Temporal and spatial changes in the abundance and size structure of copepods in the southern Benguela : 1988 to 2000

Abstract

Includes bibliographical references (leaves 24-26).Copepod abundance data were collected during the annual pelagic fish spawner and recruit stock assessment surveys, in summer and winter from 1988 to 2000. These data were used to examine temporal and spatial variability in abundance and community structure of copepods in the southern Benguela, by estimating changes in abundance (No. mol) of individual species and size-classes. The relative abundance of Calanus agulhensis, Calanoides carinatus, Centropages brachiatus, Metridia lucens. Oithona spp., On cae a spp., copepod nauplii and other small copepods show that these taxa were important in driving total copepod abundance changes in the region. A general linear model (GLM) was used to help control for different factors that might affect interannual variability in copepod abundance: season, area, chlorophyll concentrations and sea surface temperature. The southern Benguela was divided into six sub-areas. Results of the GLMs show the largest abundances of copepods on the northern and central West Coast, fewer copepods on the central and eastern Agulhas Bank and fewest copepods on the south West Coast and western Agulhas Bank. Relative abundance time-series of four cope pod size-classes (cyclopoid copepods «900 μm total length (TL )), small calanoid copepods (900-1000 μm TL), medium calanoid copepods (1000-2000 μm TL) and large calanoid copepods (2000-5000 μm TL)) show that there was a possible change in control mechanism of copepods in 1996 from bottom-up to top-down. Moreover, in winter 1999 there was a unique event during which the abundance of various species showed a marked decline, but returned to normal levels in 2000. The time series of relative abundance showed reduced numbers of all four size-classes in 1992 1994. These findings indicate that factors affecting interannual variability of copepods are complex, but that, similar to previous findings, the southern Benguela system has not undergone a major change over the past two to three decades in terms of total copepod abundance in the different sub-areas