Differences between Atlantic halibut (Hippoglossus hippoglossus L.) and turbot (Scophthalmus maximus L.) in tolerance to acute low temperature exposure

Juvenile Atlantic halibut (Hippoglossus hippoglossus L.) and turbot (Scophthalmus maximus L.) acclimated to 8–9 °C sea water were transferred to sea water at 1 °C for 7 days. An immediate spasm-like response was noticed in the turbot after the transfer, but not in the halibut. None of the fish died. The turbot suffered from substantial physiological disturbances after the transfer. Their blood plasma Cl– concentration increased and muscle water content decreased markedly. The plasma glucose concentration was several times higher than the control level from day one onwards. A decrease in the haematocrit was recorded some hours after transfer, followed by a marked increase on day 7. In halibut, the plasma Cl– concentration fell somewhat during the first days, but returned thereafter to its pretransfer level. The muscle water content was unchanged. Both haematocrit and plasma glucose concentration were unchanged until day 7, when both were significantly higher. The differences in the response to low-temperature challenge between halibut and turbot are probably related to genetic differences in temperature tolerance, which reflect differences in the distribution of the species.