a strictly protected area and of a neighbouring fished area, located off a Mediterranean urbanized coast (Carry-le-Rouet, France), were compared in order to select the most evident, constant, and easily recorded indices of the ‘reserve effect’. Visual censusing was repeated simultaneously at both sites eight time a year during 3 years along four permanent transects layed on shallow rocky bottoms (9–14 m). Overall species richness was 16% higher in the reserve, but differences in average instantaneous richness were only marginally significant. The most contrasting results were shown by the occurrence frequency, abundance, and demographic structure of two types of target species. The type A target species group comprises 16 large meso- and macrocarnivores particularly threatened by spearfishing, including the common nectobenthic sparids Diplodus spp. Type B target species are represented by two small territorial fishes particularly impacted by angling: the serranid Serranus cabrilla and the terminal phase males of the labrid Coris julis. The size structure of Diplodus spp., S. cabrilla, and C.julis subpopulations strongly argues for the role of protected areas as a reservoir of large spawners. Fishing pressure is assumed to modify the social conditions of the C. julis local populations and to induce earlier sex change.
To determine how grassland, woodland, and bordering forests respond to increased aridity, we used paleoecological methods to examine past responses along a transect of three sites at the eastern boundary of the Northern Plains of North America. Our study region corresponds to the confluence of three air streams that control central North American climates and, hence, should be sensitive to climate change. Sediment cores were analyzed for evidence of Holocene vegetation and fire from tall-grass prairie in eastern North Dakota (Moon Lake), from mixed forest near the prairie border in northwestern Minnesota (Deming Lake), and from mixed forest more remote from prairie in western Wisconsin (Dark Lake). Together with pollen and charcoal analysis, we present a new method for determining 13 C of terrestrial (charred) organic matter and, thus, the relative importance of C 3 and C 4 photosynthetic pathways in past vegetation. Paleorecords were supplemented with surface charcoal accumulation and 13 C from 21 North American lakes that span boreal, deciduous, pine, and mixed forest to tall-and mixed-grass prairie. Surface charcoal and 13 C follow vegetation and climate gradients, with high charcoal accumulation and 13 C (20‰) in the Plains (Dakotas, Nebraska, and southwest Minnesota) and decreases to the east, west, and north. The 13 C pattern is consistent with observed patterns of C 3 :C 4 dominance across the region. Sediment, pollen, charcoal, and terrestrial 13 C show that vegetation response to climate change varied substantially among tall-grass prairie, bordering woodland, and forest. During maximum aridity (8000–4000 yr BP) prairie vegetation in eastern North Dakota showed a demise of woody vegetation followed by a fluctuating dominance of grasses (40% C 4) and forbs. Meanwhile, prairie expanded eastward into northwestern Minnesota, where it produced a shifting dominance between mostly C 4 grasses and woody vegetation until more humid conditions and mixed forest developed after 4000 yr BP. Mixed forest in southwestern Wisconsin showed little response to mid-Holocene aridity. Elevated 13 C values from 5000 to 3000 yr BP suggest that composition of grasses changed (to increased C 4), although pollen data indicate that the total abundance of grasses remained constant. The increase in C 4 grasses at this time is consistent with previous studies suggesting a delayed dry interval in eastern Iowa. Reduced aridity of the last 2000 yr brought increased fire to tall-grass prairie as higher primary productivity led to increased fuel load. Meanwhile, forest expanded in northwestern Minnesota, leading to decreased ignition and fine fuels, in turn resulting in decreased fire at the woodland margin. Key words: C 4 ; carbon isotopes; charcoal analysis; climate change; 13 C determination meth-odology; fire; Holocene climate change and vegetation effects; Northern Great Plains (USA); paleo-ecological data; pollen analysis; prairie; woodland.
Indicators are needed to support the implementation of an ecosystem approach to fisheries (EAF), by providing information on the state of the ecosystem, the extent and intensity of effort or mortality and the progress of management in relation to objectives. Here, I review recent work on the development, selection and application of indicators and consider how indicators might support an EAF. Indicators should guide the management of fishing activities that have led to, or are most likely to lead to, unsustainable impacts on ecosystem components or attributes. The numbers and types of indicators used to support an EAF will vary among management regions, depending on resources available for monitoring and enforcement, and actual and potential fishing impacts. State indicators provide feedback on the state of ecosystem components or attributes and the extent to which management objectives, which usually relate to state, are met. State can only be managed if the relationships with fishing (pressure) and management (response) are known. Predicting such relationships is fundamental to developing a management system that supports the achievement of objectives. In a management framework supported by pressure, state and response indicators, the relationship between the value of an indicator and a target or limit reference point, reference trajectory or direction provides guidance on the management action to take. Values of pressure, state and response indicators may be affected by measurement, process, model and estimation error and thus different indicators, and the same indicators measured at different scales and in different ways, will detect true trends on different timescales. Managers can use several methods to estimate the effects of error on the probability of detecting true trends and/or to account for error when setting reference points, trajectories and directions. Given the high noise to signal ratio in many state indicators, pressure and response indicators would often guide short-term management decision making more effectively, with state indicators providing longer-term policy-focused feedback on the effects of management action.
