Estuarine Fisheries Research Articles

Initial impacts of Microcystis aeruginosa blooms on the aquatic food web in the San Francisco Estuary

The impact of the toxic cyanobacterium Microcystis aeruginosa on estuarine food web pro- duction in San Francisco Estuary is unknown. It is hypothesized that Microcystis contributed to a recent decline in pelagic organisms directly through its toxicity or indirectly through its impact on the food web after 1999. In order to evaluate this hypothesis, phytoplankton, cyanobacteria, zooplankton, and fish were collected biweekly at stations throughout the estuary in 2005. Concentrations of the tumor-promot- ing Microcystis toxin, microcystin, were measured in water, plankton, zooplankton, and fish by a protein phosphatase inhibition assay, and fish health was assessed by histopathology. Microcystis abundance was elevated in the surface layer of the western and central delta and reached a maximum of 32 9 10 9 cells l -1 at Old River in August. Its distribution across the estuary was correlated with a suite of phytoplankton and cyanobacteria species in the surface layer and 1 m depth including Aphanizomenon spp., Aulacoseira granulata, Bacillaria paradoxa, Rhodomonas spp., and Cryptomonas spp. Shifts in the phytoplankton community composition coincided with a decrease in the percentage of diatom and green algal carbon and increase in the percentage of cryptophyte carbon at 1 m depth. Maximum calanoid and cyclopoid cope- pod carbon coincided with elevated Microcystis abundance, but it was accompanied by a low clado- cera to calanoid copepod ratio. Total microcystins were present at all levels of the food web and the greater total microcystins concentration in striped bass than their prey suggested toxins accumulated at higher trophic levels. Histopathology of fish liver tissue suggested the health of two common fish in the estuary, striped bass (Morone saxatilis), and Missis- sippi silversides (Menidia audens), was impacted by tumor-promoting substances, particularly at stations where total microcystins concentration was elevated. This study suggests that even at low abundance, Microcystis may impact estuarine fishery production through toxic and food web impacts at multiple trophic levels.

Climate Change in the U.S. Atlantic Affecting Recreational Fisheries

This review provides an examination of the consequences of climate change in the coming century to saltwater sport fishing. We emphasized recreational fisheries in the U.S. Atlantic, but draw from the broader national and international literature where appropriate. Three themes were addressed: (1) climate change in the U.S. Atlantic, with a focus on increases in temperature, precipitation, sea level, the frequency and intensity of storms, and changes in ocean circulation; (2) the response of marine and estuarine fishes to climate change on an individual, population, and community-level; and (3) the response of marine and estuarine recreational fisheries to climate change. In addition, we provide strategies for the future of fisheries assessment and management in response to climate change.

Modelling the possible effects of climate change on an Australian multi-fleet prawn fishery

The relationship between fisheries and climate has been given renewed emphasis owing to increasing concern regarding anthropogenically induced climate change. This relationship is particularly important for estuarine fisheries, where there are documented correlations between river discharge and productivity. The commercial catch of school prawns (Metapenaeus macleayi) has been shown to be positively correlated with the rates of river discharge in northern New South Wales, Australia. In the present study, a simulation model was developed to analyse the dynamics of the stock for 10 years under alternative river discharge scenarios, and the effectiveness of a series of management strategies under these scenarios was examined. A size-based metapopulation model was developed that incorporated the dynamics of school prawn populations in three habitats being harvested by three different fishing methods. The model indicated that both the growth and movement of prawns were affected by the rates of river discharge, and that higher rates of river discharge usually generated increased commercial catches, but this outcome was not certain. It was concluded that the population does not appear to be overexploited and that none of the three alternative management strategies performed better within the model than the current spatio-temporal closures, even under a wide range of river discharge scenarios.

Short-term climate variability and the commercial barramundi (Lates calcarifer) fishery of north-east Queensland, Australia

The sustainable productivity of estuarine fisheries worldwide is threatened by over-fishing, habitat destruction and water impoundment. In some cases, the natural variability of freshwater inputs has been shown to affect catch when low flows reduce nutrient input and inundated nursery habitats. Historically, the annual commercial catch of barramundi (Lates calcarifer) in Queensland has been highly variable for reasons not fully understood. In conjunction with a life-cycle model, statistical analyses of climate variables and barramundi catch data from the Princess Charlotte Bay area identified several significant relationships. Warm sea surface temperatures, high rainfall, increased freshwater flow and low evaporation (all measures of an extensive and productive nursery habitat) were significantly correlated with barramundi catch 2 years later and suggest that young barramundi survival is enhanced under these conditions. Catchability was significantly increased with high freshwater flow and rainfall events in the year of catch. A forward stepwise ridge regression model that included a measure of rainfall and evaporation 2 years before catch explained 62% of the variance in catch adjusted for effort. It is recommended that the impact of climate variability be considered in the management of wild barramundi stocks and possibly other species not yet examined.

