EFFECTS OF PREY DENSITY ON THE GROWTH AND MORTALITY OF WEAKFISH CYNOSCION REGALIS (BLOCH AND SCHNEIDER) LARVAE - EXPERIMENTS IN-FIELD ENCLOSURES

Weakfish larvae (Cynoscion regalis (Bloch and Schneider)) were reared in large enclosures (1.4 m(3)) deployed near adult spawning grounds in Delaware Bay, USA. Larvae were fed wild zooplankton, and experimental prey levels encompassed naturally occurring densities (10, 100 and 10 00 prey items l(-1)). Separate experiments were conducted for early-st age and late-stage larvae in 1990, and the late-stage experiment was r epeated in 1994 using an improved statistical design. In 1990, there w as a trend toward elevated growth rates with increasing prey levels. F or early-stage larvae, larval notochord lengths (NL) varied with prey density and ranged from 3.65 to 4.30 mm. Larval dry weight (W) also va ried with prey density and ranged from 39.5 to 61.3 mu g. Calculated g rowth rates varied accordingly. Growth among late-stage larvae followe d a similar trend, and NL ranged from 4.65 to 5.25 mm, while W ranged from 73.7 to 139.7 mu g. In the repeated late-stage experiment (1994), a nested ANOVA showed significant effects of prey level on growth. No tochord length (4.33 mm) and dry weight (72.3 mu g) were significantly depressed at the lowest prey density. Rates of instantaneous mortalit y were variable among individual enclosures and were not related to pr ey density in either year. Our results differ from previous field encl osure studies, which have shown little effect of prey density on the g rowth of larval fish. By contrast, our data indicate that low food lev els may result in depressed growth rates among weakfish larvae in nurs ery areas like Delaware Bay. These lower growth rates may lead to an i ncreased duration of larval development, potentially culminating in hi gh mortality from increased exposure to predation or from advection fr om suitable juvenile habitats.