Effect of rapid changes in temperature and salinity on availability of the rotifers Brachionus rotundiformis and Brachionus plicatilis.

Euryhaline rotifers are an important food for rearing marine fish larvae. Their availability to fish larvae in the water column may be reduced if they are transferred to fish larval rearing tanks with different temperatures and salinities. The rotifers, Brachionus rotundiformis (formerly called S-type) and Brachionus plicatilis (formerly called L-type), were mass-cultured at 23°C and 35‰ salinity, and then abruptly transfered into tanks at different temperatures (18°C, 23°C, 28°C) and salinities (10‰, 15‰, 20‰, 25‰, 30‰, 35‰). Their availability in the water column was then measured in non-turbulent beakers over time. Both species showed initial transfer shock. The number of rotifers 1 h after stocking was approximately 50% of that potentially available, but increased to approximately 75% after 6 h. Availability was reduced when rotifers were transferred from 23°C to 18°C, but was not affected by transfer from 23°C to 28°C and remained relatively constant over time. B. rotundiformis was slightly more tolerant at higher temperatures than B. plicatilis. Salinity had a greater effect on availability than temperature. Availability of rotifers decreased as salinity was reduced. The availability of rotifers increased over time indicating that some rotifers had acclimated to the transfer conditions. B. plicatilis was slightly more tolerant at lower salinity than B. rotundiformis. Rotifers should be cultured at lower temperatures and similar salinities to the fish larval rearing tanks or acclimated for at least 6 h to larval rearing conditions before transfer. (C) 2000 Elsevier Science B.V.