The Spatial Organization of the Nocturnal Fish Fauna of a Tropical Seagrass Feeding Ground

Robblee, Michael

A diver-operated encircling net was used to make discrete, quantitative collections of grass bed fishes in Tague Bay, St. Croix. Coupled with these collections were measurements of variables characterizing the grass bed habitat. These data were used to describe spatial variation in the grass bed fish assemblage emphasizing reef fish species visiting the seagrass meadow at night to feed and the processes organizing them. Diel variation in fish density was evident at the familial level at three Thalassia-dominated stations. For the major species found density reflected accurately patterns of feeding activity. Small permanently resident fishes characterized the diurnal fish assemblage with parrotfishes (Scaridae), primarily Sparisoma radiana, and wrasses ( Labridae) of the genus Halichoeres dominating numerically. At night, grunts (Haemulidae), squirrelfishes (Holocentridae) and cardinal fishes (Apogonidae) were the abundant active fishes. Among those fishes which fed within the grass canopy the grunts. Haemulon Plumieri, and H. flavolineatum, and the squirrelfish Holocentrus rufus dominated numerically. A twilight changeover, similar to that observed on coral reefs, marked the transition between these day and night fish assemblages.