Piscivory in age-0 summer flounder Paralichthys dentatus with a focus on predator-induced mortality of post-settlement winter flounder Pseudopleuronectes americanus

摘要

We examined the piscivorous diet of age-0 summer flounder Paralichthys dentatus in southern New England tidal rivers, with a focus on their predatory impact on post-settlement winter flounder Pseudopleuronectes americanus. The population density, size-structure, and growth of age-0 summer flounder and winter flounder were evaluated in the Seekonk and Taunton Rivers (Rhode Island and Massachusetts, USA, respectively) between May and August/September 2009 through 2015. For a subsample of summer flounder collected during this time (20-181 mm total length, TL; n = 743), diet was assessed using direct visual analysis and PCR-based assays that detect winter flounder mitochondrial DNA within predator stomach contents. Summer flounder were generalist piscivores consuming 8 distinct fish prey taxa from both epibenthic and pelagic guilds. The most frequently observed fishes in the diet of summer flounder were age-0 winter flounder and herring (Clupeidae) with frequencies of occurrence, %F, of 2.6 and 2.0%, respectively, and overall %F of fish equal to 13.6%. Fish were absent in the stomachs of summer flounder 44 mm TL, beyond which piscivory increased significantly with increasing predator size. Summer flounder 50-153 mm TL preyed on winter flounder ranging from 19-54 mm TL, resulting in predator-to-prey size ratios of 2.2-3.6 (mean +/- SD = 2.8 +/- 0.3). Incidences of summer flounder predation on winter flounder were positively related to body size ratios, and this relationship was attributed to the enlarged mouth gape and improved prey capture abilities of larger predators. Summer flounder predation on fishes, including winter flounder, also demonstrated significant spatiotemporal variability, reflecting riverine and seasonal differences in flounder population size structure and dynamics in prey composition and availability. Deterministic model simulations estimated that age-0 summer flounder account for 0.7 % of the daily mortality of post-settlement winter flounder (range = 0.0-2.9%), and consumed 3.0% of the total winter flounder year-class annually (range = 0.0-12.8 %). Therefore, relative to other predatory fishes and decapod crustaceans, age-0 summer flounder likely have a nominal effect on winter flounder populations in tidal river nurseries. Summer flounder predation may be substantial, however, when multiple age-classes are considered and elevated age-0 summer flounder densities elicit a strong effect on winter flounder survival, albeit at local scales.