Predation on the early life history stages of winter flounder (Pseudopleuronectes americanus) by the sand shrimp (Crangon septemspinosa)

Winter flounder (Pseudopleuronectes americanus) stock complexes have been heavily exploited in the recent past, consequently decreasing abundances to record low levels. Despite management efforts to reduce further stock losses due to fishing mortality, discrete sub-populations of flounder continue to decrease for reasons that are unknown. Predation on the early life history stages of flatfish is recognized as the most important factor regulating year-class formation and recruitment. Winter flounder eggs and juveniles possess few morphological qualities that benefit the species in predator defense. Subsequently, survival of the early life stages of winter flounder depends on the effectiveness of spatial and temporal refuges in minimizing epibenthic predation. There is circumstantial evidence that increasing seasonal water temperature in several northern-temperate estuaries compromise these prey refuges, thereby exposing early-stage flounder to intense predator-induced mortality. The objectives of this study were twofold: (1) identify the importance of sand shrimp (Crangon septemspinosa) as a predators of winter flounder eggs and post-settlement juveniles; and (2) assess the implications of increasing water temperature on predator-prey dynamics and the regulation of flounder year-class strength and recruitment. Immunological assays were used in this study to analyze the stomach contents of field-collected C. septemspinosa. Accordingly, the first tangible evidence is given indicating that crangonid shrimp are predators of winter flounder eggs and juveniles in natural populations. By extrapolating results from immunoassays, C. septemspinosa were projected to consume 34.3% and 56.5% of the total number of flounder in the egg and juvenile life-stage, respectively. Moreover, a series of laboratory experiments elucidated how variations in thermal regimes potentially impact the behavioral mechanisms underlying the shrimp-flatfish predator-prey interaction. Specifically, increasing water temperature exacerbates the predator-induced mortality of early-stage flounder and possibly causes the destabilization of the predator-prey interaction with C. septemspinosa. Thus, the recent warming trend experienced in northwest Atlantic estuaries may explain the failure of the winter flounder stocks to recover in certain regions of the fish's geographic distribution.