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Fishing Down Marine Food Webs
Abstract
The mean trophic level of the species groups reported in Food and Agricultural Organization global fisheries statistics declined
from 1950 to 1994. This reflects a gradual transition in landings from long-lived, high trophic level, piscivorous bottom
fish toward short-lived, low trophic level invertebrates and planktivorous pelagic fish. This effect, also found to be occurring
in inland fisheries, is most pronounced in the Northern Hemisphere. Fishing down food webs (that is, at lower trophic levels)
leads at first to increasing catches, then to a phase transition associated with stagnating or declining catches. These results
indicate that present exploitation patterns are unsustainable.
... Some of these species, which can have a massive presence in bottom trawl catches, are starting to be marketed by some vessels in the study area (e.g. the largest individuals of L. depurator; Lucchetti, personal communication). This is clear evidence of the fishing down the marine food webs [43] as demonstrated by stock assessments in the Mediterranean, which highlight a situation of overexploitation for around 75% of the assessed commercial species [7]. ...
Bottom trawl fisheries often catch several species simultaneously. However, most studies addressing the catch performance and selectivity of a specific trawl focus on a few commercially important or most vulnerable species requiring management measures. By contrast, the present study considers the multispecies nature of Mediterranean bottom trawl fisheries through a holistic approach that accounts for the full species community in the catches. Specifically , we evaluated and compared the catch performance of the two codends allowed for this fishery, made of 40 mm square (SM40) and 50 mm diamond (DM50) meshes. Results showed that 50 and 80% of the catch in weight and count numbers, respectively, consisted of species without commercial value, demonstrating that large proportions of the catch are not considered when using the existing approach to evaluate the ecological impact of the fishing activity. Significant differences in catch profiles between the two codends were observed, especially for two commercial flatfish species, Arnoglossus laterna and Citharus linguatula, with larger contributions in the SM40. Further, the SM40 codend had a significantly higher retention, compared to DM50 codend, for specific sizes of Merluccius merluc-cius and Mullus barbatus. The outcomes of the study can be useful for the Mediterranean bottom trawl fisheries management.
... Biological traits such as large body size, late sexual maturity, and long generation time, among others, predict species' extinction vulnerability to fishing (Reynolds et al. 2005;Juan-Jordá et al. 2013;Mellin et al. 2016). Welcomme (1999) integrated insights from life history theory with the history of some SSF to propose the 'fishing down process', which differs from the fishing down marine food webs concept of Pauly et al. (1998), which focuses on trophic level. The fishing down process attempts to describe the historical dynamics of tropical multispecies fisheries, most of which are smallscale. ...
Small-scale fisheries (SSF) account for much of the global fish catch, but data to assess them often do not exist, impeding assessments of their historical dynamics and status. Here, we propose an approach to assess 'data-less' SSF using local knowledge to produce data, life history theory to describe their historical multispecies dynamics, and length-based reference points to evaluate stock status. We demonstrate use of this approach in three data-less SSFs of the Congo Basin. Fishers' recalls of past fishing events indicated fish catch declined by 65-80% over the last half-century. Declines in and depletion of many historically important species reduced the diversity of exploited species, making the species composition of the catch more homogenous in recent years. Length-at-catch of 11 of the 12 most important species were below their respective lengths-at-maturity and optimal lengths (obtained from Fishbase) in recent years, indicating overfishing. The most overfished species were large-bodied and found in the Congo mainstem. These results show the approach can suitably assess data-less SSF. Fishers' knowledge produced data at a fraction of the cost and effort of collecting fisheries landings data. Historical and current data on fish catch, length-at-catch, and species diversity can inform management and restoration efforts to curb shifting baselines of these fisheries. Classification of stock status allows prioritizing management efforts. The approach is easy to apply and generates intuitive results, having potential to complement the toolkits of researchers and managers working in SSF and engage stakeholders in decision-making processes.
... To investigate long-term variations of LMEs, we calculated weighted mean values of the trophic level, optimal temperature, the lowest latitude, the highest latitude, and the maximum body length of each LME as an integrated index of LME. The mean trophic level (MTL) represents changes in ecosystem structure and owns integrative nature, which is widely used in the assessment of sustainability in fisheries (Pauly et al., 1998). The mean temperature of the catch (MTC) is used to evaluate the potential effects of climate change on fisheries locally that represents the temperature preference of species in fisheries catch (Cheung et al., 2013); The maximum body length (MBL) represents size compositions of ecosystems and can also be used to reflect ecosystem structure (McCay et al., 1935); The mean value of the lowest latitude (MLL) and the mean value of highest latitude (MHL) represents the lowest and highest latitude, respectively, where fishes are distributed (Perry et al., 2005). ...
