Copepods are considered the best initial live food item for the initial feeding of many marine fish. Their small size is compatible with the mouth gape of small fish larvae and their high nutritional value leads to improved growth rate, improved survival, and reduced rates of deformities compared to rotifers. In fact, many commercially important marine fish species that have small larvae including many grouper, snapper and marine ornamentals have been successfully reared past their larval stage only by using a diet of live copepods. However, the culture of copepods on a large scale has been problematic due to the high variability of productivity of cultures and the relatively low density that can be applied in culture. Understanding the impact that different environmental conditions have on copepod population demographics and production characteristics will facilitate the optimization of copepod culture methods.
In this study, the commonly cultured calanoid copepods Acartia tonsa and Parvocalanus crassirostris were reared at four salinities (20, 25, 30, 35 ppt). After acclimation to the treatment salinities for a least three generations, replicated experiments were conducted to assess the impact of culture salinity on sex ratio, egg production, egg hatching rate, and mortality post hatch. The temperature was maintained at 25°C for A. tonsa and 27.5°C for P. crassirostris, and live Tisochrysis lutea was fed twice daily to maintain food availability above estimated carbon saturation densities for the two species (1,500 μg C L-1 for A. tonsa and 1,000 μg C L-1 for P. crassirostris).
For A. tonsa, the percentage of females varied significantly and inversely (p=0.025) with salinity from 68.24% ± 2.13% [SD] at 20 ppt to 62.53% ± 3.01% at 35 ppt. For P. crassirostris the percentage of females did not differ significantly among salinity treatments and averaged 50.69% ± 6.21%. Survival from initial stocking of early nauplii (N1-N2) to the adult stage (A. tonsa - day 8, P. crassirostris - day 5) was not affected significantly by salinity in either species (58.41% ± 10.48% for A. tonsa, 70.51% ± 10.67% for P. crassirostris). For both species, daily egg production by individual females significantly decreased (p=<0.001) over 7 days. For A. tonsa, mean egg production decreased from 36.28 ± 13.83 on day 1 to 23.58 ± 13.54 on day 7. Likewise, egg production in P. crassirostris decreased from 41.84 ± 8.69 on day 1 to 23.58 ± 13.54 on day 7. Fecundity significantly increased (p=0.02) in 30 ppt for A. tonsa; salinity had no effect on fecundity in P. crassirostris. Egg hatching rate (mean of 67.13% ± 32.68% for A. tonsa, 91.79% ± 15.69% for P. crassirostris) was not impacted by salinity or age for either species. The optimal salinity regime for production may be specific to individual species or culture methods, but this study suggests that production by A. tonsa may be optimized at 30 ppt whereas the production by P. crassirostris is minimally impacted within the salinity range tested.