Mangrove Fish Research Articles

Importance of mangrove shorelines for rainbow parrotfish Scarus guacamaia: habitat suitability modeling in a subtropical bay

Rainbow parrotfish Scarus guacamaia is a coral-reef herbivore that requires both mangrove and coral-reef habitat to complete its life cycle. This species is listed as 'vulnerable' by the International Union for Conservation of Nature. The present study used a long-term visual sur- vey of mangrove fishes to develop a predictive habitat model for juvenile S. guacamaia. The fac- tors tested were temperature, dissolved oxygen, salinity, average depth, distance from offshore channel openings, temperature variation (ΔT), and salinity variation (ΔS). The average depth, dis- tance from offshore channel openings, ΔT, and ΔS emerged as significant within the model evalu- ations. The results suggested that high variation in salinity reduces mangrove habitat suitability for this species. Salinity variation along many of south Florida's coastal bays is largely driven by water management-related freshwater canal discharges. Everglades restoration efforts seek to reduce ΔS along south Florida's mainland shoreline; thus, if successful, restoration may also confer benefits to Scarus guacamaia through the subsequent expansion of suitable mangrove habitat.

Weight-length relationships of mangrove fishes from Bahía de La Paz, Gulf of California, Mexico

The weight (W) – length (L) relationship was calculated for 24 fish species captured in three mangrove systems of the southwest Gulf of California, Mexico.

Fish ( Arius heudelotii Valenciennes, 1840) as bioindicator of heavy metals in Douala Estuary of Cameroon

This study assessed and monitored accumulation levels of heavy metals (Fe, Pb, Cu, Zn and Mn) in a prominent benthic mangrove fish species ( Arius heudelotii Valenciennes, 1840) from the Douala Estuary, Cameroon. Fish species selection was based on signs of deterioration in taste and texture. Sampled estuarine water and tissue parts (liver, gut, flesh, gills and bones) of the captured fish were analyzed using atomic absorption spectrophotometer. The results show that concentration of the heavy metals except Mn in water was below the permissible limits imposed by WHO and USEPA for aquatic life. The metal levels in fish collected were significantly higher (p<0.05) than the levels in water, indicating bioaccumulation. Metal levels in the fish organs varied considerably with the highest mean concentrations of Pb and Zn found in the flesh and gills. The high concentration of heavy metals in these parts makes the fish unsafe for consumption as the majority of consumers depend on these parts for their livelihood. This calls for environmental surveillance and monitoring of these water bodies and their surrounding environment. Key words : Arius heudelotii, bioaccumulation, heavy metals, estuary, environmental surveillance.

Fish (Arius heudelotii Valenciennes, 1840) as bio-indicator of heavy metals in Douala Estuary of Cameroon

This study assessed and monitored accumulation levels of heavy metals (Fe, Pb, Cu, Zn and Mn) in a prominent benthic mangrove fish species (Arius heudelotiiValenciennes, 1840) from the Douala Estuary, Cameroon. Fish species selection was based on signs of deterioration in taste and texture. Sampled estuarine water and tissue parts (liver, gut, flesh, gills and bones) of the captured fish were analyzed using atomic absorption spectrophotometer. The results show that concentration of the heavy metals except Mn in water was below the permissible limits imposed by WHO and USEPA for aquatic life. The metal levels in fish collected were significantly higher (p<0.05) than the levels in water, indicating bioaccumulation. Metal levels in the fish organs varied considerably with the highest mean concentrations of Pb and Zn found in the flesh and gills. The high concentration of heavy metals in these parts makes the fish unsafe for consumption as the majority of consumers depend on these parts for their livelihood. This calls for environmental surveillance and monitoring of these water bodies and their surrounding environment. Key words: Arius heudelotii, bioaccumulation, heavy metals, estuary, environmental surveillance.

