Investigations into the causes of early larval mortality in cultured summer flounder (Paralichthys dentatus L.)

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Experiments were conducted to investigate larval mortality in cultured summer flounder during the first 2 weeks after hatch. The influence of feeding success, parentage, addition of algae, water quality, and the microbial community on mortality during this period were investigated. Larvae were raised in 2-1 bowls at initial densities of 50 or 75/l with mild aeration, 12 L:12 D photoperiod, and regular 50% water changes. In all experiments, mortalities were recorded and removed daily. In the first two experiments, daily samples of larvae were taken to assess feeding success and to relate that to survival. The second experiment investigated the effects of both feeding success and the addition of algae to larval culture bowls on larval survival. The third experiment investigated the effects of water quality and bacterial load on survival during the experimental period. The first two experiments indicated that failure to establish feeding is probably not the cause of catastrophic mortality of the larvae, although a statistical relationship existed between feeding incidence and survival in two of six cases. High variability (34 ± 38%, n = 82) in survival was seen in the first two experiments (both within and between parental crosses) suggesting that catastrophic mortalities were due to rearing conditions in addition to gamete quality. The addition of algae to larval cultures increased survival from 13 ± 24% (n = 33) in the absence of algae during the first experiment to 46 ± 39% (n = 49) with the addition of algae during the second experiment. The final experiment indicated that larval mortality was not linked to the measured microbial or water quality conditions. The relationship between the percentage of floating eggs at time of fertilization and survival at 10 days after hatch was not significant.

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