Marine Stragglers Research Articles

Fish assemblages in an environmentally complex narrow coastal region in the Southwestern Atlantic Ocean: an alternative nursery habitat for fishery species

The narrow coastal waters south of the Río de la Plata estuary are an oceanographically dynamic area where small-scale or artisanal fisheries occur. We analyzed 95 fishing bottom trawls from research surveys, along with oceanographic data collected during 2016 and 2017. Multivariate analyses (PERMANOVA, SIMPER, CAP) were applied to determine the spatial distribution of fish density, diversity and assemblage structure, as well as their relationship with the oceanographic environment. We estimated the length- frequency distribution of Sciaenid species to determine the presence of juveniles in the area. We recorded forty-three coastal fish species (15 commercial and 28 noncommercial) with the families Sciaenidae and Rajidae accounting for approximately ∼80% of the catches. Marine migrants were the most dominant in terms of species, followed by marine stragglers and estuarine residents. We detect differences in assemblages between surveys, primarily associated with nine species. Bottom salinity and temperature had a significant influence on the spatial structuring of discriminant species and functional groups. Sciaenid juveniles were found in areas with low salinities and shallow habitats, while adults were distributed in deeper and saltier waters. This work highlights the importance of physical conditions on fish assemblage dynamics in a complex environment, as well as the significant role of a narrow coastal band (∼5 km) as a nursery habitat for commercially important fish species.

Occurrence of larval fishes sampled by drifting light traps in the lower reaches of a South African estuary.

The tidal occurrence of larval fishes was investigated in the permanently open Kowie Estuary on the warm-temperate coast of South Africa. Larval fishes were sampled in the mouth region using two drifting light traps deployed on the ebb and flood tides every second night for two consecutive 14-day periods, coinciding with the dark moon phase. A total of 553 larval fishes were caught, representing nine families and 26 species, of which Blenniidae and Clupeidae dominated. The prevalence of different estuarine association fish guilds was also tide-specific. Marine and estuarine species, such as Omobranchus woodi, were more dominant during flood tides, while marine straggler species, such as Sardinops sagax, which are not dependent on estuaries, were dominant on the ebb tide. Marine estuarine-dependents were only present during flood tides, potentially indicating ingress and entrainment within the estuary. The results confirm that light trap catches yield a different composition of species compared to towed ichthyoplankton net studies. Additionally, drifting light traps allow for better targeting of species with a phototactic response and reduction of incidental catch. Consequently, a mixture of gear is encouraged for more comprehensive surveys of larval fish occurrence.

Fish assemblage and guild structure in the Ashtamudi Estuary, a tropical Ramsar site in India

A first hand study on guild structure of fish species (n = 148) in Ashtamudi Estuary, a Ramsar Site of International importance, was carried out from May 2019 to April 2020 based on temporal (monsoon, pre-monsoon and post-monsoon seasons) and spatial (upper, middle and lower zones) data. Guild composition consisted of eight estuarine use guilds (marine estuarine opportunist, marine estuarine dependent, estuarine resident species, anadromous species, freshwater migrants, freshwater stragglers, catadromous and marine stragglers) and eight feeding guilds (zooplanktivore, zoobenthivore, carnivores, herbivores, invertebrate feeders, omnivores, detritus feeders and piscivores). The most dominant estuarine use guilds comprised of marine migrants (marine estuarine opportunist and marine estuarine dependent), whereas carnivore, omnivore and zooplanktivore formed the dominant feeding guilds in Ashtamudi Estuary. The species diversity decreased from upper to lower zone and was highest during the post-monsoon period. The present study has seen a decrease in estuarine resident and freshwater species due to reduced freshwater inflow, saltwater intrusion, unsustainable fishing and inadequate environmental management practices. Hence, we propose a management framework to improve the ecosystem of Ashtamudi Estuary with a special focus on its fish species and their estuarine use.

