Article

Effects of flood and drought events on multi‐species, multi‐method estuarine and coastal fisheries in eastern Australia

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Abstract

Abstract Multivariate patterns in commercial fisheries landings, effort and revenue from three adjacent estuarine and coastal systems were examined in eastern Australia between 9-month periods of flood (September 2000–May 2001) and drought (September 2002–May 2003). Patterns in species landings, methods of fishing effort and revenue per species were significantly different between flood and drought. Spearman’s rank correlations between Bray–Curtis similarity matrices for landings, effort and revenue indicated that patterns in fisheries metrics represented a mixed signal of ecological response and fishers’ harvesting behaviour. Flood and drought events were associated with shifts in the species composition of landings that were reciprocated between estuarine and coastal systems. Estuarine migrant species (e.g. school prawn Metapenaeus macleayi Haswell) primarily contributed to landings during flood, whilst marine estuarine-opportunist species (e.g. yellowfin bream Acanthopagrus australis Owen) primarily contributed to landings during drought. Flood and drought events redistributed fisheries resources between estuarine and coastal systems, modifying the bioeconomic productivity of commercial fisheries. Results indicated that flood and drought events influence commercial fisheries by modifying landings composition, fishers’ harvesting behaviour and revenue generation.

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... River runoff plays a main role in physical, chemical and biological processes in coastal regions Testa et al., 2018;Saldías and Lara, 2020). Many studies report relationships between river runoff and subsequent fish landings (Quiñones and Montes, 2001;Lloret et al., 2004;Gillson et al., 2012;Meynecke et al., 2012), reproductive migrations, spawning and recruitment success (Gillson, 2011;Reinfelds et al., 2013;Walsh et al., 2013). ...
... Rivers located in the central macrozone reduce their runoff, plume areas and nutrient export by about 50% in comparison to values observed before 2010, which translates into a marked reduction in the chlorophyll pool within their plumes . Drought events or reduced river runoff are commonly accompanied by changes in the abundance of neritic fisheries, which in some cases can lead to recruitment failure (e.g., Gillson et al., 2012;Costalago et al., 2018). Our results reinforce the idea that fluctuations of freshwater entry to the coastal zone due to decreased river runoff is a significant factor that affects the recruitment of anchovy. ...
Article
River runoff to the coastal zone is one of the most important environmental variables that influences the structure and functioning of the neritic trophic web and modulates temporal fluctuations of coastal fishery production in many ecosystems worldwide. The relationship between recruitment of anchovy ( Engraulis ringens ) and common sardine ( Strangomera bentincki ) in central-south Chile (34°–40°S) and fluctuations in river runoff was analyzed during the last two decades (1999–2018) using linear and nonlinear regression models. River runoff was also incorporated as a co-variate in Beverton-Holt (BH) stock-recruitment models. Anchovy recruitment was found to be positive and significantly associated with river runoff for all analyzed rivers both for the runoff calculated during its main reproductive period (July–October) and for each single month during this period. This detected nonlinear relationship remained significant when runoff was analyzed for the central macrozone (CMZ) and southern macrozone (SMZ). In contrast, no relationship between runoff and recruitment was detected for the common sardine in any river or macrozone (CMZ, SMZ), for each month of the main reproductive season or for the whole reproductive period (July–October). No density-dependent regulation was detected using BH stock-recruitment models for either species, with or without total freshwater input (i.e., the joint discharge of all rivers; RIVSUM index) as a covariate. Our results highlight the relevance of river runoff as a major driving factor in anchovy recruitment variability, suggesting that high (low) monthly runoff anomalies during the main reproductive period of anchovy populations markedly affect the survival of its early stages, altering recruitment success. For common sardine the runoff effect was not significant, which suggests that other oceanographic/climatic factors (wind-driven upwelling, sea surface temperature, El Niño) are more relevant in driving the variability of its recruitment than river runoff.
... Freshwater ecosystems are particularly vulnerable to reduced streamfows (Gillson et al. 2012) because drought conditions reduce surface water runoff and groundwater discharge into streams (Strauch et al. 2015(Strauch et al. , 2017a. Drought may also cause increased concentrations of fecal bacteria, with higher loads immediately after rain events (Strauch et al. 2014). ...
... A reduced freshwater supply could affect both the ability of a community to remain in that location and its ability to irrigate agricultural crops. Coastal fsheries and reefs that are integral for both subsistence and the social structure of a community can be set back long after drought events because droughts can shift estuarine and coastal fsh species composition (Gillson et al. 2012), slow fushing times and increase the chance of algal blooms (Alber and Sheldon 1999), and alter coastal vegetation communities for years afterward (White and Alber 2009). In Hawai'i, traditional loko i'a (fshpond) aquaculture helps to cultivate fsh and supports the intergenerational teaching of local fshing practices. ...
... 2352-4855/© 2020 Elsevier B.V. All rights reserved. of literature has reported the influence of flow or discharge on the river-estuarine fisheries (Gillson et al., 2009(Gillson et al., , 2012Schwalb et al., 2014;Castello et al., 2015;Isaac et al., 2016;Bower et al., 2019). Freshwater flow has had direct or indirect influence on the fish production, which has been duly established for the drought period in the Minho and the Mondego estuaries of the Iberian coast. ...
... Hydrological drivers influence the fish catch in the riverestuary system to a great extent. Several studies have documented such influences using various techniques, such as, Non Table 4 Estimated regression parameters, standard errors, t values, lower (low.CI) and upper (up.CI) confidence interval (CI) and p values of hydrological variables (GRBF, ASG) on five commercial fish species of Narmada river-estuary system using DFA model on catch time series data from 1991-2013.. Metric Multi-Dimensional Scaling (NMDS), Canonical Correspondence Analysis (CCA), etc., to study the effect of hydrology on fish diversity and assemblage (Bower et al., 2019;Rabuffetti et al., 2017;Abrial et al., 2019;Arthington et al., 2016;Hossain et al., 2012); Multivariate Analysis of Variance (MANOVA) to assess the hydrological factors like flood and drought on multispecies landings (Gillson et al., 2012) etc. But those techniques have not provided any quantitative estimate of the effects and their statistical significance. ...
Article
Hydrological regime is the key determinant of fish species diversity and production in the river–estuary system; thereby it influences the catch dynamics of multispecies fisheries of the system. It is of significant importance to quantify the effect of the river hydrological regime on commercial fisheries in the river–estuarysystem, for developing strategic management plans. This article presents an analytical framework to accomplish the same for commercial fisheries of multiple species, including Macrobrachium rosenbergii, Mugil spp., Harpadon neherus, Wallago attu and Tenualosa ilisha, of a tropical river–estuary system. The prime tool of the framework is ‘Dynamic Factor Analysis’, which identifies the common trends in catch, and quantifies the influence of hydrological factors on multiple commercial fisheries. Annual hydrological and multispecies catch data of the Narmada river–estuary system for the period 1991 to 2013 have been used in the present study. The results reveal two significant trends: Trend 1, representing mainly the Tenualosa ilisha catch; and Trend 2, representing commercial fish catches of the remaining species. The annual observable flood level significantly influences the catch of all but Mugil spp. species. The annual sediment load significantly influences the catch of Tenualosa ilisha , Macrobrachium rosenbergii and Harpadon nehereus. This analytical approach has been effective in understanding the common catch trend patterns and the hydrological factors influencing them in the river–estuary system. The results can be used for formulating sustainable management plans for multispecies commercial fisheries in the river–estuary system.
... Seasonal changes in the composition of fish assemblages may occur as a result of natural fluctuations in the hydrological regime, thereby affecting the distribution and abundance of freshwater species [1][2][3]. These effects have been linked to factors such as the duration, frequency, and periodicity of the hydrological regime, local and regional geomorphology and zoogeography, changes in habitat, human interventions, and climate change [4][5][6]. ...
... Studies by Lamberth [35] and Gillson and Suthers [3] have shown that drought and flood events cause fluctuations in fishery resources, resulting in altered species composition of landings and changes in capture techniques employed and revenue generated. Other factors reported to be associated with differences in fishing captures between high-and low-water periods are related to changes in turbidity and dissolved oxygen availability due to a greater availability of organic matter in flooded margins [8,20,21]. ...
Article
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Seasonal changes in composition and yield of artisanal fishery catches may occur due to natural fluctuations in hydrological regime, fishers, and gear used. In this context, the present study aimed to test the hypothesis that the hydrological cycle of the Araguaia River influences the activity of small-scale commercial fishing by examining the following predictions: fishery yield is influenced by the hydrological regime; the main species exploited through artisanal fishing can change with variations in hydrological regime; and there are differences between male and female fishers in both yield and main exploited species due to differences in the sites that they explore and the gear that they use. Results from the study demonstrated a significantly higher fishery yield during the low-water season, when the catch per unit effort (CPUE) was 24.8 kg/fisher/day, compared to an ebb season CPUE of 19.9 kg/fisher/day. The main species caught by artisanal fisheries was indeed influenced by the hydrological regime of the Araguaia River and the fisher’s gender, which was associated with differences in fishing locations and gear used. These findings suggest that both Araguaia River hydrology and fisher gender should be taken into consideration in developing management strategies for achieving sustainable yields. Of note, our results for fishery yields were 305 % higher than official Brazilian estimates for the entire Araguaia basin over the same period, which suggests underestimation in the official statistics.
... Freshwater input into Port Phillip Bay, predominantly from the Yarra River, led to a drop in salinity across the bay, while dissolved inorganic nitrogen loads from key point sources increased from 60 tonnes in 2008 to 572 tonnes in 2011, substantially altering productivity in the bay (EPA Environmental Protection Agency Victoria 2012; Lee et al. 2012). Fluctuations in salinity and nutrients have substantial effects on the growth and distribution of marine organisms (Gillson et al. 2012) and may have been responsible for the observed increases in clupeoid [anchovy, pilchards, sandy sprat, and blue sprat (Spatelloides robustus)] abundance in Port Phillip Bay (Hirst et al. 2011;Parry and Stokie 2008). Under these conditions, little penguins are a good model species to investigate how small-ranging resident seabirds respond to changes in environmental conditions combined with changes in local prey availability. ...
