Post-collapse somatic growth and population recovery failure of sardine (Sardinops sagax) in the northern Benguela from otolith biochronologies

Abstract

The sardine (Sardinops sagax) population in northern Benguela, Namibia, supported high catches in the 1950s-1960s and collapsed by the late 1960s. Despite a fishing moratorium since 2018, the population has shown no signs of recovery by 2023. In this study, a 48-year (1974–2021) biochronology was developed using archived sardine otoliths. Otolith increment widths, used as a proxy for annual fish growth, were analysed using linear mixed effects models to explore intrinsic (within individual) and extrinsic (sardine biomass, sea surface temperature and upwelling) factors contributing to annual variations in fish growth. The absence of otolith data from before the population collapse in the late 1960s meant that the analysis of predicted annual sardine otolith growth post-collapse showed short-term fluctuations but no significant long-term growth rate changes. Predicted annual sardine growth was significantly negatively linked with SST in Austral spring, and positively linked with upwelling in summer for the area 17–20 ºS (northern Namibia). The results suggest environmental conditions play a dominant role in driving sardine growth, exacerbated by the extremely low sardine biomass, which may be indicative of depensation. The study provides insight on the reasons behind the lack of recovery in sardine biomass. Therefore, otolith biochronologies provides a broader understanding of how small pelagic fish stocks respond to environmental changes.