Herbivorous fishes are a key functional group on coral reefs. These fishes are central to the capacity of reefs to resist
phase shifts and regenerate after disturbance. Despite this importance few studies have quantified the direct impact of these
fishes on coral reefs. In this study the roles of parrotfishes, a ubiquitous group of herbivorous fishes, were examined on
reefs in the northern Great Barrier Reef. The distribution of 24 species of parrotfish was quantified on three reefs in each
of three cross-shelf regions. Functional roles (grazing, erosion, coral predation and sediment reworking) were calculated
as the product of fish density, bite area or volume, bite rate, and the proportion of bites taken from various substrata.
Inner-shelf reefs supported high densities but low biomass of parrotfishes, with high rates of grazing and sediment reworking.
In contrast, outer-shelf reefs were characterised by low densities and high biomass of parrotfish, with high rates of erosion
and coral predation. Mid-shelf reefs displayed moderate levels of all roles examined. The majority of this variation in functional
roles was attributable to just two species. Despite being rare, Bolbometopon muricatum, the largest parrotfish species, accounted for 87.5% of the erosion and 99.5% of the coral predation on outer-shelf reefs.
B. muricatum displayed little evidence of selectivity of feeding, with most substrata being consumed in proportion to their availability.
In contrast, the high density of Scarus rivulatus accounted for over 70% of the total grazing and sediment reworking on inner-shelf reefs. This marked variation in the roles
of parrotfishes across the continental shelf suggests that each shelf system is shaped by fundamentally different processes.
The data requirements and resources needed to develop multispecies indicators of fishing impacts are often lacking and this is particularly true for coral reef fisheries. Size-spectra, relationships between abundance and body-size class, regardless of taxonomy, can be calculated from simple sizeabundance data. Both the slope and the mid-point height of the relationship can be compared at different fishing intensities. Here, we develop size-spectra for reef fish assemblages using body size- abundance data collected by underwater visual census in each of ten fishing grounds across a known gradient of fishing intensity in the Kadavu Island group, Fiji. Slopes of the size-spectra became steeper (F9,69=3.20, p9,69=15.78, p
Ghiselin's size-advantage hypothesis has been used with considerable success to explain the adaptive significance of sequential
hermaphroditism in fishes. If the expected number of offspring produced differs between the sexes with size, then an individual
that changes sex can take advantage of these differences and have more offspring than an individual that remains exclusively
male or female. Despite its success, the size-advantage hypothesis has been criticized as inadequate or incomplete, and some
modifications have made it more consistent with common life-history theories. In general, individuals should change sex when
the other sex has a higher reproductive value. Thus individuals may change sex and suffer an initial drop in reproductive
success if this enhances their future expectations. The size-advantage model was formalized to identify the important variables
that affect selection for sex change. Many of the assumptions made (e.g., a genetically fixed age of sex change) were for
mathematical tractability and are not critical to the general premise. While the model can be used to specify the optimal
size or age of sex change in a hermaphroditic species, it is necessary to state the factors likely to be affecting sex-specific
reproductive values in the particular species in question. Inasmuch as these factors (e.g., mating system, local sex ratios,
and size distributions) vary from species to species, the general model cannot and should not include them. For the same reasons,
the model was also not intended to describe the precise cues that an animal might use to change sex. Any number of features
might be used as a predictor of reproductive value, and the size-advantage model does not require that relative size be the
actual criterion utilized. Several aspects of the model require further testing. Perhaps the most important problem is a refinement
of the predictions for the occurrence of sex change from male to female.
This paper examines how to obtain species biplots in unconstrained or constrained ordination without resorting to the Euclidean distance [used in principal-component analysis (PCA) and redundancy analysis (RDA)] or the chi-square distance [preserved in correspondence analysis (CA) and canonical correspondence analysis (CCA)] which are not always appropriate for the analysis of community composition data. To achieve this goal, transformations are proposed for species data tables. They allow ecologists to use ordination methods such as PCA and RDA, which are Euclidean-based, for the analysis of community data, while circumventing the problems associated with the Euclidean distance, and avoiding CA and CCA which present problems of their own in some cases. This allows the use of the original (transformed) species data in RDA carried out to test for relationships with explanatory variables (i.e. environmental variables, or factors of a multifactorial analysis-of-variance model); ecologists can then draw biplots displaying the relationships of the species to the explanatory variables. Another application allows the use of species data in other methods of multivariate data analysis which optimize a least-squares loss function; an example is K-means partitioning.
