Effects of prey density on the growth and mortality of weakfish Cynoscion regalis (Bloch and Schneider) larvae: Experiments in field enclosures

Abstract

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 1000 prey items 1 −1). Separate experiments were conducted for early-stage and late-stage larvae in 1990, and the late-stage experiment was repeated in 1994 using an improved statistical design. In 1990, there was a trend toward elevated growth rates with increasing prey levels. For 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 varied with prey density and ranged from 39.5 to 61.3 μg. Calculated growth rates varied accordingly. Growth among late-stage larvae followed a similar trend, and NL ranged from 4.65 to 5.25 mm, while W ranged from 73.7 to 139.7 μg. In the repeated late-stage experiment (1994), a nested ANOVA showed significant effects of prey level on growth. Notochord length (4.33 mm) and dry weight (72.3 μg) were significantly depressed at the lowest prey density. Rates of instantaneous mortality were variable among individual enclosures and were not related to prey density in either year. Our results differ from previous field enclosure studies, which have shown little effect of prey density on the growth of larval fish. By contrast, our data indicate that low food levels may result in depressed growth rates among weakfish larvae in nursery areas like Delaware Bay. These lower growth rates may lead to an increased duration of larval development, potentially culminating in high mortality from increased exposure to predation or from advection from suitable juvenile habitats.