... Future directions in the study of the foraging ecology of little penguins and other coastal seabirds in relation to river outlets should address how the strength, size and spatial variability of river plumes influence productivity and the suitability of fronts for foraging. This is especially important given the anticipated effects of climate change where, in many regions, including south-eastern Australia, climate change scenarios predict decreases in rainfall and enhanced evaporation, which will have significant effects on salinity, productivity and ultimately on predator-prey interactions (Gillson et al. 2012;Lee et al. 2012). ...
Article
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Marine animals forage in areas that aggregate prey to maximize their energy intake. However, these foraging 'hot spots' experience environmental variability, which can substantially alter prey availability. To survive and reproduce animals need to modify their foraging in response to these prey shifts. By monitoring their inter-annual foraging behaviours, we can understand which environmental variables affect their foraging efficiency, and can assess how they respond to environmental variability. Here, we monitored the foraging behaviour and isotopic niche of little penguins (Eudyptula minor), over 3 years (2008, 2011, and 2012) of climatic and prey variability within Port Phillip Bay, Australia. During drought (2008), penguins foraged in close proximity to the Yarra River outlet on a predominantly anchovy-based diet. In periods of heavy rainfall, when water depth in the largest tributary into the bay (Yarra River) was high, the total distance travelled, maximum distance travelled, distance to core-range, and size of core- and home-ranges of penguins increased significantly. This larger foraging range was associated with broad dietary diversity and high reproductive success. These results suggest the increased foraging range and dietary diversity of penguins were a means to maximize resource acquisition rather than a strategy to overcome local depletions in prey. Our results demonstrate the significance of the Yarra River in structuring predator-prey interactions in this enclosed bay, as well as the flexible foraging strategies of penguins in response to environmental variability. This plasticity is central to the survival of this small-ranging, resident seabird species.
... Thirdly, the Clarence River also experiences greater freshwater flows and a lower level of flow regulation (Gillson et al., 2009), which also contributes to increased recruitment and/or catch of many of the species examined here (e.g. Gillson, 2011;Gillson et al., 2012;Loneragan and Bunn, 1999;Robins et al., 2005;Ruello, 1973;Taylor et al., 2014). Finally, species in the Hunter River are exposed to various contaminants not found in the Clarence River catchment (e.g. ...
Article
Broad-scale links between productivity of estuarine habitats (such as saltmarsh and mangrove) and the exploited species that rely on them have often been used to build a case for habitat conservation and repair. Stable isotope composition can provide a temporally and spatially integrated measure of trophic connectivity with which to quantify habitat-fishery linkages, allowing primary producers that comprise these habitats to be linked with harvested biomass with relatively few assumptions. We present a novel model that applies this approach to estimate the economic value of fisheries harvest derived from dominant estuarine habitats, in two eastern Australian estuaries. Estimated values of fisheries harvest supported by habitats within the model regions ranged from ∼AUD100,000 y−1 to ∼AUD7,200,000 y−1. Saltmarsh in the Clarence River had by far the greatest economic value per-unit-area, with an average estimated Total Economic Output (from fisheries harvest) of AUD25,741 ha−1 y−1, whereas mangrove was estimated to be AUD5,297 ha−1 y−1. Average Total Economic Output in the Hunter River was AUD2,579 ha−1 y−1 and AUD316 ha−1 y−1 for saltmarsh and mangrove habitats respectively. Estuarine habitats are key ecological indicators of fisheries productivity, and the framework presented here will be broadly useful in estimating the potential economic impacts associated with changes in these indicators.
... Given that freshwater discharge has a strong influence on the distribution, abundance and phenology of many species of fish and crustaceans in eastern Australia (e.g. [23,37] and other references herein), it would not be surprising to see these effects realised in apex predators. ...
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Understanding environmental and climatic drivers of natural mortality of marine mammals is critical for managing populations effectively and for predicting responses to climate change. Here we use a 17-year dataset to demonstrate a clear relationship between environmental forcing and natural mortality of inshore marine mammals across a subtropical-tropical coastline spanning a latitudinal gradient of 13° (>2000 km of coastline). Peak mortality of inshore dolphins and dugongs followed sustained periods of elevated freshwater discharge (9 months) and low air temperature (3 months). At a regional scale, these results translated into a strong relationship between annual mortality and an index of El Niño-Southern Oscillation. The number of cyclones crossing the coastline had a comparatively weak effect on inshore marine mammal mortality, and only in the tropics. Natural mortality of offshore/migratory cetaceans was not predicted by freshwater discharge, but was related to lagged air temperature. These results represent the first quantitative link between environmental forcing and marine mammal mortality in the tropics, and form the basis of a predictive tool for managers to prepare responses to periods of elevated marine mammal mortality.
... Given the global scale of anthropogenic alteration across riverine systems (Nilsson et al., 2005), understanding the effects of high flow disturbance on estuarine nekton assemblage structure is important for developing successful management strategies aimed at maintaining productive habitats. High flow events are known to affect estuarine ecosystems causing, for example, declines in the catches of estuarine and coastal fisheries (Drinkwater & Frank, 1994;Gillson et al., 2012) and decreases in abundances of estuarine fishes and invertebrates (Costa et al., 2007;McLeod & Wing, 2008). Our comparison of seasonal nekton assemblage dynamics in two rivers that experience Hydrobiologia contrasting hydrological flow regimes provides evidence for significant reductions in community variability under high flow conditions. ...
Article
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Modifications to riverine systems that alter freshwater inflow to downstream estuarine habitats have resulted in altered patterns of nekton distribution and abundance. To examine how nekton assemblages respond to variable hydrologic patterns, we used trawl and seine survey data to compare the seasonal trends (dry vs. wet season) expected of a natural system to those of a river with regulated flow discharges that often magnify high flow events. Nekton assemblages differed between seasons in a representative natural system, similar to other estuaries of the region. For example, assemblage differences were characterized by significantly higher abundance and richness in trawl surveys, and significantly higher richness in seine surveys in the wet relative to the dry season. These seasonal trends were dampened in the altered system. Species important in defining seasonal dissimilarities in both systems were characterized as estuarine resident species, including Anchoa mitchilli, Menidia spp., Cynoscion arenarius, and Trinectes maculatus, yet were observed largely to have opposing seasonal trends in abundance between the two rivers. Our comparison provides evidence that flow modifications result in a loss of natural seasonal variability in estuarine nekton assemblages, but additional investigations of flow-altered systems are needed to confirm these findings.
... This fishery responds to changes in freshwater inflows that affect breeding and recruitment of several commercial species ( Ferguson et al., 2013). The relationship between catch and inflows is complex and non-linear ( Gilson et al., 2012), but mean annual catch per fisher day during 1984-2008 was 246 kg (derived from total catch divided by fisher days). We estimated a conservative 20% increase in catch per unit effort over the long-term average associated with achieving the MurrayDarling Basin Plan target of average freshwater inflows of 42000 GL/y in 495% of years and maintenance of average salinity of o60 g/L in the Coorong Southern Lagoon and o 20 g/L in the Northern Lagoon (MDBA, 2012b two of these). ...
Article
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Evaluating different environmental policy options requires extensive modelling of biophysical outcomes linked with metrics to measure the magnitude and distribution of societal impacts. An integrated ecosystem services (ES) assessment has potential to provide salient, credible and legitimate information for environmental policy- and decision-makers. Here we present an ecosystem services assessment of the Murray-Darling Basin Plan, an Australian Government initiative to restore aspects of river flow regimes to improve the ecological condition of floodplains, rivers and wetlands in south-eastern Australia. We link the effect of policy intervention – reduced limits on water diversions for irrigation – to modeled changes in river flow and flood regimes, then to changes in ecological responses of flow-dependent ecosystems, assessed against a Baseline scenario. The final steps link changes in ecosystem condition and responses to marginal changes in the supply of ES and the monetary valuation of those services at the whole-of-basin scale. We show that the supply of most ES improves as a consequence of increases in water availability for the environment. For each ES assessed we assign a confidence category for both the ecological response modelling and the economic valuation steps and discuss other tools (review and outreach) to enhance legitimacy and credibility.
... Hidden behind precipitation and wind effects may appear the influence of trophic relationships, reproductive cycles (Keddy and Weiher 1999) and may also a stochastic component (Grossman et al. 1982). Moreover, this influence may be modulated by specific variables correlated to the rivers input into the coastal areas, including salinity, river flow (Gillson et al. 2012, Mitchell et al. 1999), turbidity (Castillo-Rivera et al. 2002, Cyrus and Blaber 1987, 1992, Johnston et al. 2007, Whitfield 1999 or pollution (Lekve et al. 2002). Nevertheless, evidences provided here indicated for consistent seasonal changes determined by precipitation and wind direction/intensity in the distribution and abundance of species within coastal fish assemblages. ...
Article
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Seasonal ecological effects caused by temperature and photoperiod are typically considered minimal in the tropics. Nevertheless, annual climate cycles may still influence the distribution and abundance of tropical species. Here, we investigate whether seasonal patterns of precipitation and wind speed influence the structure of coastal fish assemblages and fishing yields in northeast Brazil. Research trips were conducted during the rainy and dry seasons using commercial boats and gear to sample the fish community. Diversity was analyzed using abundance Whittaker curves, diversity profiles and the Shannon index. Principal Component Analysis (PCA) was used to analyze associations between the abundance of species and various environmental variables related to seasonality. A total of 2,373 fish were collected, representing 73 species from 34 families – 20 of which were classified as both frequent and abundant. Species richness was greater and more equitable during the rainy season than the dry season – driven by changes in the precipitation rather than to wind speed. Species diversity profiles were slightly greater during the rainy season than the dry season, but this difference was not statistically significant. Using PCA was identified three groups of species: the first associated with wind speed, the second with precipitation, and the third with a wide range of sampling environments. This latter group was the largest and most ecologically heterogeneous. We conclude that tropical coastal fish assemblages are largely influenced by local variables, and seasonally mediated by annual changes related to precipitation intensity and wind speed, which in turn influences fishery yields.
... An increase in global temperature of 0.85°C between 1880 and 2012 has led to greater evaporation and evapotranspiration rates (#7b Figure 1), reducing the volume of water delivered to estuaries (Nijssen et al., 2001;IPCC, 2014). Floods and droughts are predicted to become more severe and frequent with climate change (IPCC, 2014), altering flows (Gillson and Suthers, 2012). Storms can potentially deliver the annual average inflow to an estuary within a day or two (Steichen et al., 2020). ...