Habitat characteristics of a reef were examined as potential influences on fish assemblage structure, using underwater visual census to estimate numbers and biomass of all fishes visible on 42 benthic transects and making quantitative measurements of 13 variables of the corresponding physical habitat and sessile biota. Fish assemblages in the diverse set of benthic habitats were grouped by detrended correspondence analysis, and associated with six major habitat types. Statistical differences were shown between a number of these habitat types for various ensemble variables of the fish assemblages. Overall, both for complete assemblages and for component major trophic and mobility guilds, these variables tended to have higher values where reef substratum was more structurally or topographically complex, and closer to reef edges. When study sites were separately divided into five depth strata, the deeper strata tended to have statistically higher values of ensemble variables for the fish assemblages. Patterns with depth varied among the various trophic and mobility guilds. Multiple linear regression models indicated that for the complete assemblages and for most trophic and mobility guilds, a large part of the variability for most ensemble variables was explained by measures of holes in the substratum, with important contributions from measured substratum rugosity and depth. A strong linear relationship found by regression of mean fish length on mean volume of holes in the reef surface emphasized the importance of shelter for fish assemblages. Results of this study may have practical applications in designing reserve areas as well as theoretical value in helping to explain the organization of reef fish assemblages.
We use an approach based on phylogenetic comparisons to identify life history correlates of abundance trends in 18 intensively exploited fish stocks from the north-east Atlantic. After accounting for differences in fishing mortality, we show that those fishes that have decreased in abundance compared with their nearest relatives mature later, attain a larger maximum size, and exhibit significantly lower potential rates of population increase. Such trends were not evident in a more traditional cross-species analysis. This is the first phylogenetically independent evidence to link life histories with abundance trends, and provides a quantitative basis for assessing vulnerability of fish populations to exploitation. Our approach can be applied to the conservation and management of other exploited taxa.
An understanding of the indirect effects of fishing on predator-prey relationships is required for the development of valid multispecies yield models for reef fisheries and for determining the factors governing fish community structure at larger scales. We used an underwater visual census technique to examine the indirect effects of fishing on the biomass and diversity (species richness) of reef fishes in a series of ten traditional Fijian fishing grounds (qoliqoli) subject to a range of fishing intensities. All members of the families Chaetodontidae (butterflyfishes), Labridae (wrasses), Lutjanidae (snappers), Mullidae (goatfishes), Scaridae (parrotfishes) and the sub-family Epinephelinae (groupers and coral trout) which could be reliably identified were censused. Each species censused was assigned to one of three trophic groups: herbivore, invertebrate feeder or piscivore. The biomass of all piscivorous fishes and of large (> 30 cm) piscivorous fishes differed significantly between qoliqoli and was significantly correlated with fishing intensity. However, the biomass of piscivorous fishes was not correlated with the biomass or diversity of their potential prey (which were not targeted by the fishery). This suggested that the indirect effects of fishing did not have an important bearing on fish diversity or biomass and that predation by the target species did not play an important role in structuring these Fijian reef fish communities. The results contrast with those from a number of studies at smaller scales and provided further indications that the structure of reef fish communities is not governed by a single dominant process, but by a range of processes which operate on different scales in different circumstances.
The terms 'ecosystem-based fisheries management', 'ecosystem-based management' and 'ecosystem approach to fisheries' have recently entered the vernacular of fisheries management. Examination of contemporary guidelines and proposals for such an approach illustrates that ecosystem-based fisheries management is, for all intents and purposes, a re-expression of the objectives and processes associated with ecologically sustainable development (and associated international instruments). Since the early 1990's, all levels of government in Australia have committed themselves to the concept of ecologically sustainable development, and a complex network of policies and laws exist to support this, particularly in natural resource management. One significant Outcome of these instruments is the application of environmental impact assessment to the management of fisheries. This has forced extensive deliberation of the ecological impact of fisheries and stimulated the development and application of new research tools. Application of environmental impact assessment to the management of fisheries has been a crucial step for the implementation of ecosystem-based fisheries management in Australia. These assessments are embedded within a range of legal and policy instruments that capture the ecological, economic and social dimensions of fisheries. The scientific challenges associated with many aspects of ecosystem-based fisheries management are very significant, but it is likely that the value-based nature of the underlying environmental issues will continue to dominate the managerial agenda.
