Differences In Fish Density Research Articles

Paired comparisons with quiet surface drones show evidence of fish behavioral response to motorized vessels during acoustic surveys in Lake Superior

Acoustic surveys are important for fish stock assessments, but fish responses to survey vessels can bias acoustic estimates. We leveraged quiet uncrewed surface vessels (USVs) to characterize potential bias in acoustic surveys. Five conventional motorized ships overtook USVs from astern over 2 km transects at night in Lake Superior in 2022. We examined the difference in acoustic backscatter, average target depth, and average target strength (TS) between USV and motorized vessels. Although sound level measurements from the motorized vessels sometimes exceeded recommendations for scientific vessels, we did not detect differences in acoustic measures among survey vessels. However, the USVs recorded 2 dB higher acoustic backscatter and TS than motorized vessels, leading to ∼15% higher fish densities with drones when using in situ TS and echo integration. Differences in fish density would increase to 30%–60% if a standard TS value was applied. Target depth did not differ between USVs and motorized ships. These results are consistent with a change in orientation but not depth of insonified fish and limited horizontal avoidance of motorized survey vessels.

Hydroacoustic survey standardization: Inter-vessel differences in fish densities and potential effects of vessel avoidance

Hydroacoustics is used broadly to assess fish populations in marine and freshwater systems. Large-scale surveys often employ multiple vessels to complete a survey. Vessels can be a source of variation in multi-vessel surveys, and accounting for this variation is critical to precise and accurate assessments, whether as indices or measures of absolute abundance. We examined areal and volumetric density estimates, area backscattering coefficient (ABC), and mean target strength (TS) produced by three vessels in Lake Erie along three cross-lake transects in July 2014. Our study showed positive correlation in areal ABC, mean TS, and density among vessels; however, there was an almost 30 % higher density obtained with the largest vessel, on average. As a result, vessel choice could contribute to variation and/or bias in forage fish surveys deploying multi-vessels conducted on Lake Erie. Additionally, in some situations, differential depth patterns of fish contributed to vessel related differences in survey results suggesting a difference in vessel avoidance behaviors among targeted forage species, primarily emerald shiner Notropis atherinoides and rainbow smelt Osmerus mordax. This work demonstrates the potential magnitude of differences among vessels and highlights the need to identify and account for or otherwise control for these effects when monitoring or managing fisheries through standardized hydroacoustic assessments surveys.

Relating fish distributions to physical characteristics of a tidal energy candidate site in the Banks Strait, Australia

With the tidal energy industry moving towards commercial-scale developments, it is important to consider potential interactions between tidal energy converters (TECs) and the marine environment prior to the instalment of large-scale TEC arrays. The Banks Strait, a tidal channel located in the northeast of Tasmania, Australia, was identified as a promising candidate site for tidal energy by the Australian Tidal Energy (AUSTEn) project. To gain an understanding about potential overlap between TEC arrays and fish usage of the Banks Strait tidal channel, fish density distributions were estimated from hydroacoustic surveys during the tidal resource characterization campaign. Differences in fish density were examined according to bottom–depth, bottom – type, current speed, temperature and vertical distribution. Fish densities were significantly higher at night and displayed preferences for depths between 20 – 40 m and current speeds between 1.75 – 2 m/s. Fish density was generally highest in the bottom 10 m from the sea floor at all depths sampled. Variation by temperature and bottom–type sampled was not significant. Future studies involving long-term, stationary surveys of fish densities along with repeated surveys across different seasons would provide a more wholistic picture of fish distributions in the Banks Strait to inform developers about potential device encounter probabilities.

Partial migration alters population ecology and food chain length: evidence from a salmonid fish