Article
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Estuaries host unique biodiversity and deliver a range of ecosystem services at the interface between catchment and the ocean. They are also among the most degraded ecosystems on Earth. Freshwater flow regimes drive ecological processes contributing to their biodiversity and economic value, but have been modified extensively in many systems by upstream water use. Knowledge of freshwater flow requirements for estuaries (environmental flows or E-flows) lags behind that of rivers and their floodplains. Generalising estuarine E-flows is further complicated by responses that appear to be specific to each system. Here we critically review the E-flow requirements of estuaries to 1) identify the key ecosystem processes (hydrodynamics, salinity regulation, sediment dynamics, nutrient cycling and trophic transfer, and connectivity) modulated by freshwater flow regimes, 2) identify key drivers (rainfall, runoff, temperature, sea level rise and direct anthropogenic) that generate changes to the magnitude, quality and timing of flows, and 3) propose mitigation strategies (e.g., modification of dam operations and habitat restoration) to buffer against the risks of altered freshwater flows and build resilience to direct and indirect anthropogenic disturbances. These strategies support re-establishment of the natural characteristics of freshwater flow regimes which are foundational to healthy estuarine ecosystems.
... These impaired fish habitats may not adequately support the development of juveniles of fish species, leading to a reduction in its adult spawning biomass [44]. Natural events can also be escalating factors such as prolonged periods of drought, leading to reduced flows through estuaries and affecting dispersal of larvae to nearshore habitats for settlement and development [45,46]. ...
Article
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Determining the effectiveness of a management system to enable fisheries to harvest sustainably is a key challenge. To fully assess the likelihood that a fishery management system will not achieve its sustainability objectives, the assessment needs to include the whole pathway that leads to the consequences for management objectives. A crucial aspect of the pathway is the inclusion of management controls. Effectiveness of these management controls determines whether the effects of human pressures on ecological components and their impacts are reduced to a level that will not impede management achieving their objectives. Ecological risk assessments do not provide sufficient information to make decisions about what to change specifically in a management system to ensure a fishery is sustainably managed. Bowtie analysis (BTA) is a method that logically connects the relationships between management objectives, management controls, threats, potential impacts of threats on the fishery resource and the consequences of those impacts on achieving the management objectives. The combination of bowtie analysis and ecological risk assessment enables managers, scientists and stakeholders to evaluate different management controls and research options in response to risk factors and track the effectiveness of the management system. We applied a three-step method of bowtie analysis stage 1, quantitative ecological risk assessment and bowtie analysis stage 2 to evaluate fisheries management and science. We demonstrate these steps using a case study of a commercially fished species in New South Wales, Australia.
... In the case of the region, these are hydrologic phases (flood, spate, ebb and drought) that modify the fisher's behavior against the capture of the target species of commercialization. These natural oscillations in the hydrological regime cause seasonal changes in the composition of the fish communities, thus modifying the distribution and quantity of the species (Gillson and Suthers, 2012). Isaac et al. (2000) found that these differences are also a reflection of the capture and transport capacity of the fishing units operating in the region, and the changes in the density of the fish populations associated with the hydrological cycle and the migratory behavior of certain species. ...
Article
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Na Amazônia, as grandes flutuações do nível dos rios modulam a pesca e influenciam no quantitativo capturado pelos pescadores. Nesse sentido, objetivou-se caracterizar e avaliar o efeito da variação hidrológica sobre os recursos pesqueiros no município de Santarém, Pará. As informações sobre as espécies do pescado desembarcado foram obtidas por entrevistas realizadas com os pescadores no período de 2014 a 2015. Fases hidrológicas foram caracterizadas com base nos dados da estação hidrometeorológica de Santarém, disponibilizados pela ANA (2000 a 2015). A relação entre as espécies registradas e as variáveis ambientais (cota, precipitação e temperatura da água) foi verificada pela Análise de Correspondência Canônica (CCA). Observou-se que a flutuação sazonal do rio Tapajós é definida por quatro fases (enchente, cheia, vazante, seca), sendo que a cheia (7,77 m) e seca (2,28 m) são períodos extremos. Os menores volumes de produção de pescado ocorreram de dezembro a fevereiro, enquanto que os meses de abril e setembro apresentaram os maiores volumes. Nos meses de março a junho, prevalecem espécies das famílias Anostomidae, Prochilodontidae, Characidae e Pimelodidae, em novembro e dezembro (seca) a família Pimelodidae. Os dois primeiros eixos da CCA explicaram 52,4% da relação entre as espécies e variáveis, indicando que o período hidrológico é determinante na variação sazonal do pescado. Assim, o entendimento das variáveis hidrológicas junto aos recursos pesqueiros, são subsídios para auxiliar a gestão e o ordenamento da atividade. A B S T R A C T In the Amazon, large fluctuations in rivers’ level modulate fishing and influence the quantity captured by fishermen. The objective of this study was to characterize and evaluate the effect of hydrological variation on fishery resources in the municipality of Santarém, Pará. Information on the fish species landed was obtained by interviews with fishermen from December 2013 to January 2015. Hydrological phases were characterized based on data from the Santarém hydrometeorological station (2000 to 2015). The relationship between the recorded species and environmental variables (water height, rainfall, water temperature) was verified by Canonical Correspondence Analysis (CCA). It was observed that the seasonal fluctuation of the Tapajós River is defined by four phases (rising, flood, lowing, dry), and the flood (7.77 m) and dry (2.28 m) are extreme periods. The lower volumes of production occurred from December to February, while the months of April and September presented the highest volumes. From March to June, species of the families Anostomidae, Prochilodontidae, Characidae and Pimelodidae prevail, in November and December (dry), only the family Pimelodidae. The first two axes of the CCA explained 52.4% of the relation between species and environmental variables, indicating that the hydrological period is determinant in the seasonal variation of the fish. Thus, the understanding of the hydrological variables along with the fishing resources, are subsidies for the better management of the activity. Keywords: Hydrology, Tapajós River, Fishery.
... Multiple studies have demonstrated positive correlations between School Prawn productivity and freshwater discharge into estuaries (Ruello, 1973;Glaister, 1978;Loneragan and Bunn, 1999;Ives et al., 2009;Gillson et al., 2012), and prolonged dry weather normally adversely affects School Prawn and results in a smaller population and harvest (Ruello, 1973). As noted earlier, estuary inflow is generally important for juvenile School Prawn, but it is also important in the maturation, emigration and aggregation of sub-adults and adults (Ruello, 1973;Glaister, 1978). ...
Article
Many factors can affect growth, survival, reproduction, and fisheries productivity of estuarine species, including structural and physico-chemical habitats, and freshwater inflow to estuaries. Land-based activities can lead to poor catchment condition, and catchment-derived stressors can adversely impact estuarine systems. Using the Eastern School Prawn (Metapenaeus macleayi) in a south-eastern Australian estuary (Camden Haven Estuary) as a case study, we examine juvenile recruitment and fisheries productivity alongside a comprehensive suite of catchment-derived stressors, and interpret patterns in the context of existing studies of lethal and sub-lethal impacts of these stressors on penaeid prawns. Logged dissolved oxygen data indicated a moderate frequency of hypoxia throughout the system, with occasional periods of anoxia. Dissolved aluminium concentrations remained above the relevant marine water quality guideline for the majority of the study period, and concentrations tended to correlate with estuarine inflow. Hypoxia led to depressed prawn abundance, and both hypoxia and high estuary inflow led to decreased somatic condition in prawns. Long-term commercial catch negatively correlated with estuary inflow, which was the opposite of the expected pattern for the species. These patterns highlight the potential cumulative impacts of a complex array of catchment-derived stressors on an important exploited penaeid species. Similar patterns probably occur for prawn species across other floodplain estuaries across south-eastern Australia, and suggest a hitherto unquantified economic impact of degraded catchments through losses in fisheries productivity.
... other organisms or ecological processes such as scouring and rejuvenation of benthic producers and production of primary consumers) and beyond the reaches of interest (e.g. downstream fisheries; Gillson et al. 2012). ...
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Despite effects of dams and water extraction on river hydrology and consequences of aquatic ecosystems being broadly appreciated, empirical evidence is lacking for many regions (e.g. subtropics). Evidence is necessary to determine (i) the circumstances where environmental flows are necessary to protect or improve ecological processes and (ii) what hydrological events are required to achieve those ecological characteristics. Here, temporal variation in the size structure of two small-bodied fish species with contrasting ecological characteristics (Australian smelt, Cox’s gudgeon) was compared between two pairs of unregulated and regulated rivers in subtropical Australia. Frequency of in-channel flow pulses in each regulated river was lower compared to paired unregulated rivers. An experimental flow pulse was delivered to one regulated river to assess the ecological outcomes of in-channel flow pulses and inform future decisions about the use of environmental water allocations. Temporal changes in the population size structure of both species were similar between unregulated and regulated rivers and showed no response to the experimental pulse. While the experimental flow briefly suppressed in-stream temperature, changes in temperature were not beyond the thresholds at which spawning occurs or the thermal tolerances for either species. Similar population structure between unregulated and regulated rivers can be attributed to the magnitude of flow regulation being insufficient to alter the physico-chemical conditions, habitat and trophic mechanisms supporting population dynamics during the study period. This suggests current regulated hydrology is protective of local populations of Australian smelt and Cox’s gudgeon in these study rivers.
... Pranto river). In turn, during flood periods, recruitment conditions are less favourable due to the high hydrodynamics and low salinity (Gillson et al., 2012), which will result in lower densities. ...
