BULLETIN OF MARINE SCIENCE, 70(1): 41–54, 2002 41 LARVAL FISH COMMUNITY STRUCTURE ALONG THE WEST COAST OF BAJA CALIFORNIA DURING AND AFTER THE EL NIÑO EVENT (1983) René Funes-Rodríguez, Cesar Flores-Coto, Alfonso Esquivel-Herrera, María Ana Fernández-Alamo and Adolfo Gracia-Gásca ABSTRACT The present study deals with the changes in the larval fish community structure off the west coast of Baja California resulting from the relaxation of the environmental changes induced by the 1983 El Niño event up to August 1985. Larval fish assemblages defined two sampling station clusters, as could be determined from the corresponding dendro- grams. Their distribution pattern was primarily influenced by spawning area selection, but seasonal and interannual changes which appeared to be related with the ocean dy- namics during El Niño events, were also important. Environmental features may affect communities through changes in physiological and behavioral responses of organisms and by directly affecting the distribution and abun- dance patterns of individual species (Pearcy et al., 1996; Moser and Smith, 1993). Coastal circulation, shelf structure, and upwelling determinate biological production, the timing and extent of fish spawning, and horizontal larval distribution patterns (e.g., Smith and Lasker, 1978; Moser et al., 1987; Richardson et al., 1980; Sabatés, 1990; Doyle, 1992). Moser et al. (1987) showed that dynamic environmental changes markedly affect the distribution of larval fish in the California Current region. These geographic shifts, along with changes in the amount and the seasonal extent of spawning, result in changes in the structure and composition of larval fish assemblages. The southern region of the California Current is characterized by a marked yearly cycle of cooling and heating, and is also influenced interannually by El Niño–Southern Oscil- lation (ENSO) warming events. The main source of seasonal variation is related to the coastal circulation pattern, with the equatorward California Current as the dominant flow during the spring and summer, the inshore countercurrent during fall and winter, and the California undercurrent with subsuperficial flow throughout the year (Sverdrup et al., 1942; Hickey, 1979; Lynn and Simpson, 1987). The major interannual variation associ- ated with El Niño occurs as warm water impinges on the South American coast, causing a rise in sea level, and a depression of the thermocline along the American Pacific coast as a coast-confined wave that progresses poleward, as it occured during the 1982-1983 event (MacLain et al., 1985; Norton et al., 1985). El Niño events are irregular in terms of frequency, duration and amplitude; the 1982–1983 El Niño was one of the most intense of the past century (Norton et al., 1985; Cannon et al., 1985). Environmental changes related to El Niño reduce the biological productivity of surface waters (MacLain et al., 1985), and produce important changes in plankton and nekton distributions that are associated with intensification of normal seasonal poleward flow (MacLain and Thomas, 1983; Bailey and Incze, 1985; Smith, 1985; Moser et al., 1987; MacCall and Prager, 1988). An alternative major warm-event hypothesis (changes in the atmospheric circulation) suggests that local anomalies in the California Current are caused by variations in wind-induced onshore circulation, that result in very strong surface water onshore transport in midlatitudes (MacLain et al., 1985).