AbstractMany migratory species, from monarch butterflies to wildebeest, express partial migration, where only a subset of a population migrates. This intraspecific variation is likely to have large ecological consequences. We studied the ecological consequences of partial migration in a salmonid fish, Oncorhynchus mykiss, in coastal streams in California, USA. One ecotype, steelhead trout, migrates to the ocean, whereas the other, rainbow trout, completes its lifecycle in freshwater. Migration has a strong genetic basis in O. mykiss. In one stream, we found differences in the frequency of migration‐linked genotypes below and above a waterfall barrier (migratory allele frequency of 60% below vs. 31% above). Below the waterfall, in the migratory‐dominated region, the density of young fish (<1 yr old) was approximately twice that in the resident‐dominated region above the waterfall (0.46 vs. 0.26 individuals/m2, respectively), presumably reflecting the higher fecundity of migratory females. Additionally, there were half as many older fish (>1 yr old) in pools downstream of the waterfall (0.05 vs. 0.13 individuals/m2). In a second stream, between‐year variation in the dominance of migratory vs. resident fish allowed us to explore differences in fish density and size structure through time, and we found a consistent pattern. In brief, when migratory genotypes dominated, we found higher densities of young fish and lower densities of older fish, resulting in a simpler size structure, compared to when resident genotypes dominated. Moreover, large resident trout had a slightly higher trophic position than young fish (3.92 vs. 3.42 in one creek and 3.77 vs. 3.17 in the other), quantified with stable isotope data. The difference in fish size structure did not generate trophic cascades. Partial migration is widespread among migratory populations, as is phenotypic divergence between resident and migratory forms, suggesting the potential for widespread ecological effects arising from this common form of intraspecific variation.

Seasonal habitat use and community structure of rockfishes in the Gulf of Alaska

Research is needed to determine the seasonal importance of high-relief habitats, particularly those containing biotic structures to rockfishes within the Gulf of Alaska. We examined the density and community structure of commercially important rockfishes in low-relief, high-relief, and biotic habitats in the spring, summer, and winter seasons at three sites in the central Gulf of Alaska using stereo drop cameras and bottom trawls. The fish community structure within all three sites was dominated by dusky rockfish (Sebastes variabilis), northern rockfish (S. polyspinis), Pacific ocean perch (S. alutus), and harlequin rockfish (S. variegatus). There was no effect of season on community structure and density of rockfish in the study area. While there were local differences in fish density and community structure, rockfish densities throughout the study areas were higher in complex than bare habitats. Significant differences in densities between high-relief and biotic habitats were not found and the additional value of structure- forming invertebrates was not proven. Juvenile rockfish were also associated with high-relief habitat, particularly complex boulder habitat. This study advances our knowledge of seasonal habitat preferences of commercially important rockfish species with the Gulf of Alaska and emphasizes the need to consider these preferences in surveying and managing these species.

Assessing the performance of artificial reefs as substitute habitat for temperate reef fishes: Implications for reef design and placement

Artificial reefs (ARs) have been advocated and implemented as management tools for recreational fisheries, species conservation and habitat replacement. For ARs to function as substitute habitat for degraded natural reefs, they should perform as close as possible to local natural reefs, however this is seldom investigated. Here we evaluated the performance of new custom-designed reef structures (CDARs) as fish habitat. As a benchmark for their success, we compared fish abundance, diversity and community composition on CDARs to another commonly used AR type (Reef Balls (RBs)) and nearby natural reefs. Fish were monitored on all reef types over two recruitment seasons at three locations in Port Phillip Bay, Australia. Overall, there were no consistent differences in fish density among reef types, although densities on both AR designs were markedly lower than natural reefs at some locations. However, fish species richness on the CDARs was, on average, 2× higher than natural or RB reefs. There were large dissimilarities in fish community composition among reef types across all locations and years. These dissimilarities declined over time with the CDARs becoming more similar to natural communities than to RB reefs. Our results suggest that CDARs can play a role in reef fish conservation where natural reefs are under threat, supporting natural community structure and enhancing local biodiversity. Overall, our findings suggest that location of deployment, rather than design, has a more significant influence on fish abundances on ARs, whereas reef design is an important determinant of species diversity and community structure irrespective of location. ARs represent an important management tool for enhancing fisheries productivity and conservation in areas where reef habitat has been degraded or lost. However, failure to incorporate consideration of reef location and design into future AR deployments may lead to poor performance and failure to achieve restoration or conservation goals.