Overfishing is considered one of the three most significant threats to coral reef ecosystems. Exponentially increasing human populations in the tropics have placed enormous demands upon reefs as a food source. At high intensities, termed ecosystem or Malthusian overfishing, fishing causes major direct and indirect effects on the community structure of fishes and other organisms. It reduces species diversity and leads to local extinctions not only of target species but also of other species not fished directly. Conceivably it could also lead to global extinctions. Loss of keystone species, such as predators of echinoderms, through fishing, can lead to major effects on reef processes, such as accretion of calcium carbonate. Ultimately, sustained heavy fishing may lead to loss of entire functional groups of species, resulting in impairment of the potentially important ecosystem functions provided by those groups. Overfishing has been shown to interact with other agents of disturbance to reduce the ability of reefs to recover from natural occurrences such as hurricanes. Effective management of fishing will require a deeper understanding of the effects of exploitation than we now possess. Research initiatives are underway to examine the responses of fish populations to fishing, generally responses to protection from fishing. There is, however, an urgent need to look beyond fish communities and to consider the entire reef ecosystem. Studies that integrate population and community biology with ecosystem processes will provide a much better understanding of the effects of biodiversity loss on reef function and will improve our ability to manage these complex systems. Efecto de la pesca sobre la estructura ecosistémica de los arrecifes de coral.
Reef ecosystems extend throughout the tropics. Exploited by small-scale fishers, reefs supply food for millions of people, but, worldwide, there are growing worries about the productivity and current state of these ecosystems. Reef fish stocks display many features of fisheries elsewhere. However, habitat spatial complexity, biological diversity within and among species, ecosystem intricacy and variable means of exploitation make it hard to predict sustainable modes and levels of fishing.
In many biological studies of water quality, a diversity index is calculated in 'bits per individual' by using Shannon's Approximation to Brillouin's Formula. Difficulties associated with such use of Shannon's Formula and its associated parameters are discussed. Recent research has indicated that diversity indexes can be improved if (a) biological sample collection and analysis are standardized prior to use for among various aquatic systems diversity indexes and their associated parameters, (b) the diversity index is measured in 'sits per individual' rather than the presently used unit of 'bits per individual,' and (c) the equation e=(H - H(min)/(H(max) - H(min)) is used to evaluate the uniformity of distribution of individuals among species in a sample where e equals relative evenness. Relative evenness, a ratio, is an expression in which Brillouin's and Shannon's Equations are not arbitrarily mixed. Values of diversity indexes using Brillouin and Shannon Formulas, both in bits and sits per individual, and relative evenness are given for 16 hypothetical samples.
Fishing on tropical reefs is the preserve mainly of small-scale artisanal fishers, where increases in human power, rather than machine power, are used to generate large volumes of reef fish landings. The main forms of fishing gears deployed on reefs and in reef lagoons are handlines, traps, gill nets, seine nets and spears. Trawls can be deployed on the soft-bottom substrata adjacent to coral reefs but they catch mainly non-reef-associated species. The selectivity of nets, hooks, traps and spears is reviewed using, as far as possible, examples from reef fisheries. Although there is substantial variation, all gears catch a relatively wide range of species and selectivity is primarily size-based. However, species selectivity also results from the interaction of fish behaviour and gear characteristics. Reef fishes are also captured alive for restaurants and aquaria, and small pelagic fishes in coral reef lagoons are captured live for bait for pole-and-line tuna fishing. Observed yields from tropical reef fisheries range from around 0.2 tkm-2 year-1 in Papua New Guinea to over 40 tkm-2 year-1 from American Samoa. Yields in excess of 5 t km-2 year-1 are probably sustainable in the long term, although the upper limit for sustainable harvests from reef fisheries has yet to be accurately determined and will, in any case, vary among areas.
The chapter introduces the idea that the relationships between natural conditions and the outcome of an observation may be deterministic, random, strategic or chaotic, and that numerical ecology addresses the second type of data; it describes the role of numerical ecology among the various phases of an ecological research. The chapter includes discussion of the following topics: spatial structure, spatial dependence, and spatial correlation (independent observations, independent descriptors, linear independence, independent variable of a model, independent samples, origin of spatial structures, tests of significance in the presence of spatial correlation, and classical sampling and spatial structure), statistical testing by permutation (classical tests of significance, permutation tests, alternative types of permutation tests), computer programs and packages, ecological descriptors (i.e. variables: mathematical types of descriptors, and intensive, extensive, additive, and non-additive descriptors), descriptor coding (linear transformation, nonlinear transformations, combining descriptors, ranging and standardization, implicit transformation in association coefficients, normalization, dummy variable coding, and treatment of missing data (delete rows or columns, accommodate algorithms to missing data, estimate missing values). The chapter ends on a description of relevant software implemented in the R language.