Preliminary analysis of the fish species of the Pacific Central American Mangrove of Zancudo, Golfo Dulce, Costa Rica

The aim of the present paper is to provide first information on the mangrove fish community of the tropical fjord-like Golfo Dulce (GD), to study its importance for fish diversity and fisheries in the area and to give preliminary results on its spatial–temporal variation. Eighty-two species belonging to 30 families were identified. More than half of them belonged to the estuarine transient fish group and only 14 species were estuarine resident, highlighting the importance of the Zancudo mangrove as a transition area. The most diverse families were Sciaenidae, Carangidae and Haemulidae for both rainy and dry seasons. As much as 71% of the species captured were of first or second grade economic value and only 11% had no commercial value. Comparing our data with fisheries and scientific surveys inside and outside the gulf suggests the importance of the Zancudo mangrove for regional fisheries and fish diversity. Multidimensional scaling (MDS) indicated gradual changes in the fish community composition from the river mouth to the more internal sampling sites for rainy and dry seasons. Moreover, shifts in abundance and species composition between seasons were observed, mainly an increase in the differences between the boundaries and the central zones of the mangrove. These changes were attributed to the higher salinity differences between sites during the dry season forcing the occasional visitor fish downstream and freshwater fish taxa upstream. While the results contribute to initiatives for participative fisheries management in the region, further studies are needed to analyse the variations in detail.

The Fish Community of an East African Mangrove: Effects of Turbidity and Distance from the Sea

Mangroves are often reported as nursery grounds for fish. Fish may enter mangroves in order to avoid predators, but may not need to do so if turbidity provides a sufficient predator refuge outside the forest. This study assessed the effects of turbidity in the field and laboratory on mangrove fish community structure and behaviour. The extent to which fish penetrate into mangroves has received little attention. This study also looked at differences in fish community structure at mangrove sites near (6m) and far (200m) from the mangrove/sea boundary. Twelve field samples were taken at approximately monthly intervals from replicate 25m2 landward and seaward plots, in a Sonneratia alba stand at Gazi Bay, Kenya. A total of 25 species of fish were caught, 15 in seaward plots and 13 in landward ones. Mean abundance for all plots and sampling times was 2.15 (equivalent to 0.09 m-2). Seaward plots had a total mean abundance more than twice that of landward plots (2.75 ± 1.9 S.D. vs. 1.23 ± 0.33 S.D. respectively). There was no relationship between abundance and turbidity. Laboratory experiments showed no significant changes in behaviour of three common species in response to turbidity. The low density of fish recorded concurs with previous work, and probably reflects conditions in Gazi Bay as a whole, rather than unusual features of the mangrove environment there.

Spatial Patterns of Mangrove Shoreline Fish Communities in Relation with Environmental Variables in Caribbean Lagoons

Fish communities were studied in 38 stations located in two mangrove lagoons of Guadeloupe (Caribbean): the bays of Petit Cul-de-Sac Marin (PCSM) and Grand Cul-de-Sac Marin (GCSM). Fishes were sampled with nets during the dry season, and 14 environmental descriptors were recorded in each site. A total of 105 fish species were observed in mangrove lagoons. Ordination analyses and classifications were performed on data sets related to fish presence–absence, abundance, and biomass. The PCSM stations and GCSM mangrove islets sites are characterised by an assemblage composed by fish species either found in coral reefs, seagrass beds, or mangroves. Distinct mangrove fish assemblages are found across environmental gradients linked to salinity, proximities of reefs and river mouths, and presence of seagrass beds. Considering trophic categories, planktivores are numerically dominant in most stations, whereas second-order carnivores and omnivores dominate in biomass in river mouths stations of the GCSM Bay. First-level carnivores are mainly observed in mangrove stations with neighbouring seagrass beds. These last results suggest that food availability plays an important role in the structure of mangrove fish communities in addition to abiotic factors.

Assemblages of fish along a mangrove - mudflat gradient in temperate Australia

Mangroves are considered to support rich assemblages of fish and invertebrates. Fishes inhabiting mangrove habitats and at various distances from mangroves across mudflats were sampled to: (1) compare fish assemblages between habitats; and (2) determine the influence of mangrove proximity on fish abundance and diversity in three southern Australian estuaries between November 2005 and January 2006. Based on their distribution, fish species were classified as mangrove residents, mudflat residents, generalists or rare species. The assemblage structure of fish in mangroves differed from assemblages 500 m away; however, neither total abundance nor species richness differed significantly between mangroves and mudflats. Mangrove residents and Aldrichetta forsteri (yellow-eyed mullet) displayed strong associations with mangrove habitats, whereas mudflat residents were associated with mudflat habitats. No other fish groups or individual species occurred in higher abundances in either habitat. Total fish abundance, mangrove residents and A. forsteri were positively correlated with pneumatophore density, indicating that the structural complexity of the mangroves might influence the distributions of certain fish species. The current study demonstrated that mangrove habitats in temperate Australia support no greater abundance or diversity of fish than adjacent mudflat habitats and that mangrove proximity does not influence fish distribution at a habitat scale.

