No abstract

We measured predation on bay anchovy Anchoa mitchilli eggs and larvae by abundant scyphomedusae Chrysaora quinquecirrha and ctenophores Mnemiopsis leidyi from gut contents, digestion rates, and densities of predators and prey during 9 d in July 1991 at 4 stations in Chesapeake Bay, USA. These predation rates were compared to egg and larval mortality rates measured concurrently in ichthyoplankton surveys. Daily predation by medusae and ctenophores was 19 * 13":) (mean + SD) of the eggs over the 20 h stage duration, with medusae responsible for 26 to 100% of the predation. These gelatinous predators accounted for 21 i 17 ":, of the total estimated daily egg-stage rnortality. On average, medusae consumed 29 + 14 O/u d -' of the larval bay anchovy, which was 41 35% of total estimated larval mortality. Predation on larvae by ctenophores was not detected. These predation effects are compared with those measured concurrently in free-drifting 3 2 m3 mesocosms. We conclude that medusae, which had high feeding rates but low abundances, and ctenophores, which had lour feeding rates but high abundances, were important predators of bay anchovy eggs and larvae in the mesohaline region of Chesapeake Bay

Low dissolved oxygen concentrations, caused by density stratification of the water column and excess nutrient inputs, occur in many aquatic habitats. Laboratory experiments we conducted indicated that low dissolved oxygen has the potential to strongly alter the absolute and relative importance of a suite of estuarine predators of fish larvae. At dissolved oxygen concentrations 22 mg l-', predation on naked goby Gobiosolna bosc larvae by an important invertebrate predator of plankton in Chesapeake Bay [the sea nettle scyphoinedusa Chrysaora quinqueclrrha) increased. In contrast, at the same oxygen concentrations, predation by 2 vertebrate predators, juvenile striped bass Morone saxatllis and adult naked goby, decreased. Changes in consumption of larvae most likely resulted from impaired ability of larvae to escape the scyphornedusa, and decreased attack rates by adult and juvenile fishes. Fish predators increased gill ventilation rates even at oxygen levels higher than those leading to decreased predation. However, we could detect no comparable change in behavior of the sea nettle even at 1 mg 1-', the lowest oxygen concentration tested The observed changes in trophic interactions occurred at dissolved oxygen concentrations that are not lethal during short exposures, and that commonly occur in the Chesapeake Bay and other eutrophic estuaries during summer. Thus, low oxygen has the potential to cause significant changes in the importance of alternate trophic pathways in estuarine systems.

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.

Egg and larval abundance of bay anchovy Anchoa mitchilh from 4 field experiments in Biscayne Bay, Florida, were analyzed to determine variability in growth and mortality rates of dailyspawned cohorts. Mean egg densities varied by more than an order of magn~tude in the 4 experiments. Otolith-aged larvae, validated by laboratory rearing, were used to construct the age-frequency distributions of 71 cohorts. Mean growth rate estimates ranged from 0.43 to 0.56 mm d-' and the means of cohort-specific instantaneous mortality rates ranged from 0.30 to 0.45 (26 to 36 % d-l). Egg mortality rates were generally higher and more variable than those for larvae. An approximate mean mstantaneous mortality rate for eggs was 1.94 (85.6 % d-l). Mean larval growth and mortality rates differed significantly among experiments, although reasons for the lfferences were not identified. On average, relative s u~v a l s of cohorts varied by a factor of 20 at 20 d after hatching among the 4 experiments. Estimated mean abundances of larval cohorts at 20 d varied by a factor of 45, reflecting differences in in~tial abundances of eggs and the variable mortality rates. Models of bay anchovy growth and mortality in relation to prey concentration, developed in previous laboratory expenments, were used to partition field-expenment mortalities into nutrition-related and presumed predabon-related components. Mean daily losses of a cohort to predation (18 to 28 %) were 2 to 3 times higher than losses to starvation (10 to 11 %).