Monthly variability in the catchability of Namibian hake and its relationship with environmental seasonality

The monthly and spatial patterns of the Namibian hake fishery are analysed for the period 1994-1997. A strong, but misleading negative relationship between monthly catch rates (CPUE) and fishing effort was observed, while no spatial correlation between these two variables were found, which suggests, for this fishery, that monthly CPUE as an index of density is not distorted by fleet distribution. A strong seasonal pattern of catchability was found for the first three years but not in 1997. Catch rates could change by a factor of up to three in a period of six months. Moreover, the seasonal pattern in catchability occurs over the whole area, which excludes migratory displacements as a possible factor of this seasonality. We found a strong correlation between CPUE and sea surface temperature (SST). Thus we can conclude that hake catchability followed the seasonality of the system but failed when the seasonal cycle was weakest (i.e., 1997). The interannual variability of either the system or catchability is much larger during the summer than the winter, which has direct implications for resource monitoring and management. (C) 2000 Elsevier Science B.V. All rights reserved.