EFFECTS OF EL-NINO-SOUTHERN-OSCILLATION ON THE POPULATION-DYNAMICS OFA GARI-SOLIDA POPULATION (BIVALVIA, PSAMMOBIIDAE) FROM BAHIA-INDEPENDENCIA, PERU

Population dynamics of a Gari solida (Gray, 1828) population from Bahi a Independencia, Peru (14 degrees S) was studied between November 1989 and February 1995 (except for 1991) in order to investigate the effec ts of the climate phenomenon El Nino/Southern Oscillation (ENSO) which occurred in 1992/93. Results indicate that ENSO had negative effects on gonad production in 1992, when temperatures were very high, and pos itive effects in 1993, when temperatures were lower, but the warm peri od lasted longer than in 1992. Higher reproductive output in 1993 was achieved by several spawning events in addition to the normal pattern. Growth rate in 1990 was higher (K = 0.495) than in 1993/94 (K = 0.346 ). A possible explanation could be that more energy was allocated to r eproduction, and thus less surplus energy remained for growth. The neg ative effect of ENSO on production in 1992 is indicated by the low P/B ratio (0.298); in all other years P/B was consistently higher (1990: 0.570; 1993: 0.597; 1994: 0.597). From 1990 to 1994 a reduction of mea n annual biomass from 189.3 to 14.4 g ash-free dry weight m(-2) was al so observed which may be related to fishery activities. The consistenc y of P/B ratio is best explained by the higher somatic production of t he smaller individuals remaining after the larger individuals had been removed selectively by fishermen. Based on the results presented in t his study and from the literature, a model is proposed depicting how E NSO may negatively affect bivalve populations. Two main factors are me ntioned, increased temperature and reduction of assimilated energy.