Temperature effects on eggs and yolk sac larvae of the summer flounder at different salinities

The combined effects of temperature and salinity on eggs and yell: sac larv ae of summer flounder Paralichthys dentatus were examined under controlled laboratory conditions. Fertilized eggs (early gastrula stage), obtained by induced spawning of captive broodstock at 17 degrees C and 36 g/L salinity, were stocked (60 eggs/L) into forty-five 5-L translucent containers at tem peratures of 16, 20, and 24 degrees C and at salinities of 22, 28, and 34 g /L. Light intensity was 500 Ix, and photoperiod was 12 h light: 12 h dark. At 16 degrees C and 20 degrees C, hatching rate was moderate to high (57.8- 99.0%) at all salinities; at 24 degrees C, hatching rate was high at 34 g/L (75.2%) but poor at 28 g/L (0%) and 22 g/L (30.5%), indicating a high-temp erature-low-salinity inhibition (P < 0.001). At the first feeding stage and at the stage when 97% of the yolk sac was absorbed (YSA), notochord length s increased (P < 0.05) with decreasing temperature, from a minimum at 24 de grees C to a maximum at 16 degrees C. Yolk utilization efficiency appeared to show a similar trend (0.05 < P < 0.10). Average time from the first-feed ing to the 97% YSA stage in unfed larvae ranged from 2.4 to 4.3 times longe r at 16 degrees C (18.3 h) than at 20 degrees C (4.3 h) or 24 degrees C (7. 7 h). At a salinity of 34 gn, median survival time (MST) was moderate (140- 193 hours postfertilization, hpf) under all temperatures; at 22 and 28 g/L, MST was enhanced (222-294 hpf) at 16 degrees C and markedly reduced (43.1- 73.2 hpf) at 24 degrees C, indicating low-temperature-low-salinity synergis m, as well as high-temperature-low-salinity inhibitory effects (P < 0.001). A temperature of 16 degrees C, possibly associated with peak abundance of eggs and larvae in nature, is optimal for culture of summer flounder embryo s and yolk sac larvae. At this temperature, growth, yolk utilization effici ency, time to initiate exogenous feeding, and tolerance to reduced saliniti es are maximized. Simultaneous exposure to high temperatures (24 degrees C) and reduced salinities (22-28 g/L) may increase mortality and affect year- class strength.