Changes to fish assemblages visiting estuarine wetlands following the closure of commercial fishing in Botany Bay, Australia

A Before-After, Control-Impact sampling design was used to measure changes in fish assemblages in intertidal mangroves and saltmarsh prior to and following the closure of commercial fishing in the Botany Bay estuary, New South Wales, Australia. Of commercial species found as juveniles in the wetlands, there was a consistent pattern of decrease in numbers compared to the pre-closure surveys. Other small wetland fish (such as the Gobiidae) were also found to have decreased in numbers, with the exception of the Common Toadfish (Tetractenos hamiltoni Gray and Richardson, 1843) and the Glassfish (Ambassis jacksoniensis Macleay, 1881). Results suggest that the immediate response of an estuarine fishery to commercial fishing closure may be a predator-mediated decline in juvenile fish of both prey and predator species.

Home range of an estuarine‐dependent fish species Pomadasys commersonnii in a South African estuary

Abstract The home range and area use of spotted grunter, Pomadasys commersonnii (Haemulidae) (Lacepède), were investigated in the Great Fish estuary, South Africa. Forty individuals (263–698 mm TL) were surgically implanted with acoustic‐coded transmitters and manually tracked during two periods in 2003. Tagged fish spent on average 68 and 77% of the time in the estuary and the remaining time at sea during the two periods, respectively. While in the estuary, there was large variation in home range size, home range length and the number of home range areas amongst individuals. Larger fish had larger home ranges and more core areas than smaller fish. The home ranges of 29 individuals (74%) occurred in the lower reaches of the estuary, suggesting that the proclamation of a no‐take zone in this high use area could benefit this over‐exploited estuarine fishery species.

Supplemented Access Point Sampling Designs: A Cost‐Effective Way of Improving the Accuracy and Precision of Fishing Effort and Harvest Estimates Derived from Recreational Fishing Surveys

AbstractWe describe a cost‐effective method for improving the accuracy and precision of fishing effort and harvest estimates derived from recreational fishing surveys that include an access point method for effort estimation (e.g., traditional access point surveys and bus route designs). Validated auxiliary data that quantify recreational fishing effort at access points within the fishery are used to supplement an existing access point design. This supplemented access point survey (SAPS) method is a form of double sampling and should be viewed as an extension of an access point design. Case study data from a boat‐based estuarine fishery in Australia have been used to demonstrate changes in the relative accuracy and precision between a traditional access point design and the supplemented design. We found that the traditional design provided annual effort and harvest estimates for dusky flathead Platycephalus fuscus that were about 7% and 5% lower, respectively, than those obtained with the supplemented design. Importantly, the SAPS method provided improvements in relative precision for annual estimates of effort and harvest of about 76% and 37%, respectively. The SAPS design outperforms other access point designs in terms of relative accuracy and precision because the auxiliary data provide better coverage of the temporal sampling frame for fishing effort. Improvements in relative accuracy and precision are also reflected in harvest estimates because the SAPS design calculates harvest as the product of effort and a harvest rate. The SAPS method can easily be extended to cover multiple sites and offers a cost‐effective way to improve estimation procedures for any recreational fishing survey that uses an access point method for effort estimation.

Caracterização da pesca artesanal no complexo estuarino-lagunar de Cananéia-Iguape, Estado de São Paulo, Brasil (1995-1996)

