Unveiling Ecosystem Shifts in the Southern Benguela Through Otolith Biochronologies of Sardine (Sardinops sagax)

Abstract

Sardine (Sardinops sagax) in the southern Benguela has shown substantial changes in population size over the past 70 years. Heavy fishing pressure in the 1950s to early 1970s caused the collapse of sardine stocks in South Africa. A fishery collapse happens because of significant alterations in the marine community, hindering the recovery of valuable commercial species and leading to cascading effects across multiple trophic levels in marine food webs. In this study, a robust 58-year biochronology (1962–2019) was developed using archived sardine otoliths from the West of Cape Agulhas in South Africa. Sequential t-test analysis of regime shifts (STARS) performed on the biochronology of fish growth indicated four regimes with three alteration points in 1986, 2006 and 2015 that correspond with periods of low, high, average and low biomass, respectively; that is, high growth rates occurred during the high biomass period and vice versa. A series of mixed effects models was developed to determine increment width response to selected environmental, prey availability and sardine biomass factors based on the assumption that otolith increment growth is a proxy for somatic growth. Predicted sardine growth positively correlated with sardine biomass, sea surface temperature and copepod abundance estimates. This observation suggests that sardine population dynamics exhibit a depensation mechanism, potentially destabilizing populations after the fishery collapse. Sea surface temperature and copepod abundance have been primary factors influencing sardine growth, partly because of depensatory population dynamics. Furthermore, the study improves understanding of how different factors have affected sardine growth following the collapse of the sardine fishery.