A checklist is given below of 1162 species of shore and epipelagic fishes belonging to 111 families that occur in the islands of Tonga, South Pacifie Ocean; 40 of these are epipelagic species. As might be expected, the fish fauna of Tonga is most similar to those of Samoa and Fiji; at least 658 species of the fishes found in Tonga are also known from Fiji and the islands of Samoa. Twelve species of shore fishes are presently known only from Tonga. Specimens of Tongan fishes are housed mainly in the fish collections of the National Museum ofNatural History, Washington D.C.; Bernice P. Bishop Museum, Honolulu; Museum National d'Histoire Naturelle, Paris; and the Australian Museum, Sydney. Native Tongan names offishes, when known, are presented afterspecies names.
In order to evaluate the impact of heavy exploitation on settlement and recruitment rates of coral reef fishes, the abundances of pre-settlement and pre-recruitment fishes were monitored at Discovery Bay, Jamaica, where fish stocks are heavily exploited and at several sites around the British Virgin Islands (BVI) where exploitation is moderate. Light traps set concurrently at Discovery Bay and at three fringing reefs in BVI caught many species of pre-settlement reef fishes, except grunts and parrotfish, and one BVI site (Hans Creek) may be an area of particularly high settlement. In contrast, heavily exploited fish stocks at Discovery Bay had greatly reduced settlement rates compared with all three sites in BVI. Settlement rates of unexploited species were more similar in Discovery Bay and BVI. Abundances of juvenile fish caught in wire-mesh traps were monitored over several years in Discovery Bay and at three sites in BVI. Consistently low numbers of exploited species were taken at Discovery Bay compared to the BVI sites, indicating that settlement and recruitment rates have been low in recent years. Exploited fish populations in the small marine protected area (MPA) at Discovery Bay are expected to increase very slowly as a result of settlement and recruitment limitation.
1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape.
2. We measured the indirect effects of planktivorous larval walleye ( Stizostedion vitreum ) on phytoplankton biomass through their effects on the dominant herbivore species, Daphnia pulicaria and D. mendotae .
3. Fish had no effect on phytoplankton biomass or overall Daphnia density. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence of Daphnia species. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system. Daphnia pulicaria density decreased over time with fish predation, but D. mendotae density increased over time with fish predation.
4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends. Daphnia pulicaria were also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed on D. pulcaria over D. mendotae . The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.
Fish landing data from the Mombasa Marine National Park (MNP) and a marine reserve exploited by various gears were studied over a 5-yr period to determine the influence of the closed area and different gears in fisheries. The number fishing and boats per landing site was constant, but total and catch per unit effort progressively declined in all sites on an annual basis irrespective of the existence of a marine reserve, exclusion of the beach seines or use of gear. Differences between landing sites were most pronounced when analysed on a catch per area as opposed to the more standard catch per fisherman, suggesting compensation in human effort when catches decline. A marine reserve next to a closed area that excluded beach seines had the highest catch per area (5.5 kg ha−1 month−1) despite having the highest density of fishermen (0.07 ± 0.02 fishermen ha−1 month−1). The annual rate of decline in the catch was lower than the other sites at around 250 g day−1 compared with 310–400 g day−1 in the other sites. One landing site, which excluded beach seine landings for more than 20 yrs, had a high catch per area (∼5.3 kg ha−1 month−1), but after experiencing a doubling in the effort of other gears (line, speargun and trap), the catch per fisherman and area were reduced. Environmental or habitat degradation and excessive effort remain the most likely explanation for the overall declines in catch from 1995 to 1999. Closed areas and beach seine exclusion have the potential to increase catch rates, but the first often reduces the total fishing area and possibly leads to a loss of total catch, at least on a time scale of less than 10 yrs. The exclusion of beach seines can lead to an increase in other gear types that can also cause reductions in catch.
The interpretation of experiments, designed to reveal whether sex change is induced behaviorally, can influence the type of
physiological mechanism we propose to underlie sex change. Mechanisms involving stimulation differ from mechanisms of disinhibition
and each is inferred from different ways of conceptualizing behavioral experiments. Both types of mechanism are likely to
involve CNS, hypothalamus, pituitary, and gonad. Recent work, however, also implicates H-Y antigen and Bkm satellite DNA in
the sex-change process. These studies raise the exciting possibility that the changes in behavioral relationships between
the sexes that induce sex change also produce changes in certain aspects of DNA. The nature of the proximate causes of sex
change plays a surprisingly important role in influencing evolutionary models of sex change. Such models can be evaluated
by first examining their applicability to real populations and, if they are applicable, then testing their validity by comparing
their predictions with observed data. The size advantage model, in spite of its proven ability to stimulate and direct research,
does not fare well in either of these stages of evaluation. The basic weakness of the model is its failure to incorporate
individual adaptability into the life history of sex-changing fishes.
