Pelagic fish assemblages at the Espiritu Santo seamount in the Gulf of California during El Nino 1997-1998 and non-El Nino conditions

Given the importance of the El Bajo Espiritu Santo (EBES) seamount to the ecology of the pelagic communities, biological effects of the pelagic fish assemblages during 1997-1998 El Nino, were described by means of visual census, plankton sampling and monitoring tagged adult fish with electronic devices. The results, were compared with previous data of 1995-96 as well as data of June 1999. Diversity index (H') for adult fish (Dec 1995-Dec 1998), showed higher values during 97-98 El Nino event than those of non-El Nino years. Species of more tropical affinity were more abundant during El Nino conditions. A larger number of adult species arrive the area of the seamount during El Nino. Visual census of adult fish suggest that El Nino conditions favor migration of tropical adults species to the EBES. Diversity index of adult fish was higher than during non-El Nino conditions. Temperate species were found in limited numbers at EBES during the warm event. They probably migrate to waters of lower temperatures. Fish larvae diversity decreased during the advance of the warm waters of El Nino event. More than 50% of fish larvae belong to mesopelagic species with tropical affinity. Abundance patterns of tagged yellow fin tuna indicate two behaviour types. A residence pattern in which individual are present at the seamount on a daily basis, and homing pattern where individuals return to the seamount after longer periods of absence. Tunas seem not to be drastically affected by El Nino conditions.