Article
The increase in frequency and intensity of extreme climate events over the last few decades has been leading to profound changes in estuarine and marine ecosystems worldwide, with strong implications for the species inhabiting these ecosystems as well as for the services provided by them. In this study, we analysed the effects of climate variability on the temporal and spatial variations in population dynamics of the green crab Carcinus maenas in the Mondego estuary (Portugal), between 2003 and 2018. In this 15-year period, a greater recruitment of C. maenas was observed during drought periods, periods which was matched by an increase in secondary production. Ontogenic stage segregation was also observed, with juveniles being found mainly in the further upriver areas of the estuary. The estuarine population was mainly composed of the green morphotype, with the orange and red morphotypes present in more downstream areas of the estuary. Redundancy analysis (RDA) showed high spatial and temporal variability of C. maenas in the estuary which was related with environmental changes over the 15-year period. A correlation between C. maenas biological features and several local-scale (salinity and river runoff) and large-scale (North Atlantic Oscillation index and Eastern Atlantic pattern) environmental variables was identified through cumulative sums analysis (CUSUM), indicating a strong environmental control on C. maenas population dynamics. This paper shows the importance of relatively long-term datasets to unravel the effects of extreme weather events due to climate change on key epibenthic estuarine species, and also how they might cope with a changing marine environment.
... Multiple studies have demonstrated positive correlations between School Prawn productivity and freshwater discharge into estuaries (Ruello, 1973;Glaister, 1978;Loneragan and Bunn, 1999;Ives et al., 2009;Gillson et al., 2012), and prolonged dry weather normally adversely affects School Prawn and results in a smaller population and harvest (Ruello, 1973). As noted earlier, estuary inflow is generally important for juvenile School Prawn, but it is also important in the maturation, emigration and aggregation of sub-adults and adults (Ruello, 1973;Glaister, 1978). ...
Article
Seasonal ecological effects caused by temperature and photoperiod are typically considered minimal in the tropics. Nevertheless, annual climate cycles may still influence the distribution and abundance of tropical species. Here, we investigate whether seasonal patterns of precipitation and wind speed influence the structure of coastal fish assemblages and fishing yields in northeast Brazil. Research trips were conducted during the rainy and dry seasons using commercial boats and gear to sample the fish community. Diversity was analyzed using abundance Whittaker curves, diversity profiles and the Shannon index. Principal Component Analysis (PCA) was used to analyze associations between the abundance of species and various environmental variables related to seasonality. A total of 2,373 fish were collected, representing 73 species from 34 families – 20 of which were classified as both frequent and abundant. Species richness was greater and more equitable during the rainy season than the dry season – driven by changes in the precipitation rather than to wind speed. Species diversity profiles were slightly greater during the rainy season than the dry season, but this difference was not statistically significant. Using PCA was identified three groups of species: the first associated with wind speed, the second with precipitation, and the third with a wide range of sampling environments. This latter group was the largest and most ecologically heterogeneous. We conclude that tropical coastal fish assemblages are largely influenced by local variables, and seasonally mediated by annual changes related to precipitation intensity and wind speed, which in turn influences fishery yields.
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This review presents a synopsis of the impacts of freshwater flow on fisheries production in estuarine and coastal systems, with particular emphasis on regional examples from eastern Australia and southern Africa. Freshwater flow impacts habitat availability, trophic interactions, and fishers’ harvesting behavior in estuarine and coastal systems. Seasonal and interannual variation in freshwater flow influences the distribution and abundance of fish and invertebrates through changes in growth, survival, and recruitment. Episodic flood and drought events have pronounced impacts on fisheries production due to rapid changes in physicochemical conditions modifying species richness and diversity. Many documented reductions in fisheries production have been attributed to river regulation modifying natural variation in freshwater flow. Protecting natural flow regimes is likely to be an effective management strategy to maintain the production of estuarine and coastal fisheries. Understanding the freshwater requirements of estuarine and coastal fisheries will become increasingly important as climate change modifies the hydrological cycle and as human population growth increases demand for water resources. One major challenge for scientists seeking to explore relationships between freshwater flow and fisheries production is to understand how variable flows influence resource availability, fishing activity, and the economic performance of commercial fisheries in estuarine and coastal systems.
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Freshwater is scarce in many regions of the world. In some areas, water entitlements currently exceed the available water supply yet few proposals for regulating quantities extracted are scrutinised in terms of possible impacts or undergo any form of rigorous impact assessment. In addition, there is little understanding of the potential impacts. There is a growing need to understand better the impact of altered flows of fresh water on estuarine and open coastal marine systems. There is a perception that fresh water is lost when it enters the marine environment. We argue that freshwater-saltwater dynamics have profound influences on coastal ecosystems. The purpose of this paper is to review the nature of freshwater discharges and the effects of fresh water on the physical aspects of estuaries as well as estuarine and marine flora, fauna and habitats. Although the review focuses on decreased flows to marine systems, major increases in flow can also have a major impact on estuarine and coastal systems. Freshwater runoff is a function of numerous environmental variables, depending primarily on climate (precipitation and evaporation) and the physical characteristics of the drainage basin. Anthropogenic activities in catchments may result in diversions and reductions in freshwater flow, alterations of timing and rates of flow to estuarine and coastal systems, and/or adverse water quality conditions with major changes in nutrient loading. Sediment loads, pH, temperature, salinity, clarity, oceanography and nutrients are affected. Perturbations in coastal systems can be freshwater pulses (i.e. storms or opening of floodgates) or press scenarios (i.e. persistent flow of low variation from rivers or industry). Impacts on organisms can also be categorised as pulse events (where there is a rapid but not sustained change), or press events (where changes are sustained over long periods of time). Changes to freshwater input affect habitats and organisms within estuaries. The effects include mortality, changes in growth and development, and in some cases movement of organisms. Major mortalities are most likely during pulse events of freshwater input. There is considerable descriptive and small-scale experimental evidence to suggest that a variety of organisms may be affected by changes to freshwater input. Much of the experimental evidence focuses on single factor experiments and rarely have there been multifactorial experiments (an exception is seagrasses). In addition, there have been no large-scale experiments (e.g. size of sample unit 10's to 100's of metres), although it is acknowledged that such experiments will be difficult. We suggest that any changes in water management (e.g. removal of water for irrigation) should be treated as manipulative experiments and that estuarine and marine systems are monitored together with reference or control locations (where there has been no change) to determine the impact of such changes. At the large scale, finding suitable control locations could be difficult. Data should be collected multiple times prior to and after the change has occurred. In the first instance, objective scientific evidence should be used for decision-making and when that is not available, we suggest that the principle of precautionary action should be adhered to. In conclusion, freshwater flows have a great impact on physical and biological aspects of coastal environments. The impacts of fresh water on marine environments, as well as terrestrial environments, should therefore, be considered by managers.
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F r e s h wa t e r f l o w a n d a b u n d a n c e o f G i l c h r i s t e l l a a e s t u a r i a N a d i n e A . S t r y d o m e t a l . M F 0 1 0 7 7 Abstract. Past investigations into South African estuaries that are subject to altered freshwater flow regimes have concentrated on the effects of reduced river flow on recruiting estuary-dependent marine fish species. This study compares larval and juvenile abundance of an estuary resident, Gilchristella aestuaria, in the upper reaches of the 'freshwater-deprived' Kariega Estuary and the 'freshwater-rich' Great Fish Estuary. Catches in the Kariega Estuary were significantly greater than those in the Great Fish Estuary. Larval and juvenile densities were correlated and regressed with salinity, temperature, turbidity and river flow in each system. River flow was found to be the most probable defining factor in terms of the contrasting catches in these systems. G. aestuaria larvae and juveniles appear to be flushed out of the Great Fish system as a result of the excessive amount of river flow received from an inter-basin water transfer scheme. Although freshwater input into estuarine systems is vital for the nursery function of the lower reaches of South African estuaries, excessive supply can alter the larval composition of estuary-resident fish species in the upper reaches.
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The operating model The model developed for the present study is a stag e/size-structured metapopulation model of the schoo l prawn stocks inhabiting the Clarence River. It incl udes individual-based population dynamics and movement between compartments that are both subject to the e ffects of temperature and river discharge. Three fi sheries exploit the modelled population with the fisher dyn amics based on a combination of historical effort l evels and changes in catch per unit of effort. The model is calibrated to 20 years of past catch data from t he Clarence River fisheries using a Bayesian SIR appro ach which produces a 'posterior' distribution from likely parameter values and the model runs that best fit t he data.
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In KwaZulu-Natal on the east coast of South Africa, the commercial and recreational boat-based linefishery is the largest fishery accounting for 1235 t (40%) of the total landed mass there. In particular, the Thukela Banks are where most fishing takes place. Numerous developments ranging from dams to inter-basin transfer schemes either exist or have been proposed for the Thukela River, which is the largest of 17 catchments entering the sea in the region and provides over 40% of the mean annual runoff. Exploratory analyses of the relationships between monthly flows and catch-per-unit-effort were performed using spectral analysis and general linear models on a data set comprising 17 years of monthly commercial catch-and-effort data for 140 species and monthly flow data from 17 catchments. Catchments having a significant influence on catches were those providing the bulk of the runoff volume (75%) reaching the sea. Significant relationships existed between flow and the catches of 14 species, which provided over 90% of the total linefish catch on the Thukela Banks. Time lags between flow events (wet and drought periods) and changes in catch-per-unit-effort corresponded in many cases to age at 50% maturity and/or age-at-first-capture of the species concerned. Under a future maximum-development scenario, corresponding to a 44% reduction in flow from the Thukela River, catches of slinger Chrysoblephus puniceus and squaretail kob Argyrosomus thorpei are forecast to decline by 36% and 28% respectively. These two species currently provide over 50% of the landed mass on the Thukela Banks. Some species should respond positively to a reduction in flow, but any increases in their catches will be negligible, falling within natural variability, and will be insufficient to offset losses of other species because their abundance is naturally low or has been fished down to low levels. National legislation requires that sufficient freshwater flows be set aside to protect ecosystems, so these findings have important management implications, as marine needs for freshwater flows have not previously been addressed.