This study examined the effect of fishing on the abundance and species richness of families of coral reef fish at two islands
(Sumilon and Apo) in the Philippines from 1983 to 1993. Natural fishing experiments occurred in marine reserves at each island,
where long term estimates of fishing intensity were available. Responses to fishing were interpreted in terms of life histories
of fish. The intensity of fishing and fish life histories were generally good predictors of the differential rates of decline
and recovery of abundance in response to fishing. Large predators had vulnerable life histories (low rates of natural mortality,
growth and recruitment) and were subjected to high intensity fishing. They declined significantly in density when fished and
increased significantly but slowly when protected from fishing. Caesionidae, a family with a life history resilient to fishing
(high rates of natural mortality, growth and recruitment) but fished intensively also declined rapidly in abundance when fished.
Thus, knowledge of life history alone was insufficient to predict response to fishing. Acanthuridae were fished relatively
hard and had a life history of intermediate vulnerability but displayed weak responses to fishing. Thus level of fishing intensity
alone was also not sufficient to predict response to fishing. For Chaetodontidae, effects of fishing conformed to expectations
based on life history and fishing intensity at one island but not the other. Three families with intermediate vulnerability
and subjected to intermediate to light fishing (F. Scaridae, Labridae and Mullidae) displayed predictably weak responses to
fishing, or counter-intuitive responses (e.g., increasing in abundance following fishing). These counter-intuitive responses
were unlikely to be secondary effects of increase in prey in response to declines of predators. Two lightly-fished families
with resilient life histories (F. Pomacentridae, Sub F. Anthiinae) predictably displayed weak numerical responses to fishing
except during a period of use of explosives and drive nets.
The search for species associations is one of the classical problems of community ecology. This article proposes to use Kendall’s
coefficient of concordance (W) to identify groups of significantly associated species in field survey data. An overall test of independence of all species
is first carried out. If the null hypothesis is rejected, one looks for groups of correlated species and, within each group,
tests the contribution of each species to the overall statistic, using a permutation test. A field survey of oribatid mites
in the peat blanket surrounding a bog lake is presented as an example. In the permutation framework, an a posteriori test of the contribution of each “judge” (species) to the overall W concordance statistic is possible; this is not the case in the classical testing framework. A simulation study showed that
when the number of judges is small, which is the case in most real-life applications of Kendall’s test of concordance, the
classical χ2 test is overly conservative, whereas the permutation test has correct Type 1 error; power of the permutation test is thus
also higher. The interpretation and usefulness of the a posteriori tests are discussed in the framework of environmental studies. They can help identify groups of concordant species that can
be used as indices of the quality of the environment, in particular in cases of pollution or contamination of the environment.
This paper presents a systematic process to create frameworks of indicators for monitoring sustainable development in fisheries at the local community level. Indicators are evaluated and selected through a three-stage process: first, potential indicators for general use in fisheries are identified; second, this set is refined for suitability within a specific local context; third, indicators are evaluated according to data-specific criteria. The results suggest that indicators suitable at larger geographic scales are often unsuitable, and insufficient, for use at a local scale. Conversely, many social, economic and institutional fishery indicators appear suitable for use locally, but their implementation has received insufficient attention to date.
Habitat characteristics play a critical role in structuring reef fish communities subjected to fishing pressure. The line intercept transect (LIT) method provides an accurate quantitative description of the habitat, but in a very narrow corridor less than 1 m wide. Such a scale is poorly adapted to the wide-ranging species that account for a significant part of these assemblages. We developed an easy-to-use medium scale approach (MSA), based on a semi-quantitative description of 20 quadrats of 25 m2 (500 m2 in total). We then simulated virtual reef landscapes of different complexities in a computer, on which we computed MSA using different methods of calculation. These simulations allowed us to select the best method of calculation, obtaining quantitative estimates with acceptable accuracy (comparison with the original simulated landscapes: R2 ranging from 0.986 to 0.997); they also showed that MSA is a more efficient estimator than LIT, generating percentage coverage estimates that are less variable. A mensurative experiment based on thirty 50-m transects, conducted by three teams of two divers, was used to empirically compare the two estimators and assess their ability to predict fish–habitat relationships. Three-factor multivariate ANOVAs (Teams, Reef, Methods) revealed again that LIT produced habitat composition estimates that were more variable than MSA. Canonical analyses conducted on fish biomass data successively aggregated by mobility patterns, trophic groups, and size classes, showed the higher predictive power of MSA habitat data over LIT. The MSA enriches the toolbox of methods available for reef habitat description at intermediate scale (< 1000 m2), between the scale where LIT is appropriate (< 100 m2) and the landscape approach (> 1000 m2).
