Estimating environmental preferences of South African pelagic fish species using catch size- and remote sensing data

Abstract

We have studied the relationship between the variations in density of South African anchovy ( Engraulisi capensis), sardine ( Sardinops sagax) and round herring ( Etrumeus whiteheadi) from commercial catch records (1987–1997) and a suite of variables describing the environment. The indicator of density (local fish abundance) used was Catch-per-set, obtained from the more than 130 000 catches made during the 11-year study period. The set of environmental parameters included: temporal ( Year, Month, time of day or Hour), spatial ( Latitude, Longitude and water Depth), lunar ( Moon phase and Moon elevation), and thermal conditions of the environment (sea surface temperature together with indices of thermal frontal intensity and temporal change). Boat length was used to account for fishing gear effects. Previous investigations of this nature have tended to use simple bivariate correlation approaches, which suffer from the problem of covariance between the predictive variable and other environmental- or fisheries related variables not included in the analysis. We have, therefore, adopted a multivariate modelling approach, which identifies relationships between Catch-per-set and each environmental variable, accounting for covariation amongst predictors. Model building consisted of first constructing generalised additive models (GAM) as an exploratory tool to identify the shapes of the relationships, followed by parameterising these relationships using general linear models (GLM) to provide a robust predictive tool. Using this modelling approach, the suite of environmental variables explained 19.6% of the variation in Catch-per-set of anchovy, 33.9% of sardine, and 54.3% of round herring in the final GLM models. Temporal variables ( Year, Month, Hour), accounted for the major part of the variability in Catch-per-set but variables derived from SST and the lunar cycle provide insight into the effects of environmental factors on fish behaviour. For instance, it appears that schooling behaviour of anchovy and round herring is affected by the level of solar and lunar illumination but that sardine is not affected. Model results further indicate that anchovies prefer water cooler than about 15 °C, demonstrates a weak tendency to concentrate near thermal fronts and avoids recently upwelled water. The sardine’s preferences seem to be more or less the opposite to anchovy, i.e. it occur further from the coast and tends to concentrate in upwelled water which have been warmed to14–19 °C; it shows no discernible tendency for aggregation near fronts. Round herring Catch-per-set strongly increases with water depth and reach a maximum near the shelf edge; it also demonstrates a notable tendency to concentrate near thermal fronts. SST seems to have no influence on round herring which is most often caught in the 15–18 °C range, typical of SSTs found in the vicinity of the oceanic front.