Influence of Environmental Factors and Fishing Effort on Demersal Fish Species in Ghana

Abstract

Separating effects of fishing from responses of environmental factors is a key problem for fisheries scientists. I used data from fishery-independent trawl surveys (6 years data from 1999 – 2016) to test influences of fishing effort and environmental factors (temperature, oxygen salinity) on the abundance and spatial distribution of two species groups: 5 economically important species and 3 non-commercial species on the continental shelf of Ghana. Fishing effort, measured for the entire study area, affected year-to-year changes in the abundance of all but one species (5 species negatively, 2 positively, and 1 species unaffected). All species also showed significant spatio-temporal associations with temperature, salinity and oxygen levels. There was some year-to-year consistency in spatial distributions because each of these environmental variables was correlated with depth. Nonetheless, some inter-annual changes in species distribution appeared to reflect tracking of year-to-year shifts in climatic variables, e.g. inshore-offshore shifts in goatfish, red pandora and red cornetfish were associated with shifts in temperature and oxygen levels. The causes of other inter-annual changes in spatial distribution were not readily linked to climatic variables, and I argue that documenting spatial patterns of fishing effort in future might help explain these shifts. Overall, my results show that virtually all demersal species, targeted or not, appear impacted by fishing but also track spatial-temporal changes in environmental conditions from year-to-year. Improved management should thus incorporate spatially resolved measures of fishing effort alongside measures of climatic variables.