Upper temperature tolerance of ten bivalve species off Peru and Chile related to El Nino *

The upper temperature tolerance of 10 commercially important South American bivalve species (Gari solida, Semele solida, Semele corrugata, Protothaca thaca, Venus antiqua. Tagelus dombeii, Ensis macha, Aulacomya ater, Choromytilus chorus and Argopecten purpura tus) off Peru and Chile was determined and compared in order to study some of the effects of El Nino. Due to higher habitat temperatures in Peru, LTs0 (lethal temperatures for 50% of an experimental population) are higher than in Chile. In Chile LTs0 for 6 of 8 species studied varied only by 1.2"C. This might be explained by the similar temperatures and living conditions in the habitats of these species. Especially for Peru, observed differences in LTSo could be related to different geographical distributions. For all species temperature increases recorded during the strongest El Nino of this century (1982-83) did not exceed the temperature tolerance interval, TT1 (difference between after 24 h and mean annual water temperature). It is noted that all species studied here are tolerant of temperature conditions occurring during moderate El Niiio events.