Comparison of Thermohaline Optima for Juveniles of Two Sympatric Paralichthyid Flounders: Ecophysiological Evaluation of Estuarine Nursery Quality

Abstract

Juvenile summer flounder Paralichthys dentatus and southern flounder Paralichthys lethostigma exhibit ontogenetic segregation during spring and summer in southeastern USA estuaries, with summer flounder remaining primarily in polyhaline regions and southern flounder migrating to oligohaline habitat. To determine physicochemical conditions contributing to optimal nursery ground function for each species, growth rates, feeding rates, and gross growth efficiencies were examined in common-garden experiments at spring and summer thermohaline regimes. Experiments were conducted on young-of-the-year (45–100 mm total length; 0.9–8.9 g) summer flounder (15–25 °C, 10–30‰) and southern flounder (15–30 °C, 0–30‰) from North Carolina, and juvenile summer flounder from Delaware (15–25 °C, 30‰ only). Over the 15–25 °C and 10–30‰ conditions common to both species, means of all dependent variables were greater in summer flounder than southern flounder. Summer flounder grew best at 21–23 °C and 21–28‰. Low salinity (0–10‰) was physiologically suboptimal, especially at high (25 °C) summer temperatures. There was a significant direct effect of location (North Carolina vs Delaware) on growth efficiency of juvenile summer flounder, but not on growth rate, feeding rate, or linear growth rate. Growth potential of southern flounder was maximum at 25–26 °C and 10–30‰, and growth efficiency was highest at mesohaline salinities; yet growth was maintained at moderate levels over a broad range of oligohaline conditions, including ≤ 10‰, even at 30 °C. Differences in thermohaline conditions that affect growth potential illustrate spatiotemporal patterns and dynamics of optimal nursery potential for these two paralichthyid flounders.