Tropical river sardine, Caquetaia kraussii, captured from La Aguá lagoon (Sucre State, Venezuela) were acclimatized for four weeks at 22, 24, 30 and 32ºC and at 0, 5, 10, 15 and 17 ‰ salinity. To evaluate effects of thermal response to acclimatization level, the fish were transferred suddenly from lower temperatures (22 and 24ºC) to higher ones (32 and 30ºC) respectively. Then thermal resistance time was measured at the lethal temperature of 40.9ºC for 30 days. We considered that acclimatization process completed when resistance time was stabilized at the new temperature regime. For the saline effect, the concentrations of sodium and potassium were measured in the tissues at each treatment: gills, white muscle, gut and heart. The results showed that thermal tolerance increased rapidly in 3 h with a 6ºC rise in temperature (from 24 to 30ºC) and in 24 h with a 10ºC rise (22 to 32ºC). With decreasing temperatures, the acclimatization level reached its lowest in 11 days with a 6ºC decreases (from 30 to 24ºC) and in 14 days with a 10ºC decrease (32 to 22ºC). Caquetaia kraussii regulates as much sodium as potassium in gills and white muscle tissues at all salinity levels tested; however, gut and heart tissues showed significantly different regulations among salinities examined.
Keywords: thermal acclimatization, salinity, electrolytic concentration, caquetaia kraussii