Analysis of a hypersaline drought-prone estuary reveals low density and diversity of fish eggs and larvae

We analyzed fish eggs and larvae in an estuary under severe drought conditions. We detected an inverse salinity gradient, with values increasing from the mouth to the upper estuary. Egg densities decreased from the estuarine mouth to the upstream areas following the salinity increase for all three mesh net sizes. This pattern was also found for the density of larvae, which decreased in estuarine regions with hypersalinity (38 to 62). The low diversity constituted only nine fish species, which were classified as anadromous (Anchoa hepsetus), estuarine and marine (Bathygobius soporator, Hippocampus reidi, Eucinostomus sp., and Diapterus auratus), marine estuarine-opportunist (Caranx latus and Bardiella rochus), and marine stragglers (Echeneis naucrates and Haemulon sp.). In addition, we observed an oversimplification of the assemblage to include stress-tolerant estuarine and marine species. Our baseline results suggest that this hypersaline estuarine ecosystem has lower densities and diversity than a healthy mangrove system.

Relative importance of habitat mosaics for fish guilds in the northeastern coast of Brazil

The identification of patterns in habitat use by fish guilds may provide an integrated perspective of coastal mosaics. Thus, we used multivariate analyses to assess the relative importance of habitat types for fish guilds based on density and species composition throughout a seasonal event in the northeastern coast of Brazil. Our results showed a great variability of responses found for each functional group, with species composition of guilds that are estuarine-related being affected by habitat types (p=0.001) and seasons (p=0.008), whereas the facultative estuarine users showed no significant relationship with both variables (p=0.95). Specific habitat association patterns were not found for guilds, though solely estuarine species (p=0.041) and marine straggler fishes (p=0.027) were related to environmental conditions that varied greatly with seasonality (i.e., rainfall and salinity rates), indicating that temporal changes in the region allow species from different guilds to explore the whole coastal mosaic at different scales of space and time. For this reason, we highlight that the integrated protection of beaches, mangroves and seagrass compose an imperative strategy to sustain the complexity of inshore coastal areas that are highly productive for coastal fisheries and fundamental to maintain coastal livelihoods.

A New Approach to Functionally Assess Estuarine Fish Communities in Response to Hydrologic Change

Functional assessment approaches can identify indicators of environmental change and ecological response in estuaries. This study used a 24-year coastal fisheries dataset (1993–2016) and environmental data to evaluate fish assemblage changes in Galveston Bay, Texas (USA) in response to environmental parameters and freshwater inflows, drought, and flood. Functional classification categorized 107 fish species into eleven functional groups to determine if estuarine species serve as suitable indicators of environmental condition. Nonmetric multidimensional scaling (nMDS), distance-based linear models, and graphing techniques using annual departures from period averages determined relationships between functional group abundance, freshwater inflows, and environmental parameters. Results show that estuarine-resident carnivore, estuarine-resident omnivore, marine migrant carnivore, and marine straggler carnivore functional groups exhibit highest species richness (mean = 21 species) and abundance (mean = 48,411 individuals) compared with other functional groups (mean species richness = 3; mean abundance = 1143 individuals). Declines in abundance of functional groups occurred during a 2011–2014 drought, with rebounds in 2015–2016. Relationships between inflows, salinity, and functional group abundance were evident in upper Galveston Bay, diminishing with increasing distance from major freshwater sources. Functional abundance in lower parts of the bay was decoupled from inflows and salinity changes. Findings support the conclusion that estuarine-resident and marine functional groups have limited potential to serve as bioindicators, while freshwater migrant carnivores are potential bioindicators for upper portions of the estuary. Functional methodologies provide insight into effects of environmental change on estuarine ecosystems and serve as management tools to assess changes in fish assemblages.