Food habits of fishes in the mangrove estuary of Urauchi River, Iriomote Island, southern Japan

The food habits of 67 fish species collected from the mangrove estuary of the Urauchi River, Iriomote Island, southern Japan were investigated using gut content analysis. Ontogenetic changes in food preference were recognized in nine species, including mugilids, gerreids, mullids, gobiids and tetraodontids. In most cases, juveniles of these species fed mostly on small crustaceans (e.g. calanoid and cyclopoid copepods and gammaridean amphipods) or detritus. With their subsequent growth, larger prey items (e.g. crabs and polychaetes) became dominant. A cluster analysis based on dietary overlaps showed that the mangrove fish assemblage comprised eight trophic groups (zooplankton, small benthic crustacean, large benthic crustacean, polychaete, fish, detritus, plant and insect feeders). Of these, large and small benthic crustacean feeders, which consumed mainly crabs and gammaridean amphipods, respectively, were the most abundantly represented in terms of species, whereas polychaete and insect feeders were each represented by only two species.

Rapid changes in fish utilization of mangrove habitat in Western Madagascar

Fish use of a mangrove habitat was studied in a small mangrove forest on the West coast of Madagascar. A sand bar near the inlet retains water in parts of the channel (pools) at low tide. Fishes in four of these pools were examined daily at all phases of the tidal cycle for 3 weeks using underwater visual census. During week 1, fishes were diverse and abundant in all pools: the dominant species were cardinalfish (related to Apogonlateralis); monos, Monodactylus argenteus; black spotted snappers, Lutjanus ehrenbergi; double bar bream, Acanthopagrus bifasciatus; emperors, Lethrinus lentjan and L. sp., surgeon fish, Acanthurus nigricauda; red-lined sweetlips, Plectorhinchus plagiodesmis; and butterflyfish, Chaetodon kleini. Some species were more abundant in shaded pools; others in more open pools. During week 2 a dramatic difference was noted: the only fishes found were schools of cardinalfish and one moray eel. This week had neap tides, with high tides in the morning and low tides in the afternoon. As the week progressed and during week 3 (spring tides), fishes slowly repopulated the habitat and diversity increased. Monos, absent in week 2, now had increasing numbers of small individuals. While large emperors were scarce, small individuals appeared. The larger butterflyfish and surgeonfish seen in week 1 were replaced by small ones during week 3. Species that had been rare in week 1 were more abundant, including pipefish and small barracudas. While species richness increased during week 3, the community was strikingly different from that seen 2 weeks earlier. Only Pool 1, closest to the entrance, recovered its original species richness. Abundance was much lower than in week 1. Our snapshot study apparently captured a time when older juveniles left the mangrove forest and smaller fishes recruited into it. Utilization of this habitat will likely vary throughout the year depending on the reproductive cycle of the different species whose juveniles utilize it. Longer studies are needed to learn about cycles in fish use of the mangroves.

Fish communities of a disturbed mangrove wetland and an adjacent tidal river in Palmar, Ecuador