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Transport to and retention of early-life history stages within the estuarine turbidity maximum (ETM) region may be an important component of recruitment success of the anadromous striped bass Morone saxatilis and the white perch M, americana. This hypothesis was evaluated with spatially resolved surveys of the physical and biological characteristics of the upper Chesapeake Bay estuary during 5 cruises, 3 in May 1998 and 2 in May 1999. Gradients in depth-specific patterns of larval abundance and zooplankton prey were evaluated with respect to salinity, temperature and total suspended solids (TSS). Although physical conditions in the upper estuary differed between cruises and years, the spatial patterns in distribution of larvae and potential prey were consistent and tracked changes in salt front and ETM locations. A statistical analysis suggested that location relative to the ETM explained a significant amount of variability in concentrations of eggs, yolk-sac larvae, and larval prey, and that prey concentrations accounted for a significant amount of variability in concentrations of feeding larvae. Transport to the ETM region probably occurs during the egg (striped bass) and yolk-sac larva (white perch) stages and results in retention of early-stage feeding larvae in a zone of high prey concentrations. Physical conditions in the ETM region differed markedly between a low freshwater-flow year (1999) and a high flow year (1998), and abundances of striped bass and white perch post-yolk-sac larvae were significantly lower in 1999. Recruitment variability was poorly correlated with spawning stock biomass, but spawner-recruitment models that incorporated spring freshwater discharge explained an additional 41 % (striped bass) and 30 % (white perch) of the variability in recruitment. Annual changes in freshwater flow could control larval survival and recruitment by modifying the physical and biological characteristics of the ETM region.
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This paper relates daily mean seawater temperature and daily rainfall in northern Taiwan to the catches of A. japonica elvers during a two-year period. The results indicate a positive (P<O.05) relationship between rainfall and elver catches and a negative (P<O.05) relationship between seawater temperature and elver catches. Peaks in elver catches generally coincided with peaks in rainfall and minimums of temperature. Rainfall is clearly as important in affecting elver catch as is seawater temperature, which was previously regarded as the key factor.
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To confirm the occurrence of marine residents of the Japanese eel, Anguilla japonica, which have never entered freshwater ('sea eels'), we measured Sr and Ca concentrations by X-ray electron microprobe analysis of the otoliths of 69 yellow and silver eels, collected from 10 localities in seawater and freshwater habitats around Japan, and classified their migratory histories. Two-dimensional images of the Sr concentration in the otoliths showed that all specimens generally had a high Sr core at the center of their otolith, which corresponded to a period of their leptocephalus and early glass eel stages in the ocean, but there were a variety of different patterns of Sr concentration and concentric rings outside the central core. Line analysis of Sr/Ca ratios along the radius of each otolith showed peaks (ca 15 x 10(-3)) between the core and out to about 150 mum (elver mark). The pattern change of the Sr/Ca ratio outside of 150 mum indicated 3 general categories of migratory history: 'river eels', 'estuarine eels' and 'sea eels'. These 3 categories corresponded to mean values of Sr/Ca ratios of greater than or equal to 6.0 x 10(-3) for sea eels, which spent most of their life in the sea and did not enter freshwater, of 2.5 to 6.0 x 10(-3) for estuarine eels, which inhabited estuaries or switched between different habitats, and of < 2.5 x 10(-3) for river eels, which entered and remained in freshwater river habitats after arrival in the estuary. The occurrence of sea eels was 20% of all specimens examined and that of river eels, 23%, while estuarine eels were the most prevalent (57%). The occurrence of sea eels was confirmed at 4 localities in Japanese coastal waters, including offshore islands, a small bay and an estuary. The finding of estuarine eels as an intermediate type, which appear to frequently move between different habitats, and their presence at almost all localities, suggested that A. japonica has a flexible pattern of migration, with an ability to adapt to various habitats and salinities. Thus, anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, and this form of diadromy should be defined as facultative catadromy, with the sea eel as one of several ecophenotypes. Furthermore, this study indicates that eels which utilize the marine environment to various degrees during their juvenile growth phase may make a substantial contribution to the spawning stock each year.
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We used an electron probe microanalyzer (EPMA) to determine the migratory environmental history of the catadromous grey mullet Mugil cephalus from the Sr:Ca ratios in otoliths of 10 newly recruited juveniles collected from estuaries and 30 adults collected from estuaries, nearshore (coastal waters and bay) and offshore, in the adjacent waters off Taiwan. Mean (+/-SD) Sr:Ca ratios at the edges of adult otoliths increased significantly from 6.5 +/- 0.9 x 10(-3) in estuaries and nearshore waters to 8.9 +/- 1.4 x 10(-3) in offshore waters (p < 0.01), corresponding to increasing ambient salinity from estuaries and nearshore to offshore waters. The mean Sr:Ca ratios decreased significantly from the core (11.2 +/- 1.2 x 10(-3)) to the otolith edge (6.2 +/- 1.4 x 10(-3)) in juvenile otoliths (p < 0.001). The mullet generally spawned offshore and recruited to the estuary at the juvenile stage; therefore, these data support the use of Sr:Ca ratios in otoliths to reconstruct the past salinity history of the mullet. A life-history scan of the otolith Sr:Ca ratios indicated that the migratory environmental history of the mullet beyond the juvenile stage consists of 2 types. In Type 1 mullet, Sr:Ca ratios range between 4.0 x 10(-3) and 13.9 x 10(-3), indicating that they migrated between estuary and offshore waters but rarely entered the freshwater habitat. In Type 2 mullet, the Sr:Ca ratios decreased to a minimum value of 0.4 x 10(-3), indicating that the mullet migrated to a freshwater habitat. Most mullet beyond the juvenile stage migrated from estuary to offshore waters, but a few mullet less than 2 yr old may have migrated into a freshwater habitat. Most mullet collected nearshore and offshore were of Type 1, while those collected from the estuaries were a mixture of Types 1 and 2. The mullet spawning stock consisted mainly of Type 1 fish. The growth rates of the mullet were similar for Types 1 and 2. The migratory patterns of the mullet were more divergent than indicated by previous reports of their catadromous behavior.
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Populations of native and introduced aquatic organisms in the San Francisco Bay/Sacramento- San Joaquin Delta Estuary ("BayJDelta") have undergone significant declines over the past two decades. De- creased river inflow due to drought and increased freshwater diversion have contributed to the decline of at least some populations. Effective management of the estuary's biological resources requires a sensitive indicator of the response to freshwater inflow that has ecological significance, can be measured accurately and easily, and could be used as a "policy" variable to set standards for managing freshwater inflow. Positioning of the 27~0 (grams of salt per kilogram of seawater) bottom salinity value along the axis of the estuary was examined for this purpose. The 2% bottom salinity position (denoted by X,) has simple and significant statistical relationships with annual measures of many estuarine resources, including the supply of phytoplankton and phytoplankton-derived detritus from local production and river loading; benthic macroinvertebrates (molluscs); mysids and shrimp; larval fish survival; and the abundance of planktivorous, piscivorous, and bottom-foraging fish. The actual mechanisms are understood for only a few of these populations. X, also satisfies other recognized requirements for a habitat indicator and probably can be measured with greater accuracy and precision than alternative habitat indicators such as net freshwater inflow into the estuary. The 2%~ value may not have special ecological significance for other estuaries (in the BayJDelta, it marks the locations of an estuarine turbidity maximum and peaks in the abundance of several estuarine organisms), but the concept of using near-bottom isohaline position as a habitat indicator should be widely applicable. Although X, is a sensitive index of the estuarine community's response to net freshwater inflow, other hydraulic features of the estuary also determine population abundances and resource levels. In particular, di - version of water for export from or consumption within the estuary can have a direct effect on population abundance independent of its effect on X,. The need to consider diversion, in addition to X,, for managing certain estuarine resources is illustrated using striped bass survival as an example. The striped bass survival data were also used to illustrate a related important point: incorporating additional explanatory variables may decrease the prediction error for a population or process, but it can increase the uncertainty in parameter estimates and management strategies based on these estimates. Even in cases where the uncertainty is currently too large to guide management decisions, an uncertainty analysis can identify the most practical direction for future data acquisition.
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Freshwater outflow from rivers to the ocean has profound effects on the physical, chemical, and biological processes in coastal waters. It induces circulation patterns, modifies mixing processes, and influences nutrient supply to plants, thus affecting biomass production. The increased need for hydroelectric power, flood control, and irrigation of farmland has led to greater regulation of river flow, which may have signigicant ''down-stream'' consequences, such as changes in fish production. This book adopts a systems approach for modeling how marine ecosystems respond to natural and man-made changes in freshwater outflow, and also attempts to identify research priorities in coastal zone management.
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A stratified study of microhabitat use by grey mullet on the island of Minorca (Balearic archipelago, western Mediterranean) showed that the distribution of all the species was dramatically affected by salinity. Sites with a salinity level under 15 were positively selected in spring and summer by those species whose growth performance was the best in oligomesohaline water (Liza ramado and Mugil cephalus) but also by a species whose growth was not affected by salinity (Chelon labrosus). Liza aurata concentrated in polyhaline and euhaline sites, where growth was improved, a pattern also exhibited by Liza saliens. Both species avoided fresh water sites all year round. As a consequence, community structure was correlated with salinity. The above reported electivity patterns often disappeared in autumn, when most grey mullets migrate off-shore.
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Kirkup et al. (1998) [Australian Geographer 29, pp. 241–55] criticise our theory of alternating flood regimes and question its application to river management. A brief but critical appraisal of their work shows many errors and misrepresentations. When these are corrected, their challenge is found deficient.
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The present study describes the seasonal changes of the fish species composition in three areas of the main channel of the Caeteiver estuary, Brazil. The fish faunas of each habitat differed in density, biomass and species composition. Mean fish density and biomass for the Caeteiver estuary channel was 0Á25 individuals m À2 and 0Á9 g m À2 respectively. Analysis of catch data showed that the number of species, total density and total biomass differed significantly between areas and seasons. For the most important species, the mean density of Cathorops spixii, Aspredinichthys filamentosus, Aspredo sp. 2, Pimelodus blochii, Pseudauchnipterus nodosus and Macrodon ancylodon, differed significantly between seasons while the mean density of Stellifer rastrifer, Stellifer microps, Aspredo aspredo, Aspredo sp. 1 and Cynoscion acoupa did not. The mean biomass of these species, with exception of S. microps and Aspredo sp. 1, also differed significantly between seasons. In the Caeteéstuary seasonal salinity fluctuations appeared to be the main factor that structured the fish assemblage in the entire estuarine system. At least 85% of the species captured by the artisanal and subsistence fisheries in the Bragantine region required estuarine conditions to complete their life cycle.
