SELECTIVE PREDATION BY JUVENILE WEAKFISH: POST-CONSUMPTIVE CONSTRAINTS ON ENERGY MAXIMIZATION AND GROWTH

Abstract

Juvenile weakfish (Cynoscion regalis) prey heavily on mysids (Neomysis americana) and sevenspine bay shrimp (Crangon septemspinosa) during estuarine residency. Previous researchers have suggested that Neomysis are superior prey because the growth rate and condition factor of juvenile weakfish in Delaware Bay are positively associated with the proportion of Neomysis in the diet. Laboratory experiments were conducted to (1) examine prey preferences of juvenile weakfish foraging on Neomysis and Crangon; (2) determine whether selective predation leads to short- and long-term energy maximization; and (3) identify pre- and postconsumptive factors that influence relative prey profitability and potentially constrain weakfish foraging success and growth. Weakfish preyed selectively on Neomysis over a range of relative prey abundances, including trials where available Crangon biomass was ≈20 times greater than Neomysis biomass. Preconsumptive cost:benefit analyses based on prey energy content (E), prey biomass (M), and predator handling times (Ht) failed to predict prey selection by juvenile weakfish. These analyses indicated that fish should select Crangon over Neomysis to maximize gross E/Ht and M/Ht. The inadequacy of E/Ht and M/Ht as measures of prey profitability was revealed through consideration of postconsumptive processes. In particular, Neomysis were digested and evacuated from the stomach 1.8 times faster than Crangon. The hypothesis that differential prey digestibility influenced prey profitability was tested by comparing long-term energy intake rate and growth rate of weakfish foraging ad libitum on Neomysis vs. Crangon. Mean daily energy intake and growth rate were significantly higher for fish foraging on Neomysis, providing evidence that selective predation on Neomysis functioned as a rate-maximizing strategy. Differences in long-term energy intake were similar in magnitude to differences in gastric evacuation rate, indicating that the higher preconsumptive value of Crangon (measured as E/Ht and M/Ht) was nullified by a slower rate of digestion. Our findings demonstrate that currencies used to test optimal diet predictions of prey choice in fishes must incorporate postconsumptive processes that potentially constrain long-term energy intake rate and growth.