Coastal Ecuador has lost 20–30% of mangrove wetlands over the past 30 years. Such habitat loss can impair the ecological functions of wetlands. A paucity of information exists concerning mangrove fish communities of Ecuador. In this study we identify the fish community of the remaining mangrove wetland in Palmar, Ecuador. Fish were sampled in the dry season of 2003 and the wet season of 2004 by seining in mangrove creeks and Main channel of Rio Palmar. For comparison, an adjacent tidal river without mangroves, Rio Javita, was also sampled. We collected a total of 12,231 individuals comprising 36 species in 16 families from Rios Palmar and Javita. Gobiidae (7 species) was the most diverse family for mangrove sites followed by Gerreidae (5 species) and Engraulidae (4 species). A total of 34 species were collected in the mangrove wetland, 21 of which were exclusive to the mangroves including three species of juvenile snook (Centropomidae), indicating that the mangrove habitat of Palmar may provide nursery habitat for these economically valued species. In Rio Javita, Carangidae (3 species) was the most diverse family followed by Engraulidae and Gerreidae (2 species each). A total of 14 species were collected in the tidal river, only two of which were exclusive to the river. Multivariate analyses of fish community data indicated significant differences in community composition between the mangrove creeks and the tidal river and between seasons in both. Juvenile white mullet, Mugil curema, were collected in high relative abundance in both Rios Palmar and Javita, as was the tropical silverside Atherinella serrivomer an ecologically important species. Although Rios Palmar and Javita are characterized by relatively low fish species richness compared to other tropical estuarine systems, they appear to provide an important habitat for several economically and ecologically valued species.

Selective enrichment of green sulfur bacteria in the presence of 4-aminobenzenesulfonate (sulfanilate)

A novel selective enrichment method is described for phototrophic green sulfur bacteria even in the presence of purple sulfur and purple nonsulfur bacteria using sulfanilate, which was discovered during efforts to selectively isolate sulfanilate-metabolizing anoxygenic phototrophic bacteria from marine habitats. Samples for these experiments were obtained from beaches, saltpans, subsurface mangrove soils, fish and prawn aquaculture ponds and backwaters of the East and West coasts of India. Photoorganoheterotrophic and photolithoautotrophic enrichments in the absence of sulfanilate predominantly yielded purple bacterial enrichments. In contrast, photolithoautotrophic enrichments in the presence of sulfanilate yielded green-colored enrichments from the same samples. Whole cell absorption spectra of the enrichment cultures revealed the presence of bacteriochlorophyll c and thus green phototrophic bacteria. Microscopic observation demonstrated the presence of sulfur globules outside the bacterial cells and the presence of non-motile cells, some of which had prosthecae. 16S rDNA sequences obtained from green sulfur bacterial strains isolated from enrichment cultures confirmed the presence of representatives of the green sulfur bacterial genera Prosthecochloris and Chlorobaculum. The selective pressure of sulfanilate exerted through inhibition of phototrophic purple sulfur bacteria was demonstrated by inhibition studies using the purple sulfur bacteria Marichromatium indicum JA100 and Marichromatium sp. JA120 (JCM 13533) and the green sulfur bacterium Prosthecochloris sp. JAGS6 (JCM 13299).

A conceptual model of ecological interactions in the mangrove estuaries of the Florida Everglades

A brackish water ecotone of coastal bays and lakes, mangrove forests, salt marshes, tidal creeks, and upland hammocks separates Florida Bay, Biscayne Bay, and the Gulf of Mexico from the freshwater Everglades. The Everglades mangrove estuaries are characterized by salinity gradients that vary spatially with topography and vary seasonally and inter-annually with rainfall, tide, and freshwater flow from the Everglades. Because of their location at the lower end of the Everglades drainage basin, Everglades mangrove estuaries have been affected by upstream water management practices that have altered the freshwater heads and flows and that affect salinity gradients. Additionally, interannual variation in precipitation patterns, particularly those caused to El Niño events, control freshwater inputs and salinity dynamics in these estuaries. Two major external drivers on this system are water management activities and global climate change. These drivers lead to two major ecosystem stressors: reduced freshwater flow volume and duration, and sea-level rise. Major ecological attributes include mangrove forest production, soil accretion, and resilience; coastal lake submerged aquatic vegetation; resident mangrove fish populations; wood stork (Mycteria americana) and roseate spoonbill (Platelea ajaja) nesting colonies; and estuarine crocodilian populations. Causal linkages between stressors and attributes include coastal transgression, hydroperiods, salinity gradients, and the “white zone” freshwater/estuarine interface. The functional estuary and its ecological attributes, as influenced by sea level and freshwater flow, must be viewed as spatially dynamic, with a possible near-term balancing of transgression but ultimately a long-term continuation of inland movement. Regardless of the spatio-temporal timing of this transgression, a salinity gradient supportive of ecologically functional Everglades mangrove estuaries will be required to maintain the integrity of the South Florida ecosystem.