Sharks are essential components of marine food webs, but their removal might simplify ecosystems and food webs. Comparing the isotopic niche of species between archaeological and modern communities can provide information to assess temporal changes in the ecological dynamics of communities. Here, stable isotope analysis was used to compare food web topology metrics between two shark guilds or communities, a late Holocene archaeological community (AC) dating from 724 − 542 years ago and a modern community (MC) trophic web, both from South Brazil. In the same line of comparison, we assess the trophic position of a top predator, Carcharias taurus . Results showed topological temporal differences such as higher trophic redundancy and higher patterns of niche overlap in the AC. Higher trophic redundancy could be expected in late Holocene food webs, as similar studies also observed these findings. In contrast, the MC showed dispersion of species throughout the isotopic space, thus higher niche partitioning and higher trophic diversity, suggesting changing ecological interaction dynamics. We found that Carcharias taurus has increased its trophic level, possibly due to a release from the intraspecific competition and a dietary shift towards larger prey items. We suggest that anthropic impacts, such as overfishing, may cause these differences in the trophic position of this species. We also advocate that this study method might help future trophic reconstructions using shark teeth, as information about past marine environments is scarce, and could serve as a baseline for future studies.
Human exploitation has profoundly depleted animal populations in the ocean, leading to declines in ecosystem productivity, resilience, and contributions to people 1,2 . However, it remains unclear how size structure of fish populations varies across marine habitat and levels of human exploitation while simultaneously underpinning food web architecture and ecosystem functioning ³ . Here, we process 6,701 pelagic and 10,710 benthic stereo-video deployments across all the oceans to extract body size of over 800,000 individual fish, spanning 6 orders of magnitude from zooplankton size-classes (<1 g), to large oceanic predators (>1,000 kg), in order to ultimately assess vulnerability to exploitation and responses to protection status. Size-spectra analysis reveals peaks in body size distributions which differ in their position between pelagic and benthic habitats. Typical body size of relatively small and large individuals is greater in strictly protected areas remote from human markets, than in unprotected areas close to markets. Pelagic size-spectra are more sensitive to market remoteness and respond more strongly to protection than their benthic counterparts which are more species-rich, and therefore more likely to provide trophic replacements ⁴ under exploitation, owing to body size redundancy ⁵ . This contrasted result suggests that fully protected areas can partly mitigate human impacts close to markets in benthic habitats while in pelagic habitats effective protection requires remoteness from humans. Thus, the restoration of fish body-size spectra, including some of the world’s largest and most endangered vertebrates ⁶ , can be promoted by implementing fully protected areas in pelagic habitats, particularly in remote locations.
Global patterns of management and governance of small-scale fisheries:contributions towards the implementation of the SSF Guidelines
The ninth International Symposium Monitoring of Mediterranean Coastal Areas: Problems and Measurements Techniques was organized by CNR-IBE in collaboration with Italian Society of Silviculture and Forest Ecology, and Natural History Museum of the Mediterranean and under the patronage of University of Florence, Accademia dei Lincei, Accademia dei Geogofili, Tuscany Region, The North Tyrrhenian Sea Ports System Authority, Livorno Municipality and Livorno Province. In the Symposium Scholars had illustrated their activities and exchanged innovative proposals, with common aims to promote actions to preserve coastal marine environment. Despite the COVID 19 pandemic, the success of this edition is attested by the 170 contributions selected by the Scientific Committee from among those received. Participation involved all the thematic lines envisaged by the sessions, involving many countries of the Mediterranean Sea. A big endeavor for a costal environment of paramount importance but threatened by global changes. The importance of this Proceedings is attested by the fact that this volume is the first issue of a new FUP Series.
Cruise tourism is expanding almost everywhere, despite the pandemic generated by COVID-19 in this sector as well. According to estimates and reported by the CLIA (2021), between mid-March and September 2020, the economic damage was around 77 billion dollars, with a loss of jobs that exceed 500 000. The CLIA itself, however, records a significant increase in travelers who aspire to join a cruise, even among those who have never been there. Cruise ships and their economic impact have been the subject of research for many years and the data, which can also be obtained from various agencies and shipping companies, denote the importance of this particular sector, in the more general context of tourism. It is therefore increasingly appropriate to direct research towards an interdisciplinary and multidisciplinary analysis that considers the concept of environmental impact (now in many cases somewhat emptied of its deep meaning and become a sort of opportunistic slogan), so as to consider, in addition to the economic aspects, also the anthropogeographic ones (both negative and positive). In fact, the opportunity to enrich the technical and organizational changes required by the pandemic with second thoughts that also have a cultural impact, in addition to the usual environmental impact, is a delightful opportunity. Through the MSC case study, this contribution aims to partecipate in the reflection on the theme of sustainability, which is also abused lexically, to look at the possibility of directing cruise tourism towards a new way of introducing Man into the landscape.
