Seasonal variations and trophic relationships among concentrated populations of small fishes in seagrass meadows




Huh, Sung-Hoi

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Temporal and experimental changes in populations of potential competitors, their resources used, and their available resources can illustrate periods of competition. Dense populations and foods of small fishes were analyzed during 1982-1983 for day-night and monthly changes in an assemblage concentrated naturally in seagrass meadows of Redfish Bay, Texas. Samples totalled 10,223 fishes, in 23 families and 40 species. Fish densities averaged 15.1/m² in shoalgrass and 6.2 individuals/m² in turtlegrass meadows, which were somewhat deeper. The darter goby, pinfish, code goby, and Gulf pipefish were the most common fish species in both shoalgrass and turtlegrass meadows throughout the year. The darter goby predominated in shoalgrass, while the pinfish and code goby predominated in turtlegrass meadows. No clear day-night differences in densities of each common fish were detected during most of the year. Peak abundance of total fishes occurred during spring, with a secondary peak in fall and a minimum in winter. Each of the four common species showed its own seasonal abundance pattern, and had a different larval recruitment and peak abundance separated 1-3 months from other species, with some overlap. Seasonal feeding data for the four most abundant fish species were compared with respect to prey availability to illustrate how resource partitioning of food could mediate possible competition among these abundant consumers. When prey (mainly amphipods) were abundant, during spring, many fish species showed high overlap in food use. Regardless of food availability, the code goby and Gulf pipefish fed mainly on amphipods and copepods. The more common darter goby and pinfish were carnivorous during spring, but they showed herbivorous feeding habits during summer, when they consumed mainly epiphytic algae during periods of lower prey availability. These changes in resource use also resulted during depletion of major foods by fishes concentrated experimentally in cages. These shifts in diets reflect a temporary partitioning of available foods. Different seasonal abundance patterns with different times of peak recruitment among seagrass fish species thus seem to permit use of the seagrass meadow habitats with reduced, seasonal competition for major foods among these concentrated fishes