Using otolith microchemistry ofHaemulon flavolineatum (French grunt) to characterize mangroves and coral reefs throughout Turneffe Atoll, Belize: Difficulties at small spatial scales

We investigated whether the otolith chemistry ofHaemulon flavolineatum (French grunt), a nocturnally active fish, could be used as a means to differentiate individuals occupying mangrove and coral reef habitats. In 2003, adults were collected from 9 mangrove and 10 coral reef sites throughout Turneffe Atoll, Belize. Concentrations of trace elements were measured at the edge of sagittal otoliths by laser ablated inductively coupled plasma mass spectrometry. Results of a two-factor nested MANCOVA (sites nested within habitat and covariate of fish size), used to investigate whether significant differences in otolith elemental concentrations existed between habitats (i.e., mangrove versus reef) and among sites, indicated significant differences between habitats, in terms of lithium, magnesium, zinc, and rubidium (fish from mangroves had greater concentrations than those from coral reefs), as well as among sites (for several elements). Because elemental variability existed between habitats and among sites, we asked whether this variability was sufficient to differentiate habitats and sites using separate linear discriminant function analyses (LDFA). LDFA indicated that fish were classified to the habitat (mangrove or reef) from which they were collected with a moderate degree of accuracy (correct classification of 74% and 79% for mangrove and coral reef fish, respectively), but were poorly classified to the site from which they were collected (average correct classification of 46% with a range of 0–89%). Otolith microchemical investigations ofH. flavolineatum at Turneffe Atoll can be used to identify movement between habitats, yet due to the lack of unique site-specific chemical signatures likely caused by the nocturnal movement of individuals, it will not be possible to identify specific sites from which reef fish originated.

Assessing patterns of fish zonation in temperate mangroves, with emphasis on evaluating sampling artefacts

Mangroves and mudflats are a conspicuous element of temperate estuaries in south- eastern Australia, but our understanding of their use by fish is poor. A combination of experimental and survey methods were used to investigate (1) the utility of pop nets in sampling fishes and (2) pat- terns in fish assemblages among mangrove zones (forest, edge, mudflat) in 2 large embayments between October 2003 and January 2004. In the experimental assessment of pop net artefacts, there were no statistically significant differences (p >> 0.05) in assemblage structure, species richness (number of species) or abundances of fish among net disturbance treatments regardless of structure (seagrass or unvegetated sand). These results were not simply due to type II errors. The statistical power to observe an effect of net disturbance on fish abundances and species richness generally exceeded 80%. In the pop net survey of fish using mangrove zones, only 15 species of fish (9 families) were caught, but assemblages varied strongly among zones. Small (<30 mm) benthic gobiids, such as Pseudogobius olorum Sauvage 1880 and Mugilogobius paludis Whitley 1930, and juveniles of the atherinid Atherinasoma microstoma Gunther 1861 dominated catches in the forest. The sillaginid Sil- laginodes punctata Cuvier 1829, the tetraodontid Tetractenos glaber Freminville 1813 and a further 2 species of gobiid, Arenigobius frenatus Gunther 1861 and Favonigobius lateralis Macleay 1881, dominated catches along the edge and over the mudflat, but these species were more abundant along the edge. Fish densities were significantly lower (p < 0.05) in the mudflats (0.37 ± 0.05 fish m -2 ) than in the forest (1.98 ± 0.36) or at the edge (1.42 ± 0.43); species richness was greater along mangrove edges (0.25 ± 0.19 species m -2 ) than in the forest (0.17 ± 0.06) and in the mudflats (0.12 ± 0.02); bio- mass was greater (but not significantly) at the edge (4.64 ± 2.09 g m -2 ) and in the mudflats (4.06 ± 1.79) than in the forest (1.20 ± 0.38). Seine nets caught fewer individuals (0.95 ± 0.17 fish m -2 ) and species (0.03 ± 0.01 species m -2 ), and lower biomass (1.15 ± 0.21 g m -2 ) than pop nets (see above) in mudflats. Differences in the fish assemblage structure among zones suggest that overall fish biodi- versity may increase in fragmented mangrove landscapes as relative amounts of edge to area increase, but this could detrimentally affect abundances and species richness of resident mangrove fishes.

