PREY SELECTION BY LARVAL WEAKFISH (CYNOSCION-REGALIS) - THE EFFECTS OF PREY SIZE, SPEED, AND ABUNDANCE

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. Under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 items l-1 weakfish, larvae always chose large rotifers (length = 216 mum) over small rotifers (length = 160 mum). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 mum); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 items l-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10 000 items-1. Swimming speeds of prey items, which ranged from 1 to 6 mm s-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.