Wilhelm, M.Brinkman, Faye R.V.2026-03-232026-03-232025http://hdl.handle.net/11070/4212A thesis submitted in fulfillment of the requirements for the Degree of Doctor of Philosophy in Fisheries and Aquatic ScienceAssessing how fish populations reacted to previous environmental changes is valuable for predicting their future behaviours. However, this process is often constrained by the limited availability of long-term biological data. Two sardine (Sardinops sagax) stocks are found in Namibia (northern Benguela) and South Africa (southern Benguela). Sardine in the northern and 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 South Africa's and Namibian sardine stocks. A fishery collapse happens due to 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. Despite a fishing moratorium since 2018 in Namibia, the population has shown no signs of recovery as of 2024. Cape horse mackerel (Trachurus capensis) is the most abundant commercial fish species in Namibia and has been heavily exploited since the early 1970s. The overall objective of the study was to understand how growth rates of sardine (Sardinops sagax) and Cape horse mackerel (Trachurus capensis) are affected by environmental variability, long-term environmental changes and human exploitation within the Benguela region. Otolith increment widths, indicating alterations in fish growth, were analysed using linear mixed models to explore intrinsic (within individual) and extrinsic (fish biomass, sea surface temperature, upwelling, prey availability