IMPACTS OF SUBLETHAL AND LETHAL HIGH TEMPERATURES ON CLAMS EXPLOITED IN EUROPEAN FISHERIES

In Galicia, three venerid bivalves are exploited in the intertidal zone by local fishery guilds: Venerupis corrugata, Ruditapes philippinarum, and Ruditapes decussatus. The precise effects of the duration and intensity of high-temperature episodes on these bivalves remain largely unknown, especially when the animals are in sediment which acts as a thermal buffer. Clams in sediment were exposed to recorded summer sediment temperatures: 21 degrees C(control), 27 degrees C, 32 degrees C, and 36 degrees C(measured at 2 cmdepth in the sediment) on the diurnal low tide during three consecutive days. Burrowing activity of R. decussatus relative to temperature was a negative quadratic response, typical of performance curves where the upper limit to activity has been exceeded. The venerid bivalve V. corrugata showed evidence of sublethal stress with reduction in burrowing activity, starting at 27 degrees C. Temperatures greater than 32 degrees C are common on days with hot low tides on clam fisheries grounds in Galicia and are expected to increase in frequency and magnitude as the climate warms. Consistent with the sublethal response, all V. corrugata were dead after 2 days of exposure to 36 degrees C and after 3 days at 32 degrees C. No mortality was recorded at 21 degrees C and 27 degrees C. The venerid bivalves R. decussatus and R. philippinarum suffered no mortality at 21 degrees C, 27 degrees C, and 32 degrees C, but had 25% and 33% mortality, respectively, after 2 days of tidal exposure to 36 degrees C. On the basis of the results, the fishery for V. corrugata in the intertidal would appear to be most at risk due to the predicted increase of temperature, and this species would be expected to be restricted to the subtidal in the near future with negative economic consequences.