Multilevel decomposition of spatial and environmental effects on nearshore fish assemblages in tropical semi-enclosed ecosystems

We investigated the effects of local- and landscape-scale environmental variables and spatial structure at three hierarchical levels (i.e. type of system, individual systems, and zones within each system) on the fish species composition and abundance in bays and coastal lagoons in southeastern Brazil. The effects were assessed at the assemblage level, and between and within habitat use groups. Exclusive and shared effects of environmental, hierarchical, and spatial variable sets were quantified by variation partitioning, and individual environmental effects were assessed by partial canonical correspondence analyses. The hierarchical and spatial structures were more relevant for habitat use groups less widespread due to the lower dependence on estuarine areas, and influenced primarily the landscape-scale environmental differences between individual systems. At the assemblage level, the exclusively environmental effects were primarily indicative of the marine (e.g., higher depth, pH, and transparency) versus continental (e.g., greater distance from the ocean and mangrove cover) influence. Estuarine, marine migrant, and marine straggler species were more widespread across these gradients, but the former species were primarily associated with greater mangrove cover and the marine stragglers with a higher marine influence. Semi-diadromous and freshwater species were less associated with greater mangrove cover and primarily restricted to coastal lagoons. The different multi-scale environmental effects within each group reinforced the relevance of intra- and inter-specific differences in environmental tolerances and requirements, and evidenced the relevance of the connectivity between alternative habitats in maintaining the structure of fish assemblages. Disentangling multilevel spatial and environmental effects, this study evidenced the critical roles of the maintenance of the environmental heterogeneity in nearby estuarine habitats for nearshore fish assemblages in different types of tropical semi-enclosed estuarine ecosystems.

Fish assemblages of the Marchica lagoon (Mediterranean, Morocco): Spatial patterns and environmental drivers

The fish assemblages in the Marchica lagoon were studied in June 2015, by purse seine netting, according to a systematic sampling design of 32 stations covering the whole lagoon area. The fish community was analysed both in terms of its taxonomic and functional structure, and the spatial patterns in the community distribution analysed in relation to environmental gradients in the lagoon. A total of 25,674 individuals, belonging to 43 fish species and 20 families were recorded, consisting of 20 marine migrant, 12 marine straggler and 11 lagoon residents species. Numerically, the fish assemblage was largely dominated by pelagic marine planktivore species. A spatial gradient in the bentho-demersal component of the fish assemblage structure was observed, with more marine species occurring near sea inlet and more resident species in the lagoon inner margins. No clear spatial structure was identified for the pelagic fish component, which was homogeneously distributed within the lagoon. Depth was the most important abiotic factor affecting the abundance distribution of both bentho-demersal and pelagic species, along with other factors such as vegetation cover, pH, temperature, dissolved oxygen, salinity and suspended matter. To maintain high biodiversity and sustainable fishing in the lagoon, management measures based on the protection of shallow habitats are recommended.

Composition of the fish fauna in a tropical estuary: the ecological guild approach

Ecological guilds have been widely applied for understanding the structure and functioning of aquatic ecosystems. This study describes the composition and the spatio-temporal changes in the structure of the fish fauna and the movements between the estuary and the coast of a tropical estuary, the Itapissuma/Itamaracá Complex (IIC) in northeastern Brazil. Fish specimens were collected during the dry and rainy seasons in 2013 and 2014. A total of 141 species of 34 families were recorded. Almost half of the species (66 species, 47%) were exclusive to the estuary and 50 species (35%) to the coast; 25 (18%) were common to both environments. Marine species were dominant in both richness and biomass as they explore the environment during part of their life cycle, whereas estuarine species were dominant in abundance. Marine stragglers displayed a higher richness, abundance and biomass in the coastal waters. The estuarine environment was dominated by zoobenthivores in terms of richness, while detritivores prevailed in abundance and biomass. Zoobenthivores had the highest richness and abundance in coastal waters, while piscivores had the highest biomass. The IIC supports a rich fauna with a diverse trophic structure and is an important feeding and development area for migratory species.