Amateur fishing gear and methods, and fishing production in the Cananeia-Iguape lagoon-estuarine complex region, southernmost coast of the state of Sao Paulo (25oS; 48oW), Brazil, were analyzed during 1995 and 1996. Consisting mainly of small wooden boats, only a small part of the amateur fishing fleet, currently in operation, is motor-powered. The activity is, however, important to the region and contributes with 10 to 32% of total landing in Cananeia. The local amateur fishery is divided into two main groups: the near-shore fishery and the estuarine fishery. The former mainly exploits the Atlantic seabob ( Xiphopenaeus kroyeri ), the white shrimp ( Litopenaeus schmitti ). Fishermen fish with gillnet or long line in a typical small boat bottom trawl called as “bateira”. The fishing gear and techniques of the local estuarine fishery include the fence trap, the gillnet, and the floating and vertical long line. A typical small drift beam trawl net, locally known as “gerival”, is also used to catch juvenile shrimp Farfantepenaeus paulensis and F. brasiliensis , highly valuable bait for recreational fishing. The fence trap is the main fishing gear and more than 90 traps were set inside the estuary, where the target species is mullet ( Mugil platanus ), although many other species are also important catch items, e.g. Mugil curema , snook ( Centropomus spp.), whitemouth croaker ( Micropogonias furnieri ), king weakfish ( Macrodon ancylodon ) and mojarras ( Diapterus rhombeus ). The production of Atlantic seabob, the most important species of Cananeia’s amateur fishery, fluctuated from year to year. The amateur fishery landings of Atlantic seabob were 34 t. in 1995 and 12 t. in 1996

Role of an estuarine fisheries reserve in the production and export of ichthyoplankton

Role of an estuarine fisheries reserve in the production and export of ichthyoplankton

Development of fishery indicators for local management initiatives — a case study for Plettenberg Bay, South Africa

In attempts to achieve sustainability, ecosystem-based approaches to fisheries management are becoming increasingly applicable. Indicators to assess the success of management measures are an important component of this approach. Data from a recreational linefishery are used to develop and propose a simple set of indicators to assess local fishery sustainability. The indicators have been organised within a framework based on the ecological, institutional and social sustainability domains and scored through a rapid assessment matrix. Results indicate that the estuarine, rock-and-surf (coastal) and skiboat-based fisheries in Plettenberg Bay on the south coast of South Africa are presently non-sustainable. The institutional domain had very low scores whereas the socio-economic domain had the highest scores. The validity and usefulness of the indicators as a tool for the development of local fishery management plans are discussed.

Assessment of recreational boat-angling in a large estuarine embayment in KwaZulu-Natal, South Africa

An access point study was conducted from June 2002 to May 2003 to collect fishing effort, catch, catch composition and socio-economic information from the recreational estuarine boat-fishery in Richards Bay Harbour on the east coast of South Africa. An estimated 10 977 individual angler-outings were undertaken annually by 1 497 anglers during the study period. Numerically, retained catches were dominated by spotted grunter Pomatomus saltatrix, rockcod Epinephelus andersoni and stumpnose Rhabdosargus sarba. By mass, the most dominant species were pike conger Muraenesox bagio, dusky shark Carcharhinus obscurus and E. andersoni. The prevalence of marine species reflects the marine nature of the estuary, although many estuarine species also occurred in the catches. Catch rates were low, at 0.08 fish angler−1 h−1 or 0.11kg angler−1 h−1. The total annual retained catch was estimated at 3 827 fish with a mass of 5 355kg. Total annual expenditure on boat-angling in the harbour is estimated at R6 million, and together with the high proportion of local anglers, this underlines the importance of the estuarine fishery both socially and economically to residents of the uMhlathuze Municipality. Although there is a high presence of conservation officers in Richards Bay Harbour, contravention of regulations imposed on the recreational estuarine linefishery still occurs because estuarine boats are seldom inspected. Improved monitoring will generate a long-term data series of estuarine catches that will facilitate assessment of sustainability of the fishery.

Evaluating the selective performance of the encircling gillnet used in tropical estuarine fisheries from Colombia

Experimental estimates of gear selectivity by age used for estimating stock size and measuring fishing impact have been scarcely documented in small-scale fisheries from Latin-America and the Caribbean. In order to improve stock assessment and fisheries management in such fisheries, an experimental approach was designed to estimate the selective performance of the encircling gillnet “boliche” used in the tropical small-scale fishery of the Ciénaga Grande de Santa Marta, Colombia. Similar to covered cod-end hauls, an experimental net was enclosed by a control small-mesh net in 70 hauls. Both contact and available-selection processes affecting the selectivity of the “boliche” were modelled by fitting two selection curves: the logistic (a retention curve) and the encounter-probability (probability that a fish contacts the net) models. The encounter-probability model gave the most appropriate results in terms of the deviance statistic and the lowest Akaike's Information Criterion. Selectivity parameters were estimated for the main targeted fish species, highlighting differential avoidance reactions to the “boliche” for Eugerres plumieri (0.28), Diapterus rhombeus (0.24), Elops saurus (0.22), Micropogonias furnieri (0.13) and Cathorops spixii (0.16), but not Mugil incilis (all fish retained). To provide an overall (aggregated) effect of the “boliche” on the fish assemblage, the multi-species selection curve showed 61% retention at 15 cm, indicating that 39% of the multi-specific component may avoid the “boliche”.

