Factores oceanográficos relacionados con la abundancia relativa de anchoíta (Engraulis anchoita) sobre la plataforma bonaerense (34°S-41°S) durante la primavera

Abstract

Research has been conducted to study the relationship between some physical variables and the relative abundance of the northern stock of the Argentine anchovy (Engraulis anchoita) within the main area of distribution of the stock during springtime. Basic data were recorded during four research surveys mainly aimed at the acoustic assessment of the stock between 34°S and 41° 30'S (Buenos Aires prov., Argentina - Uruguay). The cruises were always carried out in October from 1993 to 1996. Several environmental variables were simultaneously input into a General Linear Model to analyze the variations on fish abundance; however, the model did not account for more than 17-20% of the total variance. Nevertheless most of those factors (sea surface temperature, surface salinity, stability of the water column, hydrographic environments, or the combined ranges of both temperature and salinity at the sea surface) were significant when considered as single factors. Neither the horizontal temperature gradient nor the difference in water temperature between the sea surface and the bottom were significant, whereas both the horizontal salinity gradient and the difference in water salinity between the sea surface and the bottom were significant. The effect of the stability factor on the Bonaerensis anchovy abundance was almost the opposite as previously observed in Patagonian anchovy, as the highest mean fish densities were not recorded within tidal or thermal front zones, but in regions where water was medium or highly stratified. As a rule, the Bonaerensis anchovy preferred salinity front regions at intermediate or high temperatures (>=.12.5 °C). In salinity fronts but at lower temperatures (<12.5°C) the abundance averages sharply decreased, and they were minimal at salinity equal or above 33.5 psu if combined with low or intermediate temperatures. Inter-annual variations showed an inverse relationship between mean fish abundance and the extension of the water masses combining low temperatures with relatively high salinity. This combination usually represented a limit for the anchovy shoals occurrence. In the years when anchovy was particularly abundant, the region showed wide saline front zones, and relatively high temperatures. Moreover, in zones showing low or intermediate surface salinity (<33.5 psu), a stratified water column at temperature above 11 °C-12 °C was clearly favourable for fish shoals, either within or above the thermocline layer. On the other hand, both in the regions or the years showing no thermal stratification, the anchovies were recorded at almost every depth level.