Mangrove fish: a comparison of community structure between forested and cleared habitats

The fish communities of mangrove and cleared sites were investigated in Gazi Bay, Kenya. Five forested sites were compared with paired sites that had been cleared of mangroves by human activity. Forested sites included plantations and natural stands of Sonneratia alba and natural Rhizophora mucronata stands. Two methods of stake netting were used to take quantitative samples; method one used a single 100-m-long, 18-mm mesh net, method two used paired 24-m-long, 1-mm mesh nets—samples were taken during seven different months in 2002. Mean abundances of fish found in mangrove and cleared sites, respectively, were 0.004 m −2 and 0.014 m −2 (method 1) and 0.21 m −2 and 0.25 m −2 (method 2). Thirty species were sampled, 12 of which were found exclusively in mangrove habitats and 10 of which were limited to cleared sites. The most abundant species in mangrove plots was Atherina afra (although it was only found in two, large catches); the most abundant in cleared plots was Gerres oyena (found frequently). Mean abundance (using data pooled for all sites) was significantly higher in cleared, compared with forested, sites, and multivariate analysis showed significantly different community structures in the two habitat types. There was large variation in catch rates between dates and sites, with one forested site recording no catches at all. These results do not support the predator refuge hypothesis (which predicts higher abundance of juvenile fish inside mangroves). The low abundance of fish recorded in the mangrove sites may have been due to site-specific factors determining fish abundance within mangrove forests, to the sampling techniques used or to relatively high turbidities at these sites.

Mangrove fish: a comparison of community structure between forested and cleared habitats

The fish communities of mangrove and cleared sites were investigated in Gazi Bay, Kenya. Five forested sites were compared with paired sites that had been cleared of mangroves by human activity. Forested sites included plantations and natural stands of Sonneratia alba and natural Rhizophora mucronata stands. Two methods of stake netting were used to take quantitative samples; method one used a single 100-m-long, 18-mm mesh net, method two used paired 24-m-long, 1-mm mesh nets—samples were taken during seven different months in 2002. Mean abundances of fish found in mangrove and cleared sites, respectively, were 0.004 m −2 and 0.014 m −2 (method 1) and 0.21 m −2 and 0.25 m −2 (method 2). Thirty species were sampled, 12 of which were found exclusively in mangrove habitats and 10 of which were limited to cleared sites. The most abundant species in mangrove plots was Atherina afra (although it was only found in two, large catches); the most abundant in cleared plots was Gerres oyena (found frequently). Mean abundance (using data pooled for all sites) was significantly higher in cleared, compared with forested, sites, and multivariate analysis showed significantly different community structures in the two habitat types. There was large variation in catch rates between dates and sites, with one forested site recording no catches at all. These results do not support the predator refuge hypothesis (which predicts higher abundance of juvenile fish inside mangroves). The low abundance of fish recorded in the mangrove sites may have been due to site-specific factors determining fish abundance within mangrove forests, to the sampling techniques used or to relatively high turbidities at these sites.

Observations of fish migration in a macrotidal mangrove channel in Northern Brazil using a 200-kHz split-beam sonar

A 200-kHz split-beam echosounder (BioSonics, DT6000) with a 6° circular-beam transducer was applied in a mangrove channel in Northern Brazil to study the migratory patterns of intertidal fish. Acoustic sampling was conducted horizontally across the channel perpendicular to the tidal current during two lunar cycles in the dry season 2000 and the wet season 2001. The complex acoustic environment of the mangrove channel was characterized by small target sizes (juvenile fish), multiple targets (aggregated fish), high reverberation and background noise levels due to sediment loads, plankton and mangrove litter transport. Dry seasons provided less noisy acoustic conditions resulting in clearer echo data than wet seasons. Neap tide data were less complex than spring tide data. During a tidal cycle, low water provided the clearest acoustic conditions. Mangrove leaves generated fish-like echoes. Analysis of two dry season wax moon cycles revealed fish flux maxima at low water, flood start and high water in the daytime and the night cycle. Night fish fluxes were significantly higher than at daylight. Throughout the tidal cycles, 60% of the fish traveled with the tide and 40% against, suggesting active foraging against the tide to be a major component of fish movements. Resident mangrove fish entered the intertidal creeks at early flood tide, leaving at late ebb tide at fairly shallow-water depths. Estuarine fish required a minimum water depth (about 2 m) for tidal migration. Since time delays during spring tides between immigration of resident and estuarine fish were reduced, foraging time and habitat accessibility would be enhanced and fish catches and fishes’ feeding success would be greater.