Status of the recreational fisheries in two Australian coastal estuaries following large fish-kill events

Severe flooding occurred during early 2001 over a widespread region of northern New South Wales (NSW), Australia. This flooding was followed by large scale mortalities of invertebrates and finfish in the Richmond and Macleay estuaries. These estuaries were then closed to all forms of recreational and commercial fishing to enhance the recolonisation and recovery of their fish populations. The lower reaches of these estuaries were re-opened to limited fishing after closure periods of 4.5 months (Richmond) and 3.5 months (Macleay). Complemented angler surveys, based on stratified random sampling procedures, were used to assess the subsequent recovery of these important estuarine recreational fisheries. Historical information from the Richmond and Clarence estuarine fisheries and data collected during recent estuarine recreational fishing surveys in other areas of NSW were used as relative benchmarks to compare the post-fish-kill survey data. These comparisons indicated that at the time of the post-fish-kill surveys these fisheries were characterised by normal temporal trends in fishing effort and typical seasonal harvests. Similarly, three indicators of recreational fishing quality were shown to be of similar magnitude to comparable measures from other estuarine fisheries in NSW. All comparisons indicated that the recreational fisheries in the lower Richmond and Macleay estuaries had recovered and were providing good quality recreational fishing opportunities. Luderick ( Girella tricuspidata), yellowfin bream ( Acanthopagrus australis) and dusky flathead ( Platycephalus fuscus) together accounted for 93.9% (Richmond) and 91.7% (Macleay), by weight, of the recreational harvest. Length frequency distribution data for these species taken by boat-based fishers were compared between estuaries. No significant differences between estuaries were detected for luderick or yellowfin bream indicating that legal sized recruits recolonising these estuaries after the fish-kill events were derived from larger meta-populations which spanned a spatial scale of at least 200 km (the distance between the estuaries). These results suggest that for luderick and yellowfin bream the deleterious impacts of localised catastrophes, even on the spatial scale of a large estuary, can be mitigated rapidly by immigration from other areas. In contrast, the length frequency distributions of dusky flathead that were present in the Richmond and Macleay estuaries during the survey period were highly significantly different indicating that they had originated from different source populations. The more localised scale of recolonisation evident for dusky flathead indicates that this species may be less resilient to localised catastrophic events than luderick or yellowfin bream. This study highlights the value of recreational fishing surveys for providing insights about the factors that may influence the recolonisation and recovery of estuarine finfish populations following large fish-kill events.

Linkages between seagrass, mangrove and saltmarsh as fish habitat in the Botany Bay estuary, New South Wales

Shallow-water vegetated estuarine habitats, notably seagrass, mangrove and saltmarsh, are known to be important habitats for many species of small or juvenile fish in temperate Australia. However, the movement of fish between these habitats is poorly understood, and yet critical to the management of the estuarine fisheries resource. We installed a series of buoyant pop nets in adjacent stands of seagrass, mangrove and saltmarsh in order to determine how relative abundance of fishes varied through lunar cycles. Nets were released in all habitats at the peak of the monthly spring tide for 12 months, and in the seagrass habitat at the peak of the neap tide also. The assemblage of fish in each habitat differed during the spring tides. The seagrass assemblage differed between spring and neap tide, with the neap tide assemblage showing greater abundances of fish, particularly those species which visited the adjacent habitats when inundated during spring tides. The result supports the hypothesis that fish move from the seagrass to the adjacent mangrove and saltmarsh during spring tides, taking advantage of high abundances of zooplankton, and use seagrass as a refuge during lower tides. The restoration and preservation of mangrove and saltmarsh utility as fish habitat may in some situations be linked to the proximity of available seagrass.