The dynamics of competing populations are investigated using discrete non-linear models with age structure. Necessary and sufficient stability conditions for the coexistence equilibrium between two competing non-interbreeding species are developed. An extension of this result shows that coexistence, which is not necessarily stable, occurs in terms of a simple geometric rule on a two dimensional phase plane, provided that each species reaches a stable equilibrium state in the absence of the other species. In the interbreeding case, analytical results are derived for a one locus, two allele model. Sufficient conditions for the stability of polymorphic equilibria are obtained. It is also shown that if both the homozygous genotypes have stable equilibria at fixation, then genetic coexistence occurs when the equilibrium heterozygous zygote production rate is overdominant. Harvesting can lead to the extinction of one of a pair of competing species, or to loss of genetic diversity in a panmictic population. When catchability increases sufficiently with body size, then harvesting preferentially removes the most productive genotypes, and this causes a reduction in the maximum sustainable yield of the population. Harvesting can also reverse the relative fitness of genotypes, since a rare inferior genotype in an unexploited population may be more fit under fishing.
Societies in the South Pacific are undergoing transition from reciprocity exchange and direct sustenance systems into cash based economies. The Kingdom of Tonga was selected to represent this socio-economic change. In this context, the economic viability of Tonga's subsistence and small-scale artisanal fisheries and marketing of reef and lagoon fish is assessed.Case studies were selected to represent three major geographical regions (Ha’apai, Vava’u, Tongatapu) and four major fisher groups (simple to very specialised and multi-geared fishing systems combined with cost of motorised boat transport, operational cost and fish market price). Marketing systems included operations of agents, middleman and shop owners. Economic assessment was performed by employing net present value (NPV) to explain the logic and to valorise fisheries and marketing systems against net revenues from employment and traditional agricultural production schemes.This paper also illustrates the limitations of conventional economic analysis. While NPV proved to be a useful instrument to compare fisheries and alternative sources of income, it failed to assess social and traditional values that determine the rather non-economical and non-rational subsistence and small-scale fisheries systems in the Tongan and South Pacific context.The conflict between applying mainly non-valorisation of societal and traditional values rather than underlying normal market behaviour rendered most fishing operations economically not viable or less attractive than agricultural production schemes and salaries from low skilled labour. Under these conditions, most Tongan fisheries do not reach the following suite of parameters required to make an operation economically attractive: minimum catch volume of 90–100 kg/week, a catch per unit effort of ≥5.8 kg/h, lowest possible operational cost and easy access to market selling prices of ≥5 TOP/kg fish. It was found that productivity increases from spear fishing to handline and multi-geared fisheries.Among small-scale fish marketing strategies, selling fish as a complementary activity to normal shop operations seems to be more lucrative than using agents and middlemen.Discussions focus on explaining the rationale of Tongan fishers by extending arguments beyond the economic and rationale framework. Major economic parameters such as labour and time are not accounted for in the traditional social environment. It is further argued that social considerations are impediments to economic rationale as they do not aim at catch maximisation, risk minimisation and innovation principles but the need to satisfy subsistence, social obligations and networking for societal resilience and insurance.
