The relative predation potentials on ichthyoplankton of the scyphomedusa Chrysaora quinquecjrrha, the ctenophore Mnemiops~s leidyi and the bay anchovy Anchoa mjtchllli from Chesapeake Bay, USA, were estimated in 3.2 m3 in situ mesocosm enclosures and in 1.0 m3 laboratory tanks. For all 3 predators, averaged predation mortality (d-') and volume-specific clearance rates (1 d-' ml-') were higher and less variable when bay anchovy eggs were prey than when goby Gobiosoma bosci larvae 13.0 to ca 10.0 mm standard length (SL)] were prey. The smallest larvae (3.0 to <5.5 mm SL) were more vulnerable than eggs or larger larvae. Averaged mortality rates per scyphomedusa (0.78 and 0.32 d-' on eggs and larvae respectively) were 7 times higher than those per ctenophore (0.11 and 0.04 d-l), and almost 2 times higher than those per bay anchovy (0.37 and 0.21 d-l). However, volume-specific clearance rates by the relatively small bay anchovy predators were higher (ca 500 1 d-' ml-l) than those of the gelatinous predators. The volume-specific clearance rates of the ctenophore and medusa were only 4 and 7 % respectively of that for the anchovy. Combined species results suggest that these predators may consume 20 to 40 % daily of the fish eggs and larvae in mid-Chesapeake Bay. The scyphomedusa potentially is the most important predator on summer ichthyoplankton due to its overall abundance, high clearance rates and temporal CO-occurrence with vulnerable life stages of fish.

Numerous species of gelatinous zooplankton are known to eat ctenophores, but their predation interactions have seldom been studied. Laboratory experiments showed that Chrysaora quinquecirrha medusae ( 3 to 20 mm diameter) usually consumed entire ctenophores (Mnerniopsis leidyi) that were equal in diameter or smaller. Although ctenophores larger in diameter than medusae were sometimes consumed completely, often only the lobes of the ctenophores were eaten. These damaged ctenophores healed in the laboratory. Short-lobed ctenophores had reduced fecundity, and probably lowered feeding rates as well. Short-lobed ctenophores were abundant (24 to 76% of the population) in situ during 1990. Large medusae (40 to 120 mm diameter) in 3.2 m3 in situ mesocosms cleared ctenophores at high rates (up to 6180 1 d-'). Clearance rates of medusae decreased with increasing ctenophore density and s u e , and increased with medusa size. The laboratory-determined clearance rate equation, in combination with medusa sizes and densities in situ, predicted that medusae could eliminate ctenophores from a tributary, but not at 2 stations in the main-stem Chesapeake Bay (USA), which was in agreement with in situ population data. The multlple negative effects of C. quinquecirrha on M. leidyi populations may lead to complex community-level changes that actually may reduce mortality of zooplankton and ichthyoplankton.

We applied an individual-based population model to examine the potential compensatory scope of the bay anchovy Anchoa mitchilli in Chesapeake Bay. Model simulations were analyzed to estimate: (1) how losses of individuals in different life stages affect year-class production; (2) the changes needed in individual processes and, simultaneously in multiple processes, to offset a 50% increase in larval-stage mortality; and (3) population responses to increasing larval-stage mortality under conditions of presumed high compensatory potential. We hypothesize that, in response to lower densities, the bay anchovy population could compensate for increased larval n~ortality through increased growth rates, increased allocation of energy to reproduction, or reduced predat~on mortality as predators target other species. Simulation results indicate that late-larval and juvenile bay anchovy may be able to consume a significant fraction of their zooplankton prey, suggesting that anchovy is at abundances that can cause density-dependent growth in the Chesapeake Bay. However, densitydependent effects on prey resources alone had a limited buffering effect against a 50% reduction in larval-stage survival. The potential effect of losses of larvae on future production of a year class depended upon when during the larval stage individuals are removed from the population. Modeled alone, large changes in spawning ~ntensity (no. of batches and eggs per batch), egg survival, or mortality of juveniles and adults were required to offset increased larval mortality. When all processes were varied simultaneously, much smaller changes were required. Under a high compensation scenario, there was a strong dome-shaped response in adult production potential to increased larval mortality, such that highest adult production occurred when survival rate of larvae was reduced by as much as 60% While the information presently available to examine density-dependent population responses in bay anchovy is limiting, the modeled results indicate that the bay anchovy population in Chesapeake Ray potentially can regulate its abundance through simultaneous shifts in processes believed to be sensitive to population density.