Combined effects of hydrologic alteration and cyprinid fish in mediating biogeochemical processes in a Mediterranean stream

Flow regimes are important drivers of both stream community and biogeochemical processes. However, the interplay between community and biogeochemical responses under different flow regimes in streams is less understood. In this study, we investigated the structural and functional responses of periphyton and macroinvertebrates to different densities of the Mediterranean barbel (Barbus meridionalis, Cyprinidae) in two stream reaches differing in flow regime. The study was conducted in Llémena Stream, a small calcareous Mediterranean stream with high nutrient levels. We selected a reach with permanent flow (permanent reach) and another subjected to flow regulation (regulated reach) with periods of flow intermittency. At each reach, we used in situ cages to generate 3 levels of fish density. Cages with 10 barbels were used to simulate high fish density (>7indm−2); cages with open sides were used as controls (i.e. exposed to actual fish densities of each stream reach) thus having low fish density; and those with no fish were used to simulate the disappearance of fish that occurs with stream drying. Differences in fish density did not cause significant changes in periphyton biomass and macroinvertebrate density. However, phosphate uptake by periphyton was enhanced in treatments lacking fish in the regulated reach with intermittent flow but not in the permanent reach, suggesting that hydrologic alteration hampers the ability of biotic communities to compensate for the absence of fish. This study indicates that fish density can mediate the effects of anthropogenic alterations such as flow intermittence derived from hydrologic regulation on stream benthic communities and associated biogeochemical processes, at least in eutrophic streams.

Community stability within the St. Marys River fish community: Evidence from trawl surveys

Abstract A trawl survey was conducted in the Saint Marys River during 2010–2011 and we compared our results to a prior trawl survey conducted during 1979–1983 to look for long-term changes in the fish community, especially in terms of changes induced by invasive species. We found no substantive temporal differences in fish density, fish biomass, or fish diversity; lower trawl biomass during 2010–2011 was likely a result of day versus night trawling. The Saint Marys River remains a center of high fish diversity, invasive species remain rare, and the system continues to exhibit overall long-term stability. Trawling captured a wide range of fish species, but was likely not an effective stock assessment tool for managed game fish because catch rates were low or variable for all game species except yellow perch. Trawling appeared to be an effective tool for sampling connecting channel diversity, especially when large numbers of individuals are needed for directed studies, but annual sampling would be needed to use data to assess recruitment.

High resolution study of the spatial distributions of abyssal fishes by autonomous underwater vehicle.

On abyssal plains, demersal fish are believed to play an important role in transferring energy across the seafloor and between the pelagic and benthic realms. However, little is known about their spatial distributions, making it difficult to quantify their ecological significance. To address this, we employed an autonomous underwater vehicle to conduct an exceptionally large photographic survey of fish distributions on the Porcupine Abyssal Plain (NE Atlantic, 4850 m water depth) encompassing two spatial scales (1–10 km2) on and adjacent to a small abyssal hill (240 m elevation). The spatial distributions of the total fish fauna and that of the two dominant morphotypes (Coryphaenoides sp. 1 and C. profundicolus) appeared to be random, a result contrary to common expectation but consistent with previous predictions for these fishes. We estimated total fish density on the abyssal plain to be 723 individuals km−2 (95% CI: 601–844). This estimate is higher, and likely more precise, than prior estimates from trawl catch and baited camera techniques (152 and 188 individuals km−2 respectively). We detected no significant difference in fish density between abyssal hill and plain, nor did we detect any evidence for the existence of fish aggregations at any spatial scale assessed.

Spatial variability in habitat structure and heterogenic coral reef fish assemblages inside a small-scale marine reserve after a coral mass mortality event

Coral reefs at the inner granitic islands in the Seychelles were heavily affected by the worldwide bleaching event in 1998, which led to subsequent coral mortality and widespread phase shifts to macroalgae dominated reefs. In this study, five sites within a small, but well enforced marine reserve at Cousin Island, were investigate using various methods to explore differences in coral habitat quality, coral recruitment, fish assemblages, key invertebrate grazers, and rugosity. The objective of the study was to collect a broad set of scientific data, which could be useful to describe linkage between coral reef and fish assemblages after a large-scale disturbance, as well as for future management decisions regarding marine resources, in terms of MPA protection and recovery abilities. The results showed high spatial variation in coral coverage between sites (from 1.5% to 43.2%), which were higher than previously reported, as well as high variation in dispersal of coral recruits. Furthermore, there were large heterogenic differences in fish densities and composition, which were directly linked to coral habitat quality, e.g. total fish abundance was 15 times higher on sites with high coral coverage in comparison to sites with low coral cover. In summary, this study demonstrates that coral reef habitat and fish assemblage may display high spatial variability and heterogenic differences after large-scale disturbances and suggests that potential recovery from coral mass mortality may occur in a non-linear and patchy procedure, which in turn may depend on underlying stocastical processes that affect coral recruitment and survivorship.