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The effects of freshwater regulation and diversion on the adult and larval stages of fish and invertebrates in coastal marine waters are reviewed. Potential impacts of river modification are highlighted based on our present understanding of the role of fresh water on the physical, chemical and biological processes on the marine environment. These include effects on migration patterns, spawning habitat, species diversity, water quality and distribution and production of lower trophic levels. The effect of dams on anadromous and catadromous fish are also presented. We discuss in detail the marine response to specific river regulation projects on the Nile, Indus and rivers flowing into the Black Sea, San Francisco Bay and James Bay in Canada. A decline in some coastal fisheries with an overall negative impact on the biota is generally associated with reductions in freshwater flow. Extensive ecological considerations are needed during the planning stage of large-scale freshwater modification projects to minimize potential impacts.
Book
Over the last decade or two marine scientists have become more aware of the role of freshwater outflow in coastal waters. Some have raised the question whether or not regulation of river flow may change the biological productivity in coastal marine ecosystems. The idea of organising a workshop to deal with this problem, arose from parallel concern in Norway and Canada, but it was soon recognised that the scientific interests were also shared by research groups in other countries. After the initial contacts had been made, it was agreed to establish a committee which should assist the workshop director in organising the workshop. The Organising committee consisted of K.F. Drinkwater, Bedford Institute of Oceanography, Canada; S. Skreslet (workshop director), Nordland College, Norway; V.S. Smetacek, Institut fur Meereskunde an der Universitat Kiel, Germany; and H. Svendsen, University of Bergen, Norway. An Advisory committee was established to provide terms of reference for the Organising committee, with regard to the programme and selection of participants. This committee con­ sisted of J.B.L. Matthews (chairman), Scottish Marine Bio­ logical Association, U.K.; T.R. Parsons, University of British Columbia, Canada; R. S~tre, Institute of Marine Research, Nor­ way; M.M. Sinclair, Fisheries and Oceans, Canada; and W.S. Wooster, University of Washington, Canada.
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1. Major reductions in catchment run-off, a result of frequent and prolonged drought conditions, together with extensive impoundment of rivers and streams, has led to concern about the possible negative effects on downstream estuaries. Preliminary studies have shown that changes in river flow and associated nutrient inputs have had a predominantly negative impact on the aquatic biota of Eastern Cape estuaries. Natural successions now have human imposed trajectories which are reducing variability and forcing both freshwater ‘deprived’ and freshwater ‘enriched’ systems into artificial cycles.
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Contains 30 papers or group reports which use a systems approach for modeling how marine ecosystems respond to natural and man-made changes in freshwater outflow, and attempts to identify research priorities in coastal zone management. Most of the papers (abstracted separately) are arranged in sections covering: inshore waters; marginal seas; shelf waters. -M.A.Bass
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Intercontinental comparisons of streamflow statistics are made using a global data set, followed by a summary of the regional variations of Australian streamflow characteristics. The implications of these characteristics for Australian hydrology and river management are considered. Australia and Southern Africa have more variable annual river flows and annual floods than the rest of the world's continents because rainfall in these two continents is more variable than elsewhere. The connection between runoff variability and streamflow variability is difficult to analyse without a time series of catchment rainfalls comparable to the runoff data. In the absence of appropriate rainfall data, the relationship has been investigated using only catchment <10 000 km2, each one paired with the nearest rain gauge drawn from the NCAR tape. The relationship between Cvr and Cvp is shown for various separation distances between rain and stream gauges. -from Authors
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Gillnets were used on two occasions during 1980 and from January 1981 to January 1982 to determine the catch per unit effort for fishes in the Gamtoos estuary. The mean of the fish caught was 43 fish per net (33,3 kg), which compares favourably with other larger systems like the Sundays, Bashee and Kei estuaries. All these estuaries are subject to regular floods and have relatively small mudflats and Spartina beds in the mouth areas. Flood conditions, which occurred during seven months in 1981, caused reduced salinities and high turbidity throughout the estuary and were negatively correlated with catch returns. Mullet (Fam. Mugilidae) dominated catches numerically (12 per net) followed by kob Argyrosomus hololepidotus (11,4), sea-catfish Tachysurus feliceps (10,9) and leervis Lichia amia (5,2).Kiefnette is by twee geleenthede gedurende 1980 en vanaf Januarie 1981 tot Januarie 1982 gebruik om vangs per eenheid poging van visse in die Gamtoos-getyrivier te bepaal. Daar is gemiddeld 43 visse per net (33,3 kg) gevang, wat baie goed vergelyk met vangste in soortgelyke groter sisteme soos die Sondags-, Bashee- en Kei-getyriviere. Soos in die geval van die Gamtoos is hierdie getyriviere ook onderhewig aan periodieke vloede en word hulle gekenmerk deur die afwesigheid van modder- en slikmoerasse bedek met Spartina gras, behalwe tot ‘n geringe mate in die mondgebiede. Vloede, wat ‘n drastiese verlaging in soutgehalte en hoë troebelheid van die hele getyrivier tot gevolg gehad het, het gedurende sewe van die 15 opname maande voorgekom en het ‘n negatiewe korrelasie t.o.v. vangopbrengs getoon. Die vangste is oorheers deur harder (Fam. Mugilidae) (12 per net) gevolg deur kabeljou Argyrosomus hololepidotus (11,4), seebarber Tachysurus feliceps (10,9) en leervis Lichia amia (5,2).
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Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.
Article
Estuarine food webs are frequently altered by human interventions, including freshwater diversions, toxic compounds, and introduced species. From 1988 through 1991 we examined the external morphological and internal histopathologic condition of larval striped bass (Morone saxatilis) to evaluate the potential importance of starvation to fish recruitment in the San Francisco Bay estuary. During a recent drought (1987-1992), fish populations declined markedly, concurrent with dramatic reductions in phytoplankton and zooplankton food for larval fishes. Such patterns suggest pelagic food is limited during times of low freshwater input; therefore, larval starvation may limit recruitment. However, toxic compounds in agricultural runoff are also less diluted in low-outflow years, enhancing their potential impact. Histopathology enabled us to identify their possible effects. In the laboratory, indices of larval morphology and eye and liver tissue condition reflected starvation after 2 d of food deprivation. From 1988 through 1991 >90% of 980 field-caught specimens were classified morphologically as feeding larvae. Histopathological evaluation indicated that all field-caught specimens (N = 500) had food in their guts and lacked tissue alterations consistent with starvation. However, liver alterations consistent with toxic exposure were seen in 26-30% of the field-caught larvae from 1988 through 1990, dropping to 15% in 1991. While our findings implicate toxic exposure as a factor in the relationship between low freshwater input and poor year-class success of striped bass, reductions of toxic runoff and improvement in larval liver condition in 1991 did not improve larval survival. This suggests the potentially greater importance of interactions with food limitation and predation as well as the futility of pursuing single-factor explanations for recruitment failure. The potential obfuscation of food limitation by toxic exposure also indicates the need for interdisciplinary approaches to distinguishing anthropogenic intervention from estuarine food-web processes.
Article
River discharge has long been recognized as one of the factors that contributes to the high productivity of estuaries. Although there is little evidence that river inputs of terrestrial carbon make a direct contribution to coastal food webs, such exported nutrients may stimulate in situ production in estuaries and thus enhance the survivorship and growth of fish and crustaceans in these systems. Furthermore, fluctuations in salinity and turbidity may influence the extent of available habitat for fish and crustaceans and therefore their distribution and/or catchability. Despite these potential links between flow and the secondary production of estuaries and coastal waters, there is still a common perception that ‘water going to sea is wasted’ and a continuing trend to regulate the flow of rivers. We review the evidence for links between river flow and the productivity of estuarine/coastal fisheries, drawing on a case study of the Logan River in southeast Queensland, and explore the potential mechanisms for these linkages. Our research, and that of others, confirms that high river discharge can have a strong positive effect on the production of commercial and recreational coastal fisheries. It also shows that the seasonal pattern of flow is equally, if not more important, than the magnitude of flow. River regulation is likely to have a dramatic effect on the production of coastal fisheries and, given the current pressures for water resource development, this is an important avenue for future research and evaluation.
Article
Results from field studies in Natal estuaries show that the distribution of juvenile marine fish is influenced by turbidity. Laboratory studies on turbidity preference, with other variables excluded, showed good correlation with the field data for eight of ten species tested. The importance of turbidity and other factors to juvenile fish is discussed in relation to the role which estuaries play as nursery areas for juveniles of numerous marine species.
Article
There is strong circumstantial evidence worldwide that nutrient enriched riverine discharge enhances fishery production on adjacent continental shelves, and this appears to be the case with the Mississippi River where 70–80% of Gulf of Mexico fishery landings come from waters surrounding the Mississippi delta. Two major species groupings, estuarine dependent species (e.g., red drum, Sciaenops ocellatus; spot, Leiostomus xanthurus; and Atlantic croaker, Micropogonias undulatus) and coastal species, (e.g., king mackerel, Scomberomorus calvalla; Spanish mackerel, Scomberomorus maculatus; and bluefish, Pomatomus saltatrix) are most likely to be influenced by riverine discharge. While riverine enhancement of fishery production seems clear, the exact mechanisms through which this occurs are not. Because recruitment makes the greatest contribution to fish stock biomass, it is by enhancing recruitment that fishery production is influenced most. Waters influenced by the river discharge are a rich environment where both physical dynamics, (e.g., hydrodynamic convergence, water column stratification, and transport and retention of fish larvae) and biological dynamics (e.g., primary and secondary production and larval fish production, feeding, growth, and predation) may favor processes that regulate survival and recruitment.
Article
Spawning grounds of the soles, Solea solea and Solea senegalensis, are distant from the estuarine nurseries where juveniles concentrate. Recruitment of these species is highly dependent on the success of the larval migration towards the inshore nursery grounds. Unfavourable climate and hydrodynamic circulation may lead to high mortality rates at this stage. The relation between river drainage, NAO index and the North-South wind component intensity over the three months prior to the end of the estuarine colonization and the densities of S. solea and S. senegalensis in the nursery grounds were investigated for both species based on a discontinuous historical dataset (from 1988 to 2006) for the Tagus estuary. Multiple linear regression models were developed for sole density and environmental data (separately for each species). Results showed that river drainage is positively correlated with juveniles' densities of both species, possibly due to the existence of chemical cues used by larvae for movement orientation. NAO index and the North-South wind component intensity relations with soles densities were non-significant. It was concluded that the high complexity of the Portuguese upwelling system makes it hard to detect causal relations of the environmental variables tested. The importance of river flow for coastal ecosystems was stressed. Since climate change scenarios predict a strong decrease in rain fall over the Portuguese river basins, as well as a concentrated period of heavy rain in winter, it was hypothesised that future river drainage decrease over much of the year may lead to lower recruitment success for soles, especially for S. senegalensis.