Rare Species Enhance Niche Differentiation Among Tropical Estuarine Fish Species

Based on the assumption that rare species in estuarine systems may display a different set of traits relative to common species, we hypothesized that the extirpation of rare species would negatively affect functional diversity components of these ecosystems. Therefore, the contribution of rare species to the functional structure of estuarine fish assemblages in a tropical estuarine lagoon was tested using two different simulation scenarios of species loss: (1) losing the rare species first and (2) removal occurring at random. Overall, high functional redundancy among studied estuarine fish species was found, with many species, including some rare species performing similar functions as shown by the consistent and marked species’ overlap in the functional space. Nevertheless, the removal of rare species negatively impacted niche differentiation among assemblages, indicating that while rare species play redundant functions in the studied estuary, they also have a crucial role in increasing niche differentiation among fish assemblages. Such increase in niche differentiation granted by rare species—mostly represented by marine straggler species—appears to be a key component of ecosystem stability by allowing functions to be continued even when the estuary is faced with disturbances.

Use of a species-rich and highly eutrophic tropical estuary in the South Atlantic by Pleuronectiformes (Teleostei: Acanthopterygii)

Abstract: A total of 1,471 specimens of 16 species of flatfishes (Pleuronectiformes) were caught during 48 sampling campaigns between July 2005 and June 2007 at ten stations in Guanabara Bay, Rio de Janeiro, Brazil. Paralichthyidae was the dominant family, with Etropus crossotus as the dominant species. The outer stations, especially those on the western side of the lower estuary, were distinguished as a result of their higher abundance of flatfishes and number of species. The spatial distribution of E. crossotus and its population structure indicate that this species is an estuarine resident despite the apparent reduction in its area of occupation within the estuarine complex. Among the other species, nine were classified as marine stragglers (Achirus declivis, Bothus ocellatus, Cyclopsetta chittendeni, Etropus longimanus, Paralichthys orbignyanus, P. patagonicus, Syacium micrurum, Symphurus diomedeanus and Trinectes paulistanus) and three as estuarine opportunists (Bothus robinsi, Citharichthys macrops and Syacium papillosum); another three could not be classified due to the small number of captures or lack of previous data (S. tessellatus, A. lineatus and C. spilopterus).

Larval fish dispersal along an estuarine–ocean gradient

The present study investigated the larval fish dispersal along an estuarine–ocean gradient to explore connectivity between ocean and estuaries. During spring 2009, a combined ocean–estuarine survey was conducted along the Lima estuarine salinity gradient and in two transects off the adjacent coast (northwestern Iberian Peninsula), until the 100 m isobaths. Salinity, total particulate matter, particulate organic matter, total dissolved carbon, and dissolved organic carbon reached higher values at the ocean, and chlorophyll a and nutrients increased at the estuary. From the total 56 taxa identified, 14 were present along the gradient, including estuarine species (ES), marine stragglers (MS), and migrants (MM). Canonical correspondence analysis showed that species were separated along the gradient according to their ecological functional classification. MM associated with high salinity were separated from ES correlated with lower salinities and high chlorophyll a concentrations of inner estuary. Flounder (Platichthys flesus) showed a typical spatial gradient of MM, with abundance increasing from the ocean towards inner estuary. The dispersal of larvae along the Lima estuarine–ocean gradient was indicative of connectivity between habitats, emphasizing the need to consider this feature in management plans, mainly for species exploited by commercial fisheries.

First record of the European hake (Merluccius merluccius(Linnaeus, 1758): Actinopterygii: Gadiformes: Merlucciidae) in the coastal zone of Lithuania (southeastern Baltic Sea: eastern Gotland basin)

AbstractOn 10 September 2015, a specimen of the European Hake (Merluccius merluccius Linnaeus, 1758). It is the first documented report on this boreal-subtropic, benthopelagic fish in the brakish water area of the southeastern part of the Baltic Sea. The fish was caught during the hydrological autumn, before the disappearance of the seasonal thermocline near Sventoji. The fish assemblage was dominated by seasonally prevalent marine and freshwater straggler fishes. The total length (TL) of the specimen was 51.3 cm, and weight 901.75 g. It was a 3.5–4-year-old male fish with its ogives in maturity stage III. A roundworm Contracaecum acus (Rudolphi, 1809) was detected in its intestine. Remains of Bylgides sarsi were found in the gut. The specimen was caught after the Major Baltic Inflow in December 2014. This marine fish entered the semiclosed low-salinity zone, which is beyond the optimal feeding and reproduction area in the North Sea, indicating spatial and temporal connections between the two seas. A dist...

