Size-dependent predation on post-settlement winter flounder Pseudopleuronectes americanus by sand shrimp Crangon septemspinosa

Abstract

Juvenile flatfish that are large-at-age are assumed to have a survival advantage because of a reduction in predator-induced mortality (bigger-is-better hypothesis), but this generalization is largely unsubstantiated in fisheries research. This study examined size-dependent predation on post-settlement winter flounder Pseudopleuronectes americanus by the sand shrimp Crangon septemspinosa, and the interactive effects of flounder density and water temperature on the predator-prey interaction. Shrimp were visually observed foraging on flounder, and the major elements of the 'predation sequence' were quantified, from which the susceptibility and gross vulnerability of flounder were estimated. Flounder susceptibility generally decreased with increasing fish size until a complete refuge from shrimp predation was attained at sizes > 25 mm total length (TL). The gross vulnerability of flounder to shrimp predation was dome-shaped when analyzed graphically with respect to prey size. At the extremes of the vulnerability response curve, small flounder (8 to 12 mm TL) benefited from a decreased rate of encounters with shrimp predators, whereas large flounder (18 to 22 mm TL) profited from superior escape abilities and prolonged predator handling time once attacked and captured. Conversely, intermediate-sized flounder (13 to 17 mm TL) suffered the highest rates of mortality, and this was accentuated with increasing temperature and decreasing flounder density. Intermediate-sized flounder (17 mm TL) were the most energetically profitable for shrimp. Below this size, net energy intake of shrimp decreased because of the lower caloric content of smaller flounder. Above this size, the increased metabolic demands associated with foraging on large flounder resulted in a decrease in shrimp net-energy gain. Moreover, shrimp feeding on low densities of flounder suffered a net loss in energy intake, thus possibly explaining the observed density-dependent predation of crangonid species feeding on flatfish. Results from this study challenge the applicability of the bigger-is-better hypothesis to this specific predator-prey interaction.