Article
All ecosystems are influenced by physical forcing. Estuarine ecosystems respond most strongly on an interannual timescale to variability in freshwater flow. Several mechanisms for positive or negative flow effects on biological populations in estuaries have been proposed; however, positive effects appear to operate mainly through stimulation of primary production with effects propagating up the food web. In the northern San Francisco Estuary, abundance or survival of several common species of fish and shrimp varied positively with flow-in data through 1992. I re-examined these relationships and those of several additional taxa in an analysis of long-term (20 to 40 yr) monitoring data. The spread of the introduced clam Potamocorbula amurensis in 1987 provided an opportunity to examine simultaneously the responses of estuarine species to flow and to changes in the food web. I separated variability into a flow response, a step change after 1987 and other sources of variability. Responses of fish and shrimp contrasted with those of lower trophic levels. All but 1 species of nekton responded positively to flow, only 2 had clear declines after 1987, and none of the relationships changed in slope after 1987. In contrast with the higher trophic levels, chlorophyll a (chl a) and several species of zooplankton declined markedly after 1987, and had either weak responses to flow or responses that changed after 1987. Thus, the food web appears strongly coupled between benthos and plankton, and weakly coupled between zooplankton and fish, as has been found in other systems. More importantly, the variation with freshwater flow of abundance or survival of organisms in higher trophic levels apparently did not occur through upward trophic transfer, since a similar relationship was lacking in most of the data on lower trophic levels. Rather, this variation may occur through attributes of physical habitat that vary with flow.
Article
The freshwater-flow requirements of estuarine fisheries in tropical areas are reviewed, with reference to species important to fisheries in northern Australia. Fisheries production, in terms of catch, is often elevated during, or as a consequence of, years with higher river flow, but the causality of these relationships often remains unproven. Scientific information on the freshwater-flow requirements important to fisheries production is increasingly being sought during the planning, allocation and management of water resources within Australia and in other countries around the world. Frequently, such advice is based on the analysis of catch and freshwater flow (or rainfall), or on life-history information. Clarifying fisheries-specific goals of water management would assist in prioritising research into the freshwater-flow requirements of estuarine fisheries. A framework that integrates life-history information and correlative analyses is suggested to assist in understanding the freshwater-flow requirements of estuarine fisheries. The framework is also useful in identifying knowledge gaps and pertinent research questions. The approach is illustrated through its application to identifying key freshwater-flow events likely to be important for fisheries production in a dry tropical estuary in Queensland, Australia.
Article
Relationships between river discharge and production (catch) of the school prawn M. macleayi were examined as part of an ecological study of this species in the Clarence River region. Schooling behaviour of juvenile M. macleayi exhibited a lunar periodicity and mean daily abundance peaked 5 days after full moon. This was followed 7 days later by a peak in mean daily abundance of emigrating juvenile M. macleayi. Fluctuations in the magnitude of the oceanic component of the total annual catch were found to determine the difference between high and low production seasons. During the period examined there was a direct relationship between Clarence River discharge and the oceanic component of total production of M. macleayi, for various time periods. The absence of a persistent trend in total annual production and annual catch per unit effort of M. macleayi in the Clarence River region suggests that the stock has not been overfished. Common peaks in mean monthly oceanic production of M. macleayi off the Clarence, Evans and Richmond Rivers suggest a common enhancement of the seasonal emigration of M. macleayi from the three estuaries in November due to increased river discharges. Successive troughs and peaks in mean monthly oceanic production suggest a northwards migration of M. macleayi from the Clarence and Evans Rivers and subsequent recruitment to the Evans and Richmond ocean fisheries respectively. Results indicate the importance of river discharge to production of M. macleayi and suggest that modification of discharges by restriction of freshwater flow could adversely effect production.
Article
Multivariate analyses of estuarine geomorphic variables and fish-catch data support the hypothesis that estuaries in New South Wales can be classified in terms of measurable geomorphic characteristics, and that these geomorphic characteristics exert an influence on estuarine biota. The clearest example of this association is the extent to which the area of seagrass, mangrove and central mud basin relates to geomorphic settings and the extent to which the area of these units in turn predict commercial landings of fish and crustaceans. Infilled estuaries with sand barrier entrances, with higher areas of mangrove and lower areas of seagrass and central mud basin yield a catch of fish different to estuaries which are not infilled, and contain relatively high areas of seagrass and central mud basin, and less mangrove. The species contributing most to these differences in assemblages are those which multiple regression analysis identifies to be consistently associated with seagrass area, and which ecological studies reveal to be dependant upon seagrass in their post-settlement phase.
Article
1. Major reductions in catchment run-off, a result of frequent and prolonged drought conditions, together with extensive impoundment of rivers and streams, has led to concern about the possible negative effects on downstream estuaries. Preliminary studies have shown that changes in river flow and associated nutrient inputs have had a predominantly negative impact on the aquatic biota of Eastern Cape estuaries. Natural successions now have human imposed trajectories which are reducing variability and forcing both freshwater ‘deprived’ and freshwater ‘enriched’ systems into artificial cycles.
Article
The blue swimmer crab Portunus pelagicus was sampled monthly in Leschenault Estuary and the marine embayment (Koombana Bay) into which it opens. The trends exhibited by density and size-frequency data are consistent with those previously recorded for this portunid in this estuary, implying that the spawning period, the timing of immigration and emigration, and the pattern of growth are similar each year in Leschenault Estuary. The monthly trends exhibited by the numbers and prevalence of ovigerous female crabs, together with previous data on the distribution of crab zoea, provide strong circumstantial evidence that P. pelagicus typically release their presumptive zoea in Koombana Bay, rather than in Leschenault Estuary, and that this occurs predominantly in mid-spring to late summer. Although a few 0+ recruits enter the estuary in the ensuing months, their numbers do not start to rise markedly until the following mid-to late spring, when salinities and water temperatures in the estuary are rising appreciably. At this time, the carapace widths (CW) of the majority of these crabs lie between 40 and 100 mm. During the ensuing months, the corresponding cohort increases greatly in both numbers and body size, with the result that, by the completion of their first year of life in January, the CW of many of its females and males have reached 97 mm and 84 mm, respectively, the size at which 50% of the corresponding sexes of P. pelagicus undergo a pubertal moult. These and other data demonstrate that P. pelagicus first spawn when they are approximately one year old. Although the abundance of crabs in Leschenault Estuary rises during spring and summer, that of particularly the older crabs subsequently falls precipitously during the subsequent winter months and early spring, when the crabs are ca 18-20 months in age and salinities and water temperatures undergo a marked decline. The pronounced decline in the abundance of crabs in the estuary in winter and early spring is accompanied by a conspicuous rise in their numbers in the shallows of Koombana Bay, indicating that the crabs, which had emigrated from the estuary, had entered the alternative habitats provided by that embayment. A sub- stantial number of crabs are found in Koombana Bay throughout the year. The sex ratios of crabs in samples collected by different methods indicate that females tend to leave the estuary before males and that crab pots have a greater tendency to catch male crabs than female crabs. The catch and catch per unit effort of the commercial fishery for P. pelagicus in Leschenault Estuary is greatest in January and February. This reflects in part the fact that, during these months, crabs are particularly abundant and are starting to attain the legal minimum size for capture (CW = 127 mm). This feature, together with the predominance of undersized crabs in earlier months and the emigration of crabs from the estuary in winter, help account for the marked seasonality of the fishery for this portunid in this estuary. Furthermore, as water temperatures are at their maxima in summer, the crabs are likely to be particularly active at this time and thus also most susceptible to capture. In the Leschenault Estuary, P. pelagicus feeds mainly on slow-moving or sessile macrobenthic invertebrates, such as amphipods, polychaetes, and bivalve and gastropod molluscs, and also occasionally on gobies, and thus in this system it can be regarded as essentially a benthic carnivore.
Article
Two and a half years of data were collected from the lower Great Fish River, head region and estuary to determine the fish species composition within these areas. Gilchristella aestuaria, Liza dumerilii, Rhabdosargus holubi and Pomadasys commersonnii were the four most abundant species captured, with riverine flow rate having an important effect on both species composition and numbers of fishes in the different regions. Most marine species displayed a strong inverse relationship between catch per unit effort and elevated freshwater inputs. Euryhaline marine species dominated the catches at all sampling sites during low flows but were less common during high flow periods when catadromous species were most abundant. Based on the available evidence it is suggested that for most marine species in the river this decline in abundance is related to low conductivity levels following floods rather than avoidance of elevated flows. The impact of elevated suspensoid concentrations and lowered dissolved oxygen concentrations on freshwater and estuarine fish populations during major river flooding is also discussed.
Article
Economics for Fisheries Management, by R. Quentin Grafton, James Kirkley, Tom Kompas and Dale Squires. Ashgate, 2006. 161pp. ISBN 0-7546-3249-0
Article
Fishery systems involve complex interactions between resource stocks and the people involved in harvesting those stocks. A bio-socio-economic modelling approach is developed here to incorporate these effects within a multi-objective optimization framework. Fishery labour dynamics are determined by the decisions of individual fishermen, with net migration into and out of the fishery (and hence the fishing community) dependent on internal conditions, such as per capita incomes and employment rates, as well as on the state of the external economy. The task of fishery management is then one of balancing multiple objectives - such as conservation, income generation, employment, and community stability - subject to fish and fishermen dynamics. Control theory and simulation methods are used to study the bio-socio-economic dynamics of the fishery system and the interactions of multiple management objectives in determining the resulting fishery equilibrium. -from Author
Article
Warming of the planet is indisputable and will lead to more frequent extreme events, such as droughts. From June 2003 to March 2008, the effects of variations in river flow, associated with drought conditions, were studied in the fish assemblage of the Mondego estuary, Portugal. Over this time, two distinct hydrological periods were identified: non-drought years and drought years, with consistent changes in the fish assemblages. In the drought years, salinity increased inside the estuary, displacing the estuarine brackish habitats to more upstream areas. During this period, new marine adventitious species were found mainly in the most downstream areas, while the freshwater species disappeared from the Mondego estuary catchment area. For the marine estuarine-dependent species that use estuaries as nursery areas, a decrease in abundance was observed during the drought years, owing to the decrease in freshwater flow and reduced river plume to the coastal area. In non-drought years, there were higher densities of most species and the more important species of the fish community. Our work shows that extreme events such as droughts, related to climatic changes, influenced the structure and composition of the Mondego estuary fish assemblages, and should be further considered when undertaking management plans for transitional waters.