Decadal changes in the ichthyofauna of a eutrophic estuary following a remedial engineering modification and subsequent environmental shifts

This study has determined how the characteristics of the ichthyofauna of a large eutrophic microtidal estuary changed, initially in response to major structural remedial modifications, and then, during later years, as the environment became further modified, due mainly to effects of climate change. Data on the ichthyofauna of the Peel-Harvey Estuary, in south-western Australia, were derived firstly by seining seasonally in five regions over two consecutive twelve months (two years) in 2008–10. These data were then collated with those recorded previously using the same sampling regime throughout 1980 and 1981, when massive macroalgal growths were present, and throughout 1996 and 1997, soon after the opening, in 1994, of an artificial, deep and second entrance channel. The latter resulted in greater tidal flushing, consistently high salinities and reduced macroalgal biomass. Ichthyofaunal composition changed significantly overall and in four of the five regions across the three periods. Although increased tidal exchange did not lead to a rise in the number either of those marine species that typically use estuaries as nursery areas (marine estuarine-opportunists) or of those that complete their life cycle within the estuary (estuarine residents), the contributions made by the abundances of the representatives of those two groups to the total catch of fish varied markedly between periods. Those differences were largely responsible for the inter-period changes in species composition. In contrast to the situation with marine estuarine-opportunists, increased tidal exchange and higher salinities resulted in a greater number of marine straggler species entering the system, albeit in low numbers. The ichthyofauna during 1980–81 contained relatively large numbers of species that are typically associated with macrophytes, including marine estuarine-opportunists, e.g. Pelates octolineatus, and estuarine residents e.g. Ostorhinchus rueppellii and Hyporhamphus regularis. Following the opening of the artificial entrance channel, the relative abundances of these three macrophyte-associated species declined, whereas those of species, e.g. the marine estuarine-opportunist Torquigener pleurogramma and estuarine resident Favonigobius lateralis, which typically occur over unvegetated areas and in elevated salinities, increased. By 2008–10, such species had become more abundant, following declines in freshwater discharge and a longer persistence of high salinities, with macrophyte-associated species, e.g. the marine estuarine-opportunists P. octolineatus, Gymnapistes marmoratus and Haletta semifasciata and estuarine resident O. rueppellii, becoming more numerous as macroalgal and seagrass areas became better developed. This study demonstrates the great value of long-term data sets for detecting and predicting the effects of major structural changes and climate change on the faunas of estuaries.

Fish diversity in the Río de la Plata and adjacent waters: an overview of environmental influences on its spatial and temporal structure.

The fish diversity and the main environmental factors affecting the spatial distribution of species, life history stages and community structure in the Río de la Plata (RdP) and adjacent waters are reviewed and analysed, with emphasis on the functional guild classification. The functional guild classification indicated that most species in the RdP were marine stragglers, zoobenthivores and oviparous species, although the biomass was dominated by estuarine species. Salinity had a stronger influence than temperature on the spatial pattern for all life stages, shallower and fresher waters are the preferred habitats of neonates and juveniles. During the breeding season (spring-summer), adults showed an intrusion into the inner part of RdP or to its adjacent nearshore waters from the offshore waters for spawning or mating, respectively. Variations in river discharge and wind patterns greatly affected the spatial extent of estuarine water, which ultimately influenced the domain of the main life-history stages (juveniles or adults) for both marine and estuarine fishes, as well as species and fish assemblage composition. The strong environmental gradient restricts some species and life-history stages to a particular section and defines three main fish assemblage areas. The composition of the fish assemblage is indicative of the recruitment of freshwater and marine species to the estuary in opposite ways, determined by the vertical stratification. Seasonal changes in the species composition were related to migration as a result of salinity and temperature variations and reproductive migrations to spawning and mating areas. This overview reveals that the RdP is under environmental variations that are likely to produce modifications to fish distribution and abundance that affect its fisheries. This context plus fish stock declines and changes in exploitation patterns could amplify the magnitude of the variations in the fisheries resources availability and affect the sustainability of fishing communities.

