The effect of hatch date and early growth on survival rates of juvenile Southern flounder (Paralichthys lethostigma)

Abstract

Southern flounder, Paralichthys lethostigma, spawn during late fall and winter over continental shelf waters and post-metamorphosis juveniles settle in estuarine habitats along the southeast US coast during winter and early spring. Previous studies in North Carolina have documented considerable variation in body size among young-of-the-year (YOY) flounder captured in early summer, suggesting that growth rates may be highly variable during the initial post-settlement period. I estimated birth date distributions and early growth rates of initial settlement postlarvae southern flounder using otolith microstructure and backcalculation techniques and then compared these traits from initial settlement postlarvae with those of surviving juvenile fish captured in summer to assess the potential effects of hatch timing and early growth on early survival. Fish were collected in estuarine waters from January through August. Taxonomic identification was achieved using melanophore counts and spacing along the fin margins. Increment counts were made on whole sagittal otoliths for recently settled fish (≤ 20mm SL) and on polished transverse sagittal sections for late stage fish. Hatch date distributions of initial settlement postlarvae southern flounder were adjusted for cumulative mortality and ranged from late December through late February with a peak in mid- to late January. Growth of initial settlement postlarvae fish (0-2 months of age) ranged from 0.2 – 0.6 mm/d with a mean of 0.4 mm/d. Hatch dates of surviving juvenile flounder ranged from mid-February through mid-April with a peak in late February. Growth rates of surviving juvenile fish (> 2 months of age) ranged from 0.5 - 1.3 mm/d, with a mean of 0.76 mm/d. Results indicated that higher survival rates were achieved by fast growing individuals hatched later in the spawning season. I speculate that favorable environmental conditions related to prey availability and water temperature may promote fast growth and high survival for individuals settling to the estuary later in the recruitment period. Future work will include validation of aging procedures using known age hatchery-reared fish and the evaluation of growth of late stage fish during only the initial 40 days of life.