Thermal resistance and acclimation at various salinities in the sheepshead minnow (Cyprinodon variegatus Lacepede).

Cyprinodon variegatus collected in November 1969 on Galveston Island, Texas at 19.2 C and 2.7 o/oo salinity were acclimated at 30 C and 1, 10, 20, and 30 o/oo salinity and subjected to heat death at 40.8 and 41.4 C at their acclimation salinity. Samples were killed daily for 26, 29-30, 21 and 11 days respectively at 1, 10, 20, and 30 o/oo. At the end of the experiment, survival times of fish being acclimated at 1, 20, and 30 o/oo were still increasing. These results are at variance with the assumption, usually used in high temperature experiments with fishes, that acclimation is complete in 3 days. At 10 o/oo the highest level of acclimation recorded was attained within 14 and 15 days at a kill temperature of 41.4 and 40.8 C respectively. Fish were the most resistant to heat at 10 and 20 and the least at 1 and 30 o/oo. Survival times fluctuated among days. Sex and length were a factor in survival time only at 1 o/oo salinity (sex only at 40.8 C). Long and male fish had the longest survival times.