Anchovy larval distribution in the coastal zone off northern Chile: the effect of low dissolved oxygen concentrations and of a cold-warm sequence (1990-95)
This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringens) larvae off northern Chile (18°-24°S), within the coastal zone (out to 100 nm) and their relationship to the oceanographic conditions in the top layer (0-200 m depth). The data were derived from surveys undertaken from 1990 to 1995, during each winter (July-September), the season of the main spawning and higher larval abundance. During the 1995 survey, ichthyoplankton samples were also taken at shallower depths (<100 m), to assess the influence of low dissolved oxygen concentration upon the vertical distribution of the anchovy larvae. Both surface temperature and surface salinity distributions revealed that the period of this study covered a sequence from a relatively cold condition (1990) to warmer weak El Niño condition (1992), and back to cooler conditions (1995) in the area. A deepening of the base of the thermocline (15°C isotherm) occurred during the warmer period along the whole of the coast. During cooler conditions, the upper boundary of the low dissolved oxygen layer (ca. 1.0 ml 1-1) was found at shallower depths (<50 m) in most of the northern part of the area (Arica-Iquique) and also the waters above the thermocline were significantly more oxygenated, though variations with depth were greater, than during the warmer periods. Mean intergrated abundances of anchovy larvae varied significantly between the winter periods of 1990 to 1995 (range: 230-5300 larvae in 10 m²), with the highest mean values observed during 1993 and 1994. The highest abundances occurred at the most nearshore stations (1-5 nm) during all the cruises, though high values were also found at some offshore stations, mainly during the 1993-95 period. The comparison of the mean density of larvae (number m-3) over different depth ranges (0-25, 0-50, and/or 0-100 m), suggested the ocurrence of a barrier to their vertical distribution, located in the vicinity of the 0,75 ml O21-1 concentration. Overall, the warmer conditions initiated during 1992 appeared to have provided a more suitable environment for the survival and feeding of anchovy larvae in the area, whereas changes in the distribution of low oxygen concentrations did not appear to have a direct influence on larval abundance and distribution.