ENCLOSURE EXPERIMENTS ON SURVIVAL AND GROWTH OF BLACK DRUM EGGS AND LARVAE IN LOWER CHESAPEAKE BAY

Experiments in 2.2 m3, in situ mesocosm enclosures indicate that black drum, Pogonias cromis, eggs and larvae potentially can survive in the lower Chesapeake Bay at ambient microzooplankton prey levels (approximately 200 prey l-1) in the absence of predators. In growth experiments, larva mean growth rates to 10 d posthatch were similar (0.17 mm d-1 and 0.18 mm d-1) when fed at prey levels of 50 prey l-1 and 200 prey l-1. Individual growth rates, however, were more variable at 50 prey l-1. Mortality rates also were comparable in 50 (27% d-1) and 200 (23% d-1) prey l-1 enclosures. In a second experiment, the predation potentials of the hydromedusa Nemopsis bachei and the lobate ctenophore Mnemiopsis leidyi were estimated in relation to initial black drum egg prey density, presence of alternative <1 mm zooplankton prey, and estimated daily abundance of the jellyfish on the black drum spawning grounds. Mortality rates per medusa and ctenophore were similar (0.02-0.03 d-1), were not affected by presence of alternative prey, and were directly related to initial egg density. Results suggest that the gelatinous predators, especially the hydromedusa, could have cleared a high (approximately 38%) but variable fraction of the water column daily of fish eggs and yolk-sac larvae during the black drum spawning season. We hypothesize that the poor or episodic recruitment success of black drum in Chesapeake Bay results from a short spawning season that often coincides with abundance peaks of gelatinous predators and that predation on eggs and yolk-sac larvae may control recruitment.