Regime shifts in demersal assemblages of the Benguela Current Large Marine Ecosystem: a comparative assessment

Abstract

Using long‐term survey data, changes in demersal faunal communities in the Benguela Current Large Marine Ecosystem were analysed at community and population levels to provide a comparative overview of the occurrence and timing of regime shifts. For South Africa, the timing of a community‐level shift observed in the early 1990s, and of a lesser shift observed in the mid‐2000s, corresponded well with the results of other studies that showed environmental, community‐level or population‐level changes at similar times, suggesting that environmental forcing had played a role. Several population‐level shifts were detected for Namibia; these and a regime shift in the overall community identified for this country corresponded well to the timing of severe environmental perturbations and an extensive regime shift in the pelagic ecosystem of this area. However, the interpretation of these shifts was confounded by changes in sampling gear; closer scrutiny of the types of species affected and the direction of shifts (increase/decrease) in relation to the timing and nature of sampling gear modifications, revealed that the observed shifts were potentially an artefact of gear changes. This highlighted the importance of accounting for changes in sampling protocols during the analysis and interpretation of long‐term data. For Angola, a community level shift in the mid‐2000s and population‐level changes for a few species (mainly positive), could not have been influenced by gear changes which took place mainly before the onset of the time series under consideration. However, no clear environmental or anthropogenic changes that could have influenced these shifts were obvious.

Synthesis: climate effects on biodiversity, abundance and distribution of marine organisms in the Benguela

“…In spite of changes in community structure and in species abundance and distributions that were documented during the study period (Kirkman et al ., ; Yemane et al ., , ), the presence of S hotspot areas were consistently predictable for much of the study area over the 20‐ to 30‐yr study period, varying by country (Kirkman et al ., ). These included the shelf edge of northern Namibia and the shelf edge and outer shelf areas of southern Namibia and northern South Africa off the Orange River mouth.…”

“…This species expands further south than the more estuarine S. maderensis , but its occurrence in the Nambian pelagic resource surveys has not yet increased. The regime shift observed in the demersal community (Kirkman et al ., ) could have been induced by the warm event in 2001, taking into account time lags, or else by the cold event in 2004 in case of a shorter time lag. Alternatively, fishing pressure could have played a role in inducing the observed community regime shift, because it coincided with escalating catches of hakes ( M. polli and/or M .…”

“…In addition, the generation of particulate organic matter from the pelagic community may decrease the oxygen concentrations and hence contribute to habitat changes at the bottom at seasonal and interannual scales. In the demersal community, a regime shift may have occurred immediately after the 1995 Benguela Niño and the 1994 low‐oxygen event, although the interpretation is confounded by gear changes in 1994–1996 (e.g., Mafwila, ; Axelsen and Johnsen, ; Kirkman et al ., ). The absence of any clear direct links between oceanography and fish (with the exception of sardine) from the results underline the overriding importance considering the combined effects of climate and fishing as drivers of dynamics in the northern Benguela subsystem.…”

“…data) were both characterised by population regime shifts during the study period, as shown by Kirkman et al . (). Yemane et al .…”

“…With respect to the demersal community, in the absence of major changes in demersal fishing effort, the shift observed in the early 1990s (Atkinson et al ., , ; Kirkman et al . ) is attributed to environmental forcing, possibly acting in concert with long‐term effects of fishing, as suggested by Atkinson et al . (, ).…”