Does landscape context mediate the nature of density dependence for a coral reef fish?

Over-harvest and landscape change are two of the greatest threats to marine ecosystems. Over-harvest may directly affect key population regulation mechanisms (e.g., density dependence), with the magnitude of the effects being further influenced by changes in landscape structure and associated resource availability. Because resource availability and conspecific density often co-vary within the natural landscape, manipulative experiments are needed to understand how changes in these two drivers may affect density dependence in wild populations. We used a common, shoaling, coral reef fish (white grunt, Haemulon plumierii) as our model species, and manipulated fish densities and landscape context of artificial reef habitats to assess the effects of each on fish condition. We found evidence of inverse density dependence, where individual condition was positively related to conspecific density; landscape context had little effect. Mean grunt condition on natural patch reefs was similar to that for our low grunt density treatment artificial reefs, possibly due to differences in fish densities or landscape context. These findings suggest that over-harvest may have detrimental effects on wild populations that extend beyond mere reductions in population size, especially for group-living species.

Acoustic assessment of the fish spatio-temporal distribution during the initial filling of the Three Gorges Reservoir, Yangtze River (China), from 2006 to 2010

Summary The aim of this study was to quantify the fish fauna in the Three Gorges Dam (TGD) and assess the tendency for change as well as for any spatial and temporal distribution. Two types of echo-sounder equipment (HTI Model 241 and Simrad EY60) were used to detect the fish density and fish spatio-temporal distribution. Eight surveys were conducted between 2006 and 2010, with the results showing that fish abundance in the Three Gorges Reservoir (TGR) increased yearly from 15.5 to 123 ind./1000 m3, a change apparently correlated with the corresponding increase in the reservoir water level. Horizontally, fish density increased from the lower reaches to the upper reaches. Fish preferred inhabiting the area around the Zhongxian section during winter and the Chongqing section in spring. Vertically, fish density was highest in the lower layer, followed by the middle layer. The difference in fish density in different water layers observed in spring was not as evident as in the winter. Fish target strength throughout the eight investigations showed a skewed distribution with respect to fish size. The overall increasing tendency of fish density and the spatio-temporal distribution of fish species were closely related to the water level of the TGR during the impounding period. Other influencing factors included water flow rate, water depth, reservoir shape, habitat type, and protection policy.

Assessing Effects of Stocked Trout on Nongame Fish Assemblages in Southern Appalachian Mountain Streams

AbstractFisheries managers are faced with the challenge of balancing the management of recreational fisheries with that of conserving native species and preserving ecological integrity. The negative effects that nonnative trout species exert on native trout are well documented and include alteration of competitive interactions, habitat use, and production. However, the effects that nonnative trout may exert on nongame fish assemblages are poorly understood. Our objectives were to quantify the effects of trout stocking on native nongame fish assemblages intensively on one newly stocked river, the North Toe River, North Carolina, and extensively on other southern Appalachian Mountain streams that are annually stocked with trout. In the intensive study, we adopted a before–after, control–impact (BACI) experimental design to detect short‐term effects on the nongame fish assemblage and found no significant differences in fish density, species richness, species diversity, or fish microhabitat use associated with trout stocking. We observed differences in fish microhabitat use between years, however, which suggests there is a response to environmental changes, such as the flow regime, which influence available habitat. In the extensive study, we sampled paired stocked and unstocked stream reaches to detect long‐term effects from trout stocking; however, we detected no differences in nongame fish density, species richness, species diversity, or population size structure between paired sites. Our results revealed high inherent system variation caused by natural and anthropogenic factors that appear to overwhelm any acute or chronic effect of stocked trout. Furthermore, hatchery‐reared trout may be poor competitors in a natural setting and exert a minimal or undetectable impact on native fish assemblages in these streams. These findings provide quantitative results necessary to assist agencies in strategic planning and decision making associated with trout fisheries, stream management, and conservation of native fishes.