Characterization of the fish assemblage in a Mediterranean coastal lagoon: Lesina Lagoon (central Adriatic Sea)

Taxonomic and functional properties and spatial–temporal variations of the small-sized fish assemblage were investigated in the micro-tidal Lesina Lagoon. Sampling was carried out monthly for a year in 9 stations, by means of a 2-mm mesh beach seine. According to the findings, the fish assemblage of Lesina Lagoon showed a low species richness. Marine migrants were the most represented in terms of number of species, with Mugilidae as dominant family. Nevertheless, residents were numerically prevalent, with the two species Atherinaboyeri and Aphaniusfasciatus showing the highest abundances. No marine stragglers were found. In addition, the temporal and spatial analysis of the fish assemblage showed weak temporal and spatial patterns of variation. In Lesina Lagoon, the taxonomic and functional structures of the fish assemblage seem to be strongly affected by the high degree of confinement of the lagoon from the open sea, which makes the fish assemblage very susceptible to changes in hydrological and local climatic conditions. The information achieved from this study may be useful for a better management of the natural aquatic resources in Lesina Lagoon and in Mediterranean lagoons.

Guild composition and habitat use by Tetraodontiformes (Teleostei, Acanthopterygii) in a south-western Atlantic tropical estuary

Sampling of the demersal ichthyofauna of Guanabara Bay was conducted bimonthly for 2 years at 10 stations distributed along a hydrobiological gradient. A total of 16,081 Tetraodontiformes specimens were collected, representing 10 species distributed among Ostraciidae, Monacanthidae, Tetraodontidae and Diodontidae. Tetraodontiformes appear to be well adapted to hydrological variations and inhospitable conditions prompted by intense eutrophication. However, abiotic factors traditionally considered important in estuarine community structure play a secondary role in the distribution of Tetraodontiformes. The type of sediment appears to be the most important physical factor but acts only as an indicator of ecological domain. The low explanatory power of physicochemical variables, in addition to the relative stability of the bay's ichthyofauna, suggests an influence of biological parameters. The species exhibited wide variation in their use of Guanabara Bay and utilized it as a resting, feeding and growing area. Among the species captured, Stephanolepis hispidus, Lagocephalus laevigatus, Sphoeroides greeleyi, Sphoeroides testudineus, Sphoeroides tyleri, Chilomycterus reticulatus and Chilomycterus spinosus were categorized as marine estuarine opportunists, and Aluterus heudelotii and Aluterus schoepfii were classified as marine stragglers. Acanthostracion sp. could not be categorized. The boom of C. spinosus indicates an ecological misbalance and must be carefully investigated.

Inter-decadal changes in fish communities of a tropical bay in southeastern Brazil