Article
We determined how environmental variability affected distributions of seasonally recruiting fishes (i.e., transient species) in coastal lagoons of Maryland from May–October during 1996, 1997, and 1999. A total of 241 monthly sampling events were conducted in the coastal lagoons at 40 sampling stations. Fluctuations in salinity resulting from variation in stream discharge were negatively correlated with intra-annual stability of fish assemblages. Transient, or non-resident species (e.g., recruiting species), were more frequent in habitats where salinity was less variable. When stream discharge lessened during dry years, transient species were more common throughout the coastal lagoons. Thus, environmental variability influenced distributions of young-of-year fishes in coastal estuaries.
Article
Flathead grey mullet (Mugil cephalus L. 1758) are commonly found in tropical and temperate estuaries. In spite of their ecological and economical importance, little is known about the effects of salinity on growth performance and habitat selection. Laboratory experiments revealed that the metabolic rate of young specimens was negatively affected by high salinity levels and that an improved growth performance was achieved in fresh water and oligohaline water. A stratified study on microhabitat use, carried out on the island of Minorca (Balearic archipelago), demonstrated that juvenile specimens, shorter than 200 mm (total length), concentrated all year round in fresh water or oligohaline sites. Mesohaline areas were usually avoided, except in summer. Immature fish, with a total length between 201 and 300 mm, show a similar pattern although in some seasons avoided freshwater sites. The habitat selection pattern of adults, i.e., fish longer than 301 mm, changed seasonally due to their offshore migration during the spawning season (from late summer to early winter). However, they usually showed a greater preference for polyhaline areas and strongly avoided freshwater sites. Euhaline areas were also avoided in autumn and summer. These results suggest that the young of this species are highly dependent on areas of low salinity and any factor which reduces the availability of such areas will, in turn, affect their fishery. Adults depend on polyhaline areas, although the avoidance of freshwater areas might be due to their shallowness. The situation might be different in some areas of the Indo-Pacific. These differences are discussed.
Article
River discharge has long been recognized as one of the factors that contributes to the high productivity of estuaries. Although there is little evidence that river inputs of terrestrial carbon make a direct contribution to coastal food webs, such exported nutrients may stimulate in situ production in estuaries and thus enhance the survivorship and growth of fish and crustaceans in these systems. Furthermore, fluctuations in salinity and turbidity may influence the extent of available habitat for fish and crustaceans and therefore their distribution and/or catchability. Despite these potential links between flow and the secondary production of estuaries and coastal waters, there is still a common perception that ‘water going to sea is wasted’ and a continuing trend to regulate the flow of rivers. We review the evidence for links between river flow and the productivity of estuarine/coastal fisheries, drawing on a case study of the Logan River in southeast Queensland, and explore the potential mechanisms for these linkages. Our research, and that of others, confirms that high river discharge can have a strong positive effect on the production of commercial and recreational coastal fisheries. It also shows that the seasonal pattern of flow is equally, if not more important, than the magnitude of flow. River regulation is likely to have a dramatic effect on the production of coastal fisheries and, given the current pressures for water resource development, this is an important avenue for future research and evaluation.
Chapter
IntroductionDefinition of DisturbanceDisturbances and ResponsesDisturbance and RefugiaFloodsDroughtsThe Responses to FloodsResponses to DroughtSummaryHydrological Disturbances and Future ChallengesAcknowledgementsReferences
Article
The St Lucia estuarine system is the largest of its kind in Africa. Torrential rains from Cyclone Domoina during late January 1984, followed two weeks later by Cyclone Imboa, caused extensive flooding of the St Lucia system reducing salinities from a maximum of S=60 to S = 5 over a period of two weeks. The effects of this disturbance on the ichthyoplankton were monitored 3 km from the estuary mouth over the following ten months when physical conditions in the system began to stabilize. These data are compared with ichthyoplankton catch data collected at the same site immediately before the cyclones from March 1982 until January 1984. Results showed that flooding had relatively long-term effects on the distribution and numbers of some estuarine resident and marine migrant species which utilize the estuary as a nursery ground. The frequency of occurrence and proportional density of the estuarine resident Gilchristella aestuarius increased after the floods while that of Croilia mossambica and Gobius acutipennis decreased. No changes in the proportional densities of other estuarine resident species were noted. Increases were recorded in proportional densities of marine migrant species Elops machnata, Leiognathus equula, Johnius dussumierii and Pomadasys olivaceum, while proportional densities of Solea bleekeri and Rhabdosargus sarba decreased. The ecological significance of environmental disturbances in the St Lucia estuary by episodic flooding and its effect on the distribution and abundance of estuarine resident species and marine migrant species are discussed.
Article
The production of resident (Pomatoschistus minutus and Pomatoschistus microps) and marine juvenile fish species using the Mondego estuary (central Portugal) as nursery grounds (Dicentrarchus labrax, Platichthys flesus, Solea solea), was assessed in order to: (1) understand the potential of the estuary for fish production; (2) know the production of nursery fish species likely to be exported to the coastal stocks; and (3) how anthropogenic and natural stress could influence the estimated production. Sampling occurred from June 2003 to May 2006 and together the 5 species in study comprised around 70% of the whole fish community numbers and biomass. Increasing drought conditions were observed, starting with a normal hydrological year in 2003 until attaining a severe drought in 2005, which resulted in low river discharges (1/3 of the mean river discharges in 2003). Additionally, high water temperatures were observed in 2003 and 2005 (24 and 26 C, night tem-peratures). The secondary production was estimated using the increment summation method, after recognition of the cohorts. Production was in general lower in the Mondego estuary when compared to other systems, which was associated to the estuary's small area (only 3.4 km 2 , less than 1/4 of area compared to other studied systems). Dicentrarchus labrax was among the most productive species. Production decreased in the drought year for all species, especially evident for D. labrax, P. minutus and P. flesus. No direct effects could be attributable to the salinity and temperature variations and to the low freshwater discharges (resulting from the drought and high temperatures), yet these were pointed as probable major reasons for the decreased production. A significant reduction (15e45% reduction in the estuarine production) was also concluded for the potential production to be exported for coastal areas by the nursery species in the drought conditions.
Article
Time series analyses (Box–Jenkins models) were used to study the influence of river runoff and wind mixing index on the productivity of the two most abundant species of small pelagic fish exploited in waters surrounding the Ebre (Ebro) River continental shelf (north-western Mediterranean): anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus). River flow and wind were selected because they are known to enhance fertilization and local planktonic production, thus being crucial for the survival of fish larvae. Time series of the two environmental variables and landings of the two species were analysed to extract the trend and seasonality. All series displayed important seasonal and interannual fluctuations. In the long term, landings of anchovy declined while those of sardine increased. At the seasonal scale, landings of anchovy peaked during spring/summer while those of sardine peaked during spring and autumn. Seasonality in landings of anchovy was stronger than in sardine. Concerning the environmental series, monthly average Ebre runoff showed a progressive decline from 1960 until the late 1980s, and the wind mixing index was highest during 1994–96. Within the annual cycle, the minimum river flow occurs from July to October and the wind mixing peaks in winter (December–April, excluding January). The results of the analyses showed a significant correlation between monthly landings of anchovy and freshwater input of the Ebre River during the spawning season of this species (April–August), with a time lag of 12 months. In contrast, monthly landings of sardine were significantly positively correlated with the wind mixing index during the spawning season of this species (November–March), with a lag of 18 months. The results provide evidence of the influence of riverine inputs and wind mixing on the productivity of small pelagic fish in the north-western Mediterranean. The time lags obtained in the relationships stress the importance of river runoff and wind mixing for the early stages of anchovy and sardine, respectively, and their impact on recruitment.
Article
Performance measures and reference points for the management of target species are now widely used in the fisheries of the developed world. To move us closer to an ecosystem-based fisheries management framework, we look at the prospects for expanding current single-species approaches for target species, by considering nontarget species. We also examine the development of performance measures and reference points for emergent ecosystem-level properties. We conclude that the expansion of single-species reference points to take account of the nontarget species of a fishery is tractable and desirable. In contrast, the use of performance measures for a single or a small selection of ecosystem metrics is not possible at present, owing to the absence of a clear understanding of their dynamics and a lack of underlying theory to explain their behaviour. However, recent methods that aggregate a wide range of metrics to provide an overall picture of the ecosystem status show promise and have a particular attraction because they have the potential to provide a framework for establishing a dialogue on management issues between all interested parties.
Article
Many studies have recently described and interpreted the community structure and function of fishes inhabiting estuaries and other transitional waters in terms of categories or guilds. The latter describe the main features of the fishes’ biology and the way in which they use an estuary. However, the approach has been developed by different workers in different geographical areas and with differing emphasis such that there is now a need to review the guilds proposed and used worldwide. The previous wide use of the guild approach has involved increasing overlap and/or confusion between different studies, which therefore increases the need for standardization while at the same time providing the opportunity to reconsider the types and their use worldwide. Against a conceptual model of the importance of the main features of fish use in estuaries and other transitional waters, this review further develops the guild approach to community classification of fish communities inhabiting those areas. The approach increases the understanding of the use of estuaries by fishes, their interactions and connectivity with adjacent areas (the open sea, coastal zone and freshwater catchments) and the estuarine resources required by fishes. This paper gives a global perspective on this categorization by presenting new or refined definitions for the categories, lists the synonyms from the literature and illustrates the concepts using examples from geographical areas covering north and central America, north and southern Europe, central and southern Africa, Australia and the Indo-Pacific.