UPPER TEMPERATURE TOLERANCE OF 10 BIVALVE SPECIES OFF PERU AND CHILE RELATED TO EL-NINO

The upper temperature tolerance of 10 commercially important South Ame rican bivalve species (Gari solida, Semele solida, Semele corrugata, P rotothaca thaca, Venus antiqua, Tagelus dombeii, Ensis macha, Aulacomy a ater, Choromytilus chorus and Argopecten purpuratus) off Peru and Ch ile was determined and compared in order to study some of the effects of El Nino. Due to higher habitat temperatures in Peru, LT50 (lethal t emperatures for 50 % of an experimental population) are higher than in Chile. In Chile LT50 for 6 of 8 species studied varied only by 1.2-de grees-C. This might be explained by the similar temperatures and livin g conditions in the habitats of these species. Especially for Peru, ob served differences in LT50 could be related to different geographical distributions. For all species temperature increases recorded during t he strongest El Nino of this century (1982-83) did not exceed the temp erature tolerance interval, = (difference between LT50 after 24 h and mean annual water temperature). It is noted that all species studied h ere are tolerant of temperature conditions occurring during moderate E l Nino events.