Long-term evaluations of coastal fish communities worldwide have in many cases showed a decrease in the species richness and abundance as a result of anthropogenic impacts. Fish communities in two zones (inner and outer) of a tropical bay in southeastern Brazil were sampled monthly during six years over two decades (1983–1985, 1993–1995, 1999–2001) using identical sampling methods. Over time, an increase in temperature and a decrease in water transparency were detected, while salinity remained stable. Fish assemblage was dominated by benthivorous species both migrant and resident species. The fish assemblage structure differed significantly between the two bay zones. Significant decreases in the fish richness, abundance and biomass were detected over time. The highest decreases in the species richness was recorded between 1983–1985 and 1993–1995 for the marine migrants in both bay zones and for the resident and marine straggler species in the outer zone. Dominant species such as the clupeoids Anchoajanuaria, Anchoa tricolor,Harengula clupeola, the gerreid Eucinostomos argenteus,the sciaenid Micropogonias furnieri, the atherinopsid Atherinella brasiliensis and the ariid Genidens barbus decreased over time, whereas the mugilid Mugil liza increased. Persistent differences in the fish assemblage structure between the two bay zones over the three periods can be attributed at least in part to differences in environmental variables between the zones and seem to be a key ecological element to maintenance of biodiversity. This is the first study to examine long-term changes in the fish community of a tropical bay in Brazil.

Do fish larvae have advantages over adults and other components for assessing estuarine ecological quality?

Using a novel approach to the assessment of ecological quality status of estuarine ecosystems, this study hypothesizes that compared to adult fishes and other components, the younger fish stages will be more sensitive and act as an early warning and will reflect more effectively the ecological status of estuaries. Larval stages of fishes were used to assess the ecological quality status (EQS) of four NW Portuguese estuaries, with different types and magnitudes of human pressures. The larval fish assemblages, together with water column characteristics and pollution indicators (faecal contamination and nutrient load) were sampled in the Lima, Cávado, Ave and Douro estuaries, during spring and autumn 2009. The four estuaries were classified in terms of human pressures by a global pressure index that identified the Cávado estuary as the least impacted estuary, followed by the Ave and Lima, both classified as moderately impacted system, while the Douro was classified as a highly impacted system. The Ave emerged as the most polluted system, carrying the highest nutrient load and sewage contamination. Larval fish assemblages included estuarine species, marine migrants, marine stragglers and the larger estuaries had higher species richness. Compared to adult fishes, three multimetric fish-based indices classified the Cávado, Ave and Douro estuaries with a lower ecological status when fish larvae were used. Similarly, the EQS assessed by macroinvertebrates were equal or higher when compared with fish larvae results. The EQS assessed by fish larvae was negatively correlated with sewage contamination and nitrogen nutrients, but did not reflect other anthropogenic pressures expressed by the global pressure index, which was only detected by adult fish. Fish larvae assessments were able to detect short-time events of hydrological manipulations observed in the Cávado estuary, as well as a seasonal decrease of water quality especially evident in the Ave estuary. The indices used denoted some limitations to the use of fish larvae data, thus emphasising the need for new indices to test the observed tendency for lower EQS given by fish larvae. The advantages and disadvantages of using fish larvae as more sensitive and accurate bioindicators of ecosystem integrity is also discussed as a means of providing strategically important information for improved estuarine management.

Composition of functional ecological guilds of the fish fauna of the internal sector of the Amazon Estuary, Pará, Brazil.

The present study describes the spatial and temporal structure of the estuarine fish community in the internal sector of the Amazon Estuary. Samples were obtained in the main channels and tidal creeks of Guajará and Marajó Bays and Guamá River. A total of 41,516 fish specimens were collected, representing 136 taxa, 38 families and 12 orders. In the dry season, the mean salinity of the main channel increased along a limnic-marine gradient, between the Guamá River and the Marajó Bay. Species richness was lowest in the mouth of the Guamá River and in the right margin of the Guajará Bay. Fish species composition and environmental guilds differed markedly among areas: Migrants and Freshwater Stragglers were dominant in the Guamá River and the Guajará Bay, while Estuarine, Marine Stragglers and Migrants predominated in the Marajó Bay. However, the trophic guilds were still relatively well balanced, in functional terms. Piscivores and Zoobenthivores were the dominant feeding functional groups in all the studied areas. In this study, the assessment of the community and the use of the guild approach were efficient to describe the structure and functioning of the assemblages of estuarine fish also helping to assess the anthropogenic pressures in the area.