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Abstract
The effects of light intensity and salinity on eggs and yolk sac larvae of the summer flounder, Paralichthys dentatus, were examined under controlled, laboratory conditions. Fertilized eggs (early gastrula stage), obtained by induced spawning of captive broodstock, were stocked (53 eggs/L) into forty-eight 5-L, cylindrical translucent containers under light intensities of 0 (constant dark), 500, 1,000 and 2,000 lux and at salinities of 26,31 and 36g/L. Temperature was 19°C and photoperiod was 12 L: 12 D. Although hatching success (x = 94%) and larval survival (x = 85%) were not influenced by light intensity or by salinity within the ranges tested, significant (P< 0.05) additive effects on larval growth were observed. At the 97% yolk sac absorbed stage (114-131 hours post-fertilization = hpf), at the first-feeding stage (129.5-135 hpf), and at yolk exhaustion (153.5-159 hpf), notochord lengths were generally maximal at low light intensity (500 lux) and high salinity (36 g/L), and minimal at high intensity (2,000 lux) and low salinity (26 g/L). Yolk utilization efficiency declined (P< 0.01) with increasing light intensity, presumably due to light-induced activity. Largest first-feeding larvae were produced under a low light intensity (500 lux) and a high salinity of 36 g/L. Assuming that largest larvae have highest survival potential under culture, then a low light intensity of 500 lux and salinity of 36 g/L are recommended for incubation summer flounder eggs and yolk sac larvae.
In a second experiment, the effects of a broader range of light intensities (50,100,250,500,750,1,000 and 2,000 lux) on growth of first-feeding stage larvae were studied through day 15 post-hatching (d15ph) at 36 g/L salinity. Fertilized eggs were stocked into twenty-one 19-L rectangular, opaque aquaria at a density of 14/L, yielding an initial larval density of 12/L. Temperature was 19°C and photoperiod was 12 L:12 D. S-type rotifers, enriched with Nannochloropsis oculatafor 16-24 hours, were fed at a concentration of 10 ind./mL beginning on day 2 post-hatch. N. oculatawas also added at approximately 300,000 cells/mL. Highly significant (P <0.001) effects of light intensity on growth were evident from d9ph, with a clear trend toward higher growth at lower light intensities. On day 15 post-hatch, mean (SE) notochord lengths ranged from a minimum of 3.88 (0.05) mm at the highest intensity (2,000 lux) to a maximum of 5.84 (0.18) mm at the lowest intensity (50 lux). Survival to day 15 post-hatch averaged 80.2% (range = 66.4-97.3%) among treatments, with no significant differences. Excessive light intensities may increase larval activity and metabolic rates and lower prey capture and utilization efficiency.
The results demonstrate that light intensity, even within a seemingly narrow ecological range, has pronounced effects on growth of both pre-feeding and early feeding stage summer flounder larvae under intensive culture. Optimal growth of these early life stages occurred under low illumination levels, presumably consistent with shelf waters where eggs and early larvae of this species prevail in nature.