Several management strategies for ecosystems with biological interaction are discussed, including predator removal, predator-prey coexistence, prey exploitation, overexploitation, and introduction of sanctuaries. Some case studies related to ecosystem management are briefly presented; these describe Lakes Victoria and Tanganyika, discarding from shrimp trawl fisheries and the development in the North Sea that led to introduction of multispecies analysis. The concept of fishing down the food web is discussed and the average trophic levels at which the fisheries operate in different ecosystem types are estimated based on quantified trophic flow models. On a global level, while on average fisheries operate around two trophic levels above the primary producers, still one third of the catch of the 70 major fish species caught in the world is of piscivorous fish. Using exploitation-predation rate indices for different ecosystem types, the amount of finfish consumed globally by finfish is roughly estimated to be three times the catches of finfish. Finally some implications for the management of ecosystems are drawn up. It makes little difference if short-term prognoses are based on single-species or multispecies considerations. Multispecies models may, however, give the better long-term advice, and adaptive management may facilitate the move towards such long-term goals.
Several nations are beginning to harvest krill in the Southern Ocean around Antarctica. We need to know the best method of managing this ecosystem so it will sustain a yield of whales and krill without exterminating one of the whales. To estimate the maximum harvest a particular whale is capable of yielding in a sustained way one needs information on birth and death rates. Light harvest of a whale species generally leads to a rise in the birth rate and a younger age for maturity. A decrease in the blue whales Balaenoptera musculus resulted in an increase in other whales such as the minke B. acutorostrata due to competition for the same food supply, the krill and other plankton. Regulations are needed to control the harvest of whales and krill. Regulation of the krill harvest should start early before there is a large investment in ships and equipment. The Southern Ocean should be a favorable region to study the effect of whaling and krill harvesting because the ecosystem is relatively simple. -F.McElhoe Jr
Recruitment rates are determined by larval production, and both intrinsic (eg reproductive mode, larval behavior) and extrinsic (eg predation, resource availability, currents) factors that influence the geographic range over which a refuge can effectively supply recruits. The size, number, and distribution of refuges depend on patterns of larval replenishment. Since resource requirements of fish often change with ontogeny and reproductive condition, refuges may need to include a wide variety of habitats. Larval production by refuges may be enhanced by multispecies management that provides protection for or allows harvesting of nontarget species. Additionally, protection may be needed for resources located outside refuges that enhance recruitment to harvested populations. -from Authors
Despite a vast body of traditional knowledge on management of marine resources in the tropics there are few direct tests, utilizing manipulative or natural experiments, of the effect of protective management on the abundance of marine resources and, more importantly, their yield. This paper reports on a natural experiment which makes such a direct test. Approximately 25% of the sub-tidal coral reef of Sumilon Island in the central Philippines was protected from all forms of exploitation from 1974 until May 1984. Cessation of protective management led to fishing of the entire coral reef. This resulted in a significant reduction in abundance within the previously protected area of the fishes which constituted the majority of the yield from the reef. There was a decline of 54% in the total yield of reef fishes from Sumilon Island reef between 1983/1984 and 1985/1986 despite the fact that only 75% of the reef was fished in 1983/1984. Protective management maintained high abundances of fishes in the reserve and significantly higher yields to fishermen from areas adjacent to the reserve. -from Authors
Marine fisheries refugia, unaltered areas that serve as sources of replenishment, can potentially compensate for recruitment and ecosystem overfishing and enhance fishery yields for some coastal stocks. Evidence from existing marine reserves indicates that increased abundance, individual size, reproductive output, and species diversity occurred in a variety of marine species in refuges of various sizes, shapes, and histories in communities ranging from coral reefs to temperate kelp forests. Fishery yield enhancement in areas surrounding refuges occurred in the few studies where yields were examined. The export of propagules required to enhance fisheries in areas surrounding refugia adds a level of complexity to the design of fishery refugia beyond that of terrestrial reserves. Fishery refugia design should consider species life histories, oceanographic regimes, habitat quality, and socioeconomic factors. -from Authors
The ECOPATH II microcomputer software is presented as an approach for balancing ecosystem models. It includes (i) routines for balancing the flow in a steady-state ecosystem from estimation of a missing parameter for all groups in the system, (ii) routines for estimating network flow indices, and (iii) miscellaneous routines for deriving additional indices such as food selection indices and omnivory indices. The use of ECOPATH II is exemplified through presentation of a model of the Schlei Fjord ecosystem (Western Baltic).