Intertidal fish communities may make poor indicators of environmental quality: Lessons from a study of mangrove habitat modification

Utilization of fish as indicators of environmental quality in estuarine systems has received much recent attention, although some results have raised questions about using these taxa for such a role. We attempted to identify a fish response to a horticultural practice which removes portions of mangrove forest canopy to create unobstructed vistas. Fish were sampled monthly from six pairs of sites within the mangrove-forested intertidal zone of Rookery Bay, FL over two years. Following a first year of baseline sampling, the mangroves in one site from each pair of sites were pruned in accordance to Florida State law (Mangrove Trimming and Preservation Act of 1996), resulting in a mean 46% and 50% reduction in canopy coverage and litter fall, respectively. However neither the abundance nor species composition of fish in trimmed versus control plots were significantly different (Two-Way Repeated Measures ANOVA's for paired plot differences in fish density and biomass; nonmetric multidimensional scaling, and ANOSIM) suggesting that fish did not respond to mangrove canopy damage. Our results show that due to rather high spatial and temporal variability in fish abundance, it would be difficult to detect any but extreme changes of these taxa in the mangrove intertidal zone, and this may be a common feature of the fish communities of mangrove intertidal zones and other coastal systems. We suggest that improved use of fish as indicators of intertidal habitat quality would benefit from improved information on environmental factors which determine fish distribution and abundance in these areas. Moreover, since a majority of the fish in the intertidal zone is habitat generalists, we propose that individual-level measures of fish response to habitat quality may be more effective than fish abundance.

Fine‐scale spatial and temporal variations in diets of the pipefish Stigmatopora nigra within seagrass patches

Diets of the pipefish Stigmatopora nigra were analysed to determine if food availability was causing S. nigra to distribute according to habitat edge effects. Gut analysis found little difference in the diets of S. nigra at the edge and interior of seagrass patches, regardless of time of day or season. Fish diets did, however, vary with seagrass density: S. nigra in denser seagrass consumed more harpacticoid copepods and fewer planktonic copepods. The lack of difference in prey eaten by S. nigra at the edge and interior of patches suggests either that food was not determining S. nigra distribution patterns within patches or that differences in fish densities across patches meant that relative fish-prey densities were similar at edge and interior positions. Alternatively, any edge effects in diet might be masked by gradients in seagrass structure.

Effects of discharge and local density on the growth of juvenile Atlantic salmon Salmo salar

The study explored the combined effects of density, physical habitat and different discharge levels on the growth of juvenile Atlantic salmon Salmo salar in artificial streams, by manipulating flow during both summer and winter conditions. Growth was high during all four summer trials and increased linearly with discharge and mean velocity. Differences in fish densities (fish m(-3)) due to differences in stream volume explained a similar proportion of the variation in mean growth among discharge treatments. Within streams, the fish aggregated in areas of larger sediment size, where shelters were probably abundant, while growth decreased with increasing densities. Fish appeared to favour the availability of shelter over maximization of growth. Mean growth was negative during all winter trials and did not vary among discharge treatments. These results suggest that increased fish densities are a major cause of reduced summer growth at low discharge, and that habitat-mediated density differences explain the majority of the growth variation across habitat conditions both during summer and winter.

Ecological effects of marine protected areas on rocky reef communities—a continental-scale analysis

Data compiled by volunteer divers were utilised in a continental-scale investigation of ecological differences between reefs in no-take sanctuary zones in marine protected areas (MPAs) and adjacent fished zones. In a validation analysis, volunteer-generated data were found to be com- parable to data obtained by scientific dive teams for all metrics investigated: estimated total number of species, total faunal abundance, mean fish size, and faunal composition of species sighted along transects. Variation between individual divers within volunteer and professional groups also con- tributed little to total estimated variance between transects compared to residual variation between replicate transects, variation between sites, and variation between regions studied. Sites in 11 MPAs distributed around 5000 km of the Australian coastline had significantly more large (>30 cm) fishes and total fish biomass than nearby fished reference sites. For the older MPAs, fishes belonging to the largest size class (≥80 cm) were observed approximately 10 times more often in sanctuary zones than in fished zones, while fishes in the smallest size class (2.5 cm) occurred at densities approximately 4 times higher in fished zones than in sanctuary zones. Results of our empirical field surveys contrast in several respects with outcomes of published meta-analyses. No overall differences in fish densities were evident between sanctuary zones and fished zones. The response of fish species richness to pro- tection varied significantly between different MPAs, while invertebrate density and species richness were both significantly lower in sanctuary zones than in fished zones.

