Ecological mechanisms generating variable first-year growth in Paralichthys lethostigma, a coastal flatfish

Determining the relative influence of, and possible synergistic relationships among, environmental and biotic processes on individual growth rates can identify key mechanisms that contribute to variation in body size observed among similar-aged individuals. The timing of ontogenetic shifts in diet or habitat, which themselves can be size-dependent, can have considerable impacts on individual growth trajectories, survival, and year class formation. The southern flounder Paralichthys lethostigma is a coastal flatfish species with juvenile cohorts that occupy diverse estuarine settlement habitats and commonly display 2-3 fold variation in body size by the end of the first growing season. We examined abiotic and biotic factors contributing to differences in early juvenile growth for the 2018 year-class of southern flounder in a North Carolina estuary. Following peak larval settlement (Febuary/March), flounder body sizes began to diverge by April, with differences between settlement habitats contributing to roughly 3-fold variation by August. Stomach content analyses indicated a greater occurrence of fish prey in the diets of southern flounder recovered in mesohaline versus oligohaline settlement habitats. Increased piscivorous foraging in mesohaline habitats was linked to nitrogen (δ15N)-enriched tissues and faster growth rates relative to fish collected in oligohaline habitats. Results from a controlled laboratory experiment provided evidence for the strong effects of diet, with only minor direct effects of salinity on juvenile southern flounder growth. Our findings support the critical role of ontogenetic shifts to piscivorous feeding for growth in young-of-the-year southern flounder and highlights the potential for growth-mortality tradeoffs to exist among estuarine settlement habitats.