Fish assemblage structure in relation to environmental variation in a Texas Gulf coastal wetland

We described seasonal fish-assemblages in an estuarine marsh fringing Matag orda Bay, Gulf of Mexico. Habitat zones were identified by patterns of fish species abundance and indicator species optima along gradients in salinity , dissolved oxygen (DO), and depth in our samples. Indicators of the lower brackish zone (lower lake and tidal bayou closest to the bay) were gulf men haden (Brevoortia patronus), bay anchovy (Anchoa mitchilli), silver perch ( Bairdiella chrysoura), and spotted seatrout (Cynoscion nebulosus) at salini ty > 15 parts per thousand, DO 7-10 mg 1(-1), and depth < 0.5 m. Indicators of the upper brackish zone (lake and fringing salt marsh) were pinfish (La godon rhomboides) and spot (Leiostomus xanthurus) at salinity 10-20<parts p er thousand>, DO > 10 mg 1(-1), and depth < 0.5 m. In the freshwater wetlan d zone (diked wetland, ephemeral pool, and perennial scour pool), indicator s were sheepshead minnow (Cyprinodon variegatus), rainwater killifish (Luca nia parva), mosquitofish (Gambusia affinis), and sailfin molly (Poecilia la tipinna) at salinity < 5 parts per thousand, DO < 5 mg 1(-1), and depth <gr eater than or equal to> 1 m. In the freshwater channelized zone (slough and irrigation canal), indicators were three sunfish species (Lepomis), white crappie (Pomoxis annularis), and gizzard shad (Dorosoma cepedianum) at sali nity < 5<parts per thousand>. DO < 5 mg 1(-1), and depth > 1.5 m. In bracki sh zones, seasonal variation in species diversity among sites was positivel y correlated with temperature, but assemblage structure also was influenced by depth and DO. In the freshwater zones, seasonal variation in species di versity among sites was positively correlated with depth, DO, and salinity, but assemblage structure was weakly associated with temperature. Species d iversity and assemblage structure were strongly affected by the connectivit y between freshwater wetland and brackish zones. Uncommon species in diked wetlands, such as tarpon (Megalops atlanticus) and fat sleeper (Dormitator maculatus), indicated movement of fishes from the brackish zone as the wate r level rose during natural flooding and scheduled (July) releases from the diked wetland. From September to July, diversity in the freshwater wetland zone decreased as receding waters left small isolated pools, and fish move ment became blocked by a water-control structure. Subsequently, diversity w as reduced to a few species with opportunistic life histories and tolerance to anoxic conditions that developed as flooded vegetation decayed.