The effect of hook/bait size and angling technique on the hooking location and the catch of recreationally caught black bream Acanthopagrus butcheri

The black bream Acanthopagrus butcheri recreational fishery is the largest estuarine fishery in Victoria. This fishery is managed through legal-minimum length and daily bag limits. The success of this management strategy requires a high survival rate for released fish. Deep-hooking is known to reduce the chance of fish survival after recreational capture and release. This study investigated the potential to reduce deep-hooking and the number of under-size A. butcheri caught by varying angling gear and techniques. Three sizes of long shank hook (small [size 8], medium [size 4] and large [size 1/0]) and two angling techniques (slack line and tight line) were tested for their deep-hooking rates and selectivity characteristics. Increasing the hook size from small to large decreased the likelihood of deep-hooking by 6.6 times (95% CI 2.3–16.3 times). Fishing with a tight line instead of a slack line decreased the chance of deep-hooking by almost 100% (95% CI 0.8–3.6). Fishing with a large hook instead of a small hook significantly ( F = 6.71, df = 2, P = <0.001) increased the mean A. butcheri length , although this mean size increase was less than 1 cm. This study was able to identify angling gear and angling technique manipulations that reduced the rate of deep-hooking when targeting A. butcheri in Victorian estuaries.

Tuning Surface Water Management and Wetland Restoration Programs with Historic Sediment Accumulation Rates: Merritt Island National Wildlife Refuge, East-Central Florida, U.S.A.

Previous research has demonstrated that reconnection of impounded wetlands to the Indian River Lagoon benefits estuarine fisheries and emergent ecosystems. However, no effort has yet been made to quantify the effects of managed surface water flow on patterns of sedimentation. Sedimentation patterns strongly influence wetland evolution, including the distribution of open water and emergent landscapes. Thus, the enumeration of sedimentation rates provides a basis for reconstructing the processes responsible for historic landscape change. These data also provide a basis for predicting landscape change that can accompany hydrologic reconnection of impounded wetlands to the Indian River Lagoon. In this investigation, conducted in the Merritt Island National Wildlife Refuge, Florida, we used cesium activity profiles to quantify rates of historic sediment accumulation at locations subject to managed (impounded) and unmanaged (natural) surface water hydrology. The data suggest impounded wetlands subject to historic submergence are now devoid of vegetation and experiencing substrate erosion in areas in which fetch is sufficient to induce wind-driven circulation. In unmanaged wetlands, surface elevations have kept pace with historic sea level rise solely through the in situ accumulation of organic matter. These observations suggest that hydrologic reconnection of impounded wetlands subject to persistent flooding throughout historic time will result in rapid and widespread submergence to water depths in excess of 10 to 20 cm. At these depths, most wetland plant taxa will not successfully recruit via seedling exchange. Hence, attempts to restore wetland areas analogous to those investigated during this study (i.e., Impoundment D) cannot be achieved simply by removing management structures that obstruct natural surface water flow. The wetland surface must either be (1) incrementally flooded to estuarine water level elevations slowly over time or (2) raised to the appropriate hydroperiod elevation before reconnection with suitable fill material. One possible source of fill is the Intracoastal Waterway, located within 2 km of the Merritt Island National Wildlife Refuge and intermittently dredged to maintain safe navigation.

Effect of rainfall as a component of climate change on estuarine fish production in Queensland, Australia

The speculation that climate change may impact on sustainable fish production suggests a need to understand how these effects influence fish catch on a broad scale. With a gross annual value of A$ 2.2 billion, the fishing industry is a significant primary industry in Australia. Many commercially important fish species use estuarine habitats such as mangroves, tidal flats and seagrass beds as nurseries or breeding grounds and have lifecycles correlated to rainfall and temperature patterns. Correlation of catches of mullet (e.g. Mugil cephalus) and barramundi ( Lates calcarifer) with rainfall suggests that fisheries may be sensitive to effects of climate change. This work reviews key commercial fish and crustacean species and their link to estuaries and climate parameters. A conceptual model demonstrates ecological and biophysical links of estuarine habitats that influences capture fisheries production. The difficulty involved in explaining the effect of climate change on fisheries arising from the lack of ecological knowledge may be overcome by relating climate parameters with long-term fish catch data. Catch per unit effort (CPUE), rainfall, the Southern Oscillation Index (SOI) and catch time series for specific combinations of climate seasons and regions have been explored and surplus production models applied to Queensland's commercial fish catch data with the program CLIMPROD. Results indicate that up to 30% of Queensland's total fish catch and up to 80% of the barramundi catch variation for specific regions can be explained by rainfall often with a lagged response to rainfall events. Our approach allows an evaluation of the economic consequences of climate parameters on estuarine fisheries, thus highlighting the need to develop forecast models and manage estuaries for future climate change impact by adjusting the quota for climate change sensitive species. Different modelling approaches are discussed with respect to their forecast ability.

An overview of commercial fishing in Lake Illawarra: trends in reported catch and effort, bycatch and discarding

NSW estuaries have supported valuable commercial fisheries since the early 1800’s. The NSW estuarine commercial fishery is the most complex multi-method and multi-species fishery in NSW, producing in 2001/02 approximately 3900 t of finfish and 890 t of invertebrates, valued at over $18 million. The commercial fishery in Lake Illawarra is relatively productive and important to the total estuarine commercial catch in NSW. Between 1997/98 and 2001/02, the average reported commercial catch of finfish, prawns and blue swimmer crab was 120 t, 17 t, 12 t, respectively, which was 2.9%, 2.0% and 6.5% respectively, of the total estuarine catch in NSW. The finfish catch was dominated by sea mullet (47%) and luderick (10%), which is typical of the NSW estuarine fishery in general, and dusky flathead, silver biddy, bream and river garfish also contributed > 5 t each per annum. Most finfish were caught in gillnets (75% of catch), whereas most prawns were caught in running nets (76% of catch). Bycatch and discarding in gillnets and prawning gears in Lake Illawarra was similar to other estuaries in NSW. Current research to improve the selectivities of prawn gears and changes in regulations concerning gillnets should reduce bycatch and discarding in some gear types.

The differential impact of user heterogeneity in resource management: A case study from India

This paper analyses the heterogeneous users decision to participate in co-management, which is an institutional alternative proposed in the wake of state's failure in managing the Cochin estuarine fisheries in Kerala, India. Since a collective action under co-management require not only user's active participation in terms of their labor but also involve various types of organizational and managerial costs, the users were given the following choices on co-operation. Firstly, the users had the choice to contribute their labor in conservation activities which would ensure sustainability of the fisheries. Secondly, they could make a voluntary contribution towards meeting the organizational costs of collective action. Thirdly, they could contribute in terms of labor as well as in monetary terms. Finally, they had the choice not to participate at all. Keeping in view the problems of free rider and the chances for adoption of strategic behavior by users some incentives were given for each of the above ways of co-operation. A multinomial logit analysis of the decision of about 369 sample fishermen to participate in co-management as defined above shows the differential impact of user heterogeneity in resource management. While heterogeneity in terms of the present legal status of the users motivate them for contributing their labor even in the absence of any additional economic incentives, heterogeneity in economic status and membership in formal organizations matters when it comes to making monetary payments. In addition to these, the overall optimism of the users' motivates them to both physically engage in conservation activities as well as make monetary payments. Adoption of strategic behaviors by at least some users cannot, however, be ruled out. On the whole, it is seen that the heterogeneous fishermen's decision to participate depends upon their anticipation of the distribution of benefits from co-operation. However, one must be cautious that those who anticipate disproportionate benefit from co-operation are likely to take a lead role. Therefore, care must be taken to prevent them from deciding the rules of the game in such a manner that they are disproportionately in their favor.

The effect of estuarine fisheries on juvenile fish observed within the Guadalquivir Estuary (SW Spain)

We assess how estuarine (glass eel and white shrimp) fishing activities may be affecting the recruitment of young stages of marine fish species to the Guadalquivir Estuary. We estimated the spatio-temporal abundance of fish species throughout the last 50 km of the estuary during two annual cycles (May 1997–April 1999), by catching monthly samples using 1 mm mesh size at five sampling sites. At the same time we also estimated the fishing effort (boats fishing per day) in the different estuarine zones. During the second year, we also studied the abundance of larvae and juveniles of marine fish species in discards. The number of marine fish that use the estuary as a nursery ground decreased from 23 in the outer estuary, to 18 in the inner zone and there was a marked decrease in their density upstream (from 72, 322 to 702 individuals 10 5 m 3, on average). Moreover, we found considerable temporal changes in fish density, with minimum values during the colder months (the glass eel fishing season). A highly significant correlation (Spearman rank correlation coefficient, R = 0.73; P < 0.01) was observed between fish community composition and fishery discards during the second year of our study. In general, the fishing effort targeted at glass eels was greater than that targeted at white shrimp. The mean number of fish juveniles (per kilogram of target species) observed in the discard of both fishing activities was considerably higher in the outer estuary. Both fishing activities affect the nursery function of the estuary.