HABITAT USE AND DIETS OF JUVENILE SPOT (LEIOSTOMUS XANTHURUS) AND ATLANTIC CROAKER (MICROPOGONIAS UNDULATUS) IN A SMALL ESTUARY AT MAD ISLAND MARSH, TEXAS
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Spatial and temporal variation in feeding of immature stages (10-130 mm standard length) of two sciaenids, spot (Leiostomus xanthurus) and Atlantic croaker (Micropogonias undulatus), was investigated at Mad Island Marsh on the Texas Gulf coast from March 1998 through August 1999. Periods of peak abundance of each species were different (December for croaker, March for spot). The two species were dissimilar in diet and habitat use along the longitudinal gradient of a small estuary, except for the winter months of both years when postlarvae dominated samples and dietary overlap was high. Low dietary overlap occurred despite the fact that both species consumed mostly benthic organisms (amphipods, gastropods, nematodes, polychaetes), pelagic copepods, and detritus. Overall, spot consumed greater proportions of copepods, nematodes and chironomid larvae, and croakers had broader diets that included most of the abundant invertebrates in the habitat (e.g., amphipods, shrimp, blue crabs) plus smaller percentages of fish. Regardless of season and body size, the spot diet was dominated by copepods and nematodes. The croaker had a more pronounced ontogenetic diet shift, with the smallest size classes (10-25 mm) consuming mostly copepods, and larger juveniles having diets dominated by amphipods, polychaete worms, shrimp, crabs, and fish. Ontogenetic diet shifts resulted in low interspecific dietary overlap among larger size classes when they co-occurred in the estuary, which could be interpreted as niche complementarity in response to competition past or present.