Quantifying environmental effects on the recruitment of the Brazilian sardine (Sardinella brasiliensis), 1977-1993

Abstract

The recruitment process for Brazilian sardine (Sardinella brasiliensis) from 1977 to 1993 was investigated by taking into account the effects of spawning stock biomass (SSB), the intensity of the wind and the sea surface temperature (SST). Nonparametric models were applied. Best fits resulted from relating recruitment to a combination of SSB and Ekman transport. Transformation of Ekman transport is nearly dome-shaped, with lower and upper limits of 520 and 700 kg s^-1 m^-1 respectively. Fitting explained 94% of recruitment variability. Regions of maximum values in the transformations were for predictors related to wind, such as wind stress and turbulence index, whose limits were respectively 0.035-0.045 N m^-2 and 30-45 m³ s^-3. These values correspond approximately to winds of 3.0-4.5 m s^-1. SST transformations also follow an apparent dome-shaped curve optimising around 24.75 and 25.5ºC. Inclusion of SST as a third predictor variable led to the loss of the dome-shaped relationship, as shown by the transformations with two predictor variables. Biomass curves are almost logarithmic, their critical point oscillating around 200,000-250,000 t, below which value the stock becomes more dependent on recruitment. The biomass range is greater than previous estimates.