Changes in demersal fish assemblages on the west coast of South Africa, 1986–2009

Abstract

Research survey data collected over 24 years (1986–2009) were used to explore long-term changes in demersal fish assemblages on the west coast of South Africa. Differences in spatial (latitude and depth) and temporal (seasonal and annual) factors were examined using multivariate analyses. Fish assemblages are clearly influenced by depth, with a distinct change in the region of the shelf break between 300 m and 400 m. There are also geographic differences in fish assemblage from north to south. Multivariate analyses show two clear temporal changes in assemblages, first in the early 1990s and second in the mid-2000s, although the latter change may be confounded by a concurrent change in survey trawl gear. The abundance of three fast-growing, early-maturing species increased over the study period whereas that of two slow-growing, long-lived species decreased, supporting the hypothesis of an increase in fast-growing, early-maturing species and a decline in slow-growing, long-lived species in fished systems. Shifts in demersal fish assemblages coincide temporally with spatial shifts observed in West Coast rock lobster Jasus lalandii and with regime shifts in the pelagic ecosystem. The changes in demersal fish assemblages detected are probably a reflection of long-term indirect effects of fishing in combination with environmental changes.

Keywords: continental shelf, continental slope, effects of fishing, multivariate analysis, temporal change, regime shift

African Journal of Marine Science 2011, 33(1): 157–170