Although reproductive patterns in tropical fishery species are characterized by diversity, some interesting trends are clearly emerging, despite the few families studied in detail. Reproductive output is highly variable, both within and among species, years and individuals. Especially in larger species, the differential between the fecundities of different-sized conspecifics may be orders of magnitude. Environmental and biological factors influence when and where reproduction takes place, with annual spawning seasons ranging from as little as a week or two, to much of the year; the notion that tropical marine species are all characterized by protracted spawning is unfounded. Most species produce pelagic eggs and, characteristic of reef fishes in general, all produce pelagic larvae. Females may spawn a few times to many times annually. Spawning often takes place towards dusk, in some cases at specific times in the lunar or tidal cycle. Evidence for lunar cyclicity in most larger species, however, is not strong except among a number of species which aggregate to spawn. Spawning occurs in areas of residence or at well-defined aggregation sites, metres or many kilometres away from home sites. Males and females mate either pairwise or in small groups which characteristically comprise one female and several males; the size of the testis relative to the body (gonadosomatic index) of ripe males apparently accords with mating pattern - larger for group and smaller for pair spawners. Larger, more mobile, species within a family tend to migrate to reproduce in aggregations which are frequently, but by no means exclusively, located offshore and close to deep waters. Such sites may be quite distinct but there is no evidence to indicate that they are unique. It is not clear whether aggregation spawning and the preference for specific spawning locations evolved largely for the benefit of larval dispersal or survival, to enable males and females that live somewhat dispersed to come together or for some other reason. Morphological factors such as body form and size are also important determinants of reproductive output, with more compressed forms exhibiting lower relative fecundity at length than more rounded body forms. These factors influencing egg output per spawn, combined with maturation size and spawning frequency and duration, are all pertinent to our understanding of reproductive patterns and strategies and inter- and intraannual variation in reproductive output. By elucidating the variables that determine egg production we might better understand relationships between numbers and activities of spawning adults and the resulting temporal, spatial and abundance patterns of larval and juvenile appearance on reefs. From a fishery perspective, there is an urgent need to better estimate and maintain reproductive output in exploited populations. For example, spawning aggregations, because they are often consistent in time and space, may be heavily targeted by fishers. Of particular concern is evidence that aggregations can be decimated by heavy fishing with a possible concomitant loss of larger individuals or disruption of reproductive activity. Because the effects of intense aggregation-fishing, in both the short and long term, are largely unknown but may seriously compromise reproductive output, they should be accorded particular attention for research and management. Similarly, quite a few commercially exploited reef families are hermaphroditic. The impact of fishing or conventional management practices on hermaphroditic species is not understood and will be difficult to assess until we come to understand, among other things, the factors that induce sexual changeover. Research and monitoring programmes designed specifically to address these issues provide a rich and exciting challenge to the curious ecologist and the innovative fishery biologist.
Ecosystem considerations may be incorporated into fisheries management by modifying existing overfishing paradigms or by developing new approaches to account for ecosystem structure and function in relation to harvesting. Although existing concepts of overfishing have a strong theoretical basis for evaluating policy choices and much practical use, they do not provide direct guidance on issues such as biodiversity, serial depletion, habitat degradation, and changes in the food web caused by fishing. There is, however, little basis for defining optimum fishing by using related metrics such as diversity indices, slopes of size or diversity spectra, or average trophic level of the catch, and these may produce ambiguous results. If ecosystem-based overfishing concepts are to assume a greater role in management, unambiguous, quantifiable, and predictive measures of ecosystem state and flux must be developed to index: (1) biomass and production by the ecosystem and relationships among its parts, (2) diversity at different levels of organization, (3) patterns of resource variability, and (4) social and economic benefits. Ecosystem considerations do not need to substitute for existing overfishing concepts. Instead, they should be used to evaluate and modify primary management guidance for important fisheries and species. In practice, they emphasize the need to manage fishing capacity, supported by broader use of technical measures such as marine protected areas and gear restrictions.
1. An understanding of the links between life histories and responses to exploitation could provide the basis for predicting shifts in community structure by identifying susceptible species and linking life‐history tactics with population dynamics.
2. We examined long‐term trends in the abundance of species in the North Sea bottom‐dwelling (demersal) fish community. Between 1925 & 1996 changes in species composition led to an increase in mean growth rate, while mean maximum size, age at maturity and size at maturity decreased. The demersal fish community was increasingly heavily fished during this period.
3. Trends in mean life‐history characteristics of the community were linked to trends in abundance of component species. An approach based on phylogenetic comparisons was used to examine the differential effects of fishing on individual species with contrasting life histories.
4. Those species that decreased in abundance relative to their nearest relative, matured later at a greater size, grew more slowly towards a greater maximum size and had lower rates of potential population increase. The phylogenetically based analyses demonstrated that trends in community structure could be predicted from the differential responses of related species to fishing.
5. This is the first study to link exploitation responses of an entire community to the life histories of individual species. The results demonstrate that fishing has greater effects on slower growing, larger species with later maturity and lower rates of potential population increase. The comparative approach provides a basis for predicting structural change in other exploited communities.
Fishing activities affect the population structure, growth, reproduction and distribution of target species and have indirect effects on non-target fish or invertebrate populations and their reef habitats. We review the multi-farious impacts of fishing and conclude that existing knowledge of fishing effects is remarkably primitive given the burgeoning literature on reef fishery science and management. In particular, fishing effects are widely treated as synonymous with overfishing and thus many studies of fishing effects have been based upon examination of localized catastrophic events rather than changes that occur within sustainable fisheries subject to different fishing intensities or cropping regimes. An improved understanding
of fishing effects in sustainable fisheries may assist the development of a range of new monitoring, assessment and management methods. These would provide an alternative to methods based on conventional population analyses within reef fisheries of limited economic importance which are unlikely to be selected for rigorous scientific study. Initial development of the fishing effects science could be achieved at moderate cost by refining existing procedures for collection of catch and effort data and by treating observed combinations of gear, effort and catch composition as a series of experimental manipulations.