The Response of Fishes to Physicochemical Changes in the Mangroves of Northeast Florida Bay

Historically, large volumes of fresh water from the Everglades reached Florida Bay in the form of overland sheet flow. South Florida's extensive canal system has diverted fresh water from its historic course, resulting in shorter hydroperiods and higher salinities than would have occurred in an unaltered system. The mixing zone between the freshwater Everglades and euryhaline Florida Bay is primarily characterized as a dwarf red mangrove forest. The small, demersal fishes found in this habitat are an important food source for a variety of predators and are excellent bioindicators for both short-term and long-term perturbations in the system. I examine the effect of fluctuating water level, salinity, and temperature on this fish community in order to better understand the impact water diversion has had on the ecotone. Fish were collected at four sites within the ecotone over a t-yr period using a 9-m2 drop trap. Principal components analysis was used to generate 10 composite variables (PCs) from a temporal array of 59 physicochemical variables. These composite variables were used in regression analyses to evaluate spatial and temporal changes in the fish community. Regression analysis indicated fish density was significantly related to short-term and long-term changes in water level and with long-term temperature variation (r2=0.50). An ANOVA of density between sites supports the regression results, indicating that sites with longer hydroperiod had higher density than sites with shorter hydroperiod. The impact of changes in density on biomass was reflected by regression analysis, which indicated that increased water level and decreased variability in depth were correlated with higher biomass (r2=0.61). Biomass was also influenced by changes in the salinity regime, presumably through increases in individual fish body size or through a shift in the community toward heavier-bodied fish species. An ANOVA of biomass between sites indicates sites with longer freshwater periods had higher biomass than sites with shorer freshwater periods. The first two axes of a detrended correspondence analysis on community biomass explained 59.2% of the variance in the community and supported the hypothesis that salinity was a primary determinant of community structure. These results indicate historic changes in water deliver could have altered the mangrove fish community, thereby lowering prey availability for higher trophic levels.

Fishes in Mangrove Prop-root Habitats of Northeastern Florida Bay: Distinct Assemblages across an Estuarine Gradient

Seasonal changes in freshwater inflow and other environmental conditions may induce changes in density and species composition of mangrove fishes along estuarine gradients. Fishes within mangrove habitats in a subtropical estuary were sampled monthly from May 1989 to May 1990, using block nets with rotenone and visual censuses. At 18 stations, temperature ranged from 22 to 34°C, depth from 10 to 104cm and underwater visibility from 1 to 13m. Salinity ranged from 0 to 60 upstream, and 35 to 54 mid- and downstream. A total of 573191 individuals (76 species) was observed or collected, with an average density of 6·5 fish m−2. Engraulidae, Atherinidae, Poeciliidae and Cyprinodontidae numerically dominated the assemblage. Distinct assemblages occurred up-, mid- and downstream and maintained coherent groups in these gradient positions over the seasons. Residents totalled 94·5% of the individuals, estuarine transients comprised 5·1% and occasional marine visitors were less than 0·4%. Densities of resident fishes peaked in winter as temperatures and water levels fell, uncorrelated with changes in salinity. These observations suggest that mangrove habitats may sustain diverse and abundant fish communities dominated by euryhaline residents. Although estuarine transients were consistently rare in upstream sub-basins, downstream were found numerous sub-adults of species occurring as adults on nearby reefs (Lutjanidae, Haemulidae). Thus, reef-associated estuarine transients may be abundant in mangrove habitats having near-marine salinities. Contrary to expectations, mangrove habitats in northeastern Florida bay did not function as a nursery as defined under the nursery-ground paradigm: young-of-the-year juveniles of estuarine transient species did not seek low salinity sub-basins. However, northeastern Florida Bay may not be representative of most mangrove estuaries as the area: (1) is without lunar tides and related circulation; (2) has low and variable amounts of submersed vegetation; and (3) experiences severe hypersaline conditions.