Abstract
Several commercially and ecologically important species in the southern Benguela have undergone southward and eastward shifts in their distributions over previous decades, most notably the small pelagic fish sardine Sardinops sagax and anchovy Engraulis encrasicolus. Understanding these changes and their implications is essential in implementing an ecosystem approach to fisheries in the southern Benguela and attempting to appreciate the potential impacts of future environmental change. To investigate possible impacts of these shifts at an ecosystem level, distribution maps for before (1985–1991), during (1997–2000) and after (2003–2008) the shift in small pelagic fish were constructed for 14 key species from catch and survey data, and used to calculate spatial indicators including proportion east and west of Cape Agulhas, relative overlap in biomass and area, index of diversity, connectivity. Potential interactions on the south and west coasts were also compared. For several species (redeye; chub mackerel; kingklip; chokka squid; yellowtail), previously unidentified increases in the proportion of biomass east of Cape Agulhas were shown to have occurred over the same period as that of small pelagic fish, although none to the same degree. On average, overlap with small pelagic fish increased over time and overall system connectivity was lowest in the intermediate period, possibly indicating a system under transition. Connectivity declined over time on the west coast while increasing on the east coast. Distributions of other species have changed over time, with the region east of Cape Agulhas becoming increasingly important in terms of potential trophic interaction. Variations in distribution of biomass and structural complexity affect the trophic structure and hence functioning of the system, and implications should be considered when attempting to identify the possible ecosystem impacts of current and future system-level change.
Highlights
An understanding of the functioning of an ecosystem is not achievable without knowledge of the interactions involved
The data included in analyses, after points outside of the known sampling area had been cleaned from the datasets, varied across the three time periods from several hundred survey intervals for pelagic surveys, to over three million hook sets for longline data (Table 3)
M. paradoxus, chub mackerel and snoek for example increased on the south coast from Period 1 to Period 2, and declined again in Period 3, M. paradoxus and chub mackerel both remained at significantly higher levels on the south coast in Period 3 than they had been during Period 1
Summary
An understanding of the functioning of an ecosystem is not achievable without knowledge of the interactions involved. One way in which this knowledge can be expanded is by observing any changes in the relative distributions of trophically linked species over time. The system is comprised of two physically and functionally different regions (i) the west coast, a classic wind-driven upwelling system, and (ii) the south coast which includes the Agulhas Bank and as a result, has characteristics of both upwelling and shelf systems [7,8]. The small pelagic fish sardine (Sardinops sagax) and anchovy (Engraulis encrasicolus) have a migratory life history: adults spawn on the Agulhas bank and eggs and larvae are transported to the more productive west coast where recruitment and feeding takes place. The system supports a relatively high fish biomass, which in turn supports a number of commercially important fisheries, the two largest in terms of catch and in economic value being the demersal trawl fishery, targeting deep and shallow water hake (Merluccius paradoxus and M. capensis), and the small pelagic purse-seine fishery
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