Influences of temperature and dissolved oxygen on the distributin of Cape hake, Merluccius capensis off southern Namibia

Abstract

Abstract by author:

Influences of temperature and DO (dissolved oxygen) concentration on the distribution of M. capensis are investigated. Catches of M. capensis and environmental data were collected aboard the F/V BLUE SEA I and R/V ALEXANDER von HUMBOLDT respectively, off south Namibia during the first leg of the hake survey, 2004. Catches and environmental data were obtained, using a standard bottom trawl net, and by conducting a CTD at different stations, respectively. Calculated average densities of M. capensis were related to average temperatures and DO concentrations at corresponding stations using linear relationships and bivariate correlations. Results showed that distribution of M. capensis was in shallow depths with some higher densities recorded off Walvis Bay and south of Luderitz. Environmental data revealed that upwelling was taking place in areas south of 25°S and offshore in the vicinity of Luderitz and oxygen depleted/deficient waters were recorded off central areas. Influences of the physical environment was observed showing that the specie avoided warm water and low oxygen water. Specie distribution and DO were positively related, whereas temperature was negatively related to species distribution. The specie co-existed with low DO in the central region; so it is concluded that M. capensis was adapted to low DO. These influences call into question management implications. Firstly, the implication that responses of M. capensis to these parameters should be included in fishery management and secondly, that the environmental forcing could be attributable for variability of catches in trawls during surveys and subsequent effects to survey results are questionable