The eastern tropical Pacific (ETP) reefs are affected at irregular times by extremely cold temperatures that occur principally during La Nina events. The effects of these low temperatures on the survival of reef fishes were experimentally assessed by determining the critical thermal minimum (CTM) of 15 reef fish species from Gorgona Island (ETP), and comparing these CTMs with the records of temperature during past La Nina events. Among species, mean CTMs ranged from 10.8degreesC to 16.3degreesC, which were lower than the coldest temperature recorded during the last La Nina event (18degreesC during La Nina 1998-1999). However, the observed ranges of CTM for two species (Thalassoma lucassanum and Eucinostomus gracilis) extended above 18degreesC. These results suggest that most of the reef fishes we studied are physiologically tolerant to the cold temperatures encountered during La Nina, though decreases in at least two populations may be expected as a result of the mortality of less tolerant individuals. Although tolerant to cold temperatures, reef fish populations may still experience negative changes during La Nina, because other determinants in population maintenance (e.g. Reproduction and recruitment) are more temperature sensitive. The effects of other cold phenomena on reef fish survival are also discussed herein.
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