Local ecological impacts of regional biodiversity on reef fish assemblages

AbstractAim We examined comparative data for cryptobenthic reef fishes to determine how variation in regional species richness relates to local species richness, abundance, and taxonomic and trophic composition, and to test whether systems with higher species richness exhibit finer habitat partitioning.Locations Lizard Island, Great Barrier Reef (GBR), Australia; Bahía de Loreto, Gulf of California (GoC), Mexico.Methods Cryptobenthic reef fish assemblages from four habitats (coral heads, rubble, and horizontal and vertical surfaces of boulders) were collected using clove oil. Differences in density, species richness and biomass were examined between regions and among habitats. Habitat associations were identified for each habitat/location based on multivariate ordination, and the statistical significance of patterns was tested using analysis of similarity (ANOSIM). In addition, the trophic group composition of the assemblages for both regions was examined.Results A total of 91 species in 20 families were recorded (GBR, 66 species; GoC, 25 species). Total and habitat species richness were higher on the GBR, whereas biomass was higher in the GoC. No difference in fish density between regions was found. Habitat division among assemblages was greater in the depauperate GoC. Only coral head associations proved to be distinctive on the GBR, whereas three sample groups were found in the GoC (coral heads, horizontal boulders and vertical boulders/rubble). Trophic composition in the two regions was markedly different, with omnivores dominating the GBR fauna and planktivores the GoC.Main conclusions A positive regional–local relationship in fish diversity was found between regions, but fish abundance in both regions remained similar. Contrary to expectations, habitat partitioning, at a community level, was greater in the depauperate GoC. Differences in trophic composition and patterns of habitat use appear to reflect the disparate history of the regions, whereas patterns of abundance may reflect the influence of fundamental relationships between size and abundance in communities. This study highlights the potential of reef faunas to conform to universal numerical trends while maintaining an ability to respond ecologically to local/evolutionary influences. The GoC fauna appears to be exceptionally vulnerable to natural and anthropogenic disturbance owing to the high numerical dominance of habitat‐specific species and to the limited potential for functional redundancy within the system.

Hydroacoustic study of spatial and temporal distribution of Gymnocypris przewalskii (Kessler, 1876) in Qinghai Lake, China

Gymnocypris przewalskii (Kessler 1876) is an endangered and state-protected rare fish species in Qinghai Lake, China. To further understand the life history and distribution of this fish, five surveys were carried out in Qinghai Lake between 2002-2006. Results of these surveys indicate that fishes were predominantly distributed about 2 m under the surface. In July, significant differences in fish density were found between surface and bottom layers (P = 0.001), and/or between middle and bottom layers (P = 0.025). Fish density was the greatest in the surface layer. In August and October, no significant differences were found between the different layers, but the bottom layer had a greater fish density. Furthermore, there were very large differences among different zones in fish distribution density. Differences in horizontal distribution were not significantly correlated to factors such as water depth and inshore distance, possibly because of very low and uniform fish density. Feeding, changes in water temperature, over-wintering and spawning appeared to influence fish distribution. Hydroacoustic estimates of G. przewalskii biomass in Qinghai Lake increased significantly between 2002 and 2006. We attribute this increase to the management measures put in place to protect this species.

River flow influence on the fish community of the Tagus estuary (Portugal)

The influence of river flow on the fish community was assessed for the Tagus estuary (Portugal), based on sampling surveys carried out between 1979 and 2002. Four estuarine areas were sampled using similar fishing gear and effort in all the years considered in this study (1978–1980; 1995–1997; and 2001–2002). According to river freshwater flow values, sampling years were classified as wet (mean value of 714 m3 s−1, sd = 110 m3 s−1) or dry (mean value of 164 m3 s−1, sd = 19m3 s−1). Species richness varied between 22 and 39 according to the year, but no significant differences were related to river flow. The number of species per ecological guild was also similar in wet and dry years. Fish assemblage was dominated by marine occasional, estuarine resident and marine-estuarine opportunist species that represented near 90% of all fish species. The highest densities were represented by estuarine resident species. Fish density in dry and wet years differed significantly (mean density of 10.51 individuals 1,000 m−2 and 3.62 individuals 1,000 m−2, respectively), and the major differences were registered for estuarine resident, marine-estuarine opportunist and catadromous species. These differences probably reflected the estuarine habitat availability and also differences in fish densities in some estuarine areas under different flow conditions. The multivariate ordination analyses performed outlined both seasonal and spatial variation trends in fish distribution and abundance. The estuarine longitudinal gradient and its relationship with species distribution were less evident in dry years. Relationships between species abundance and river flow were different according to species, which is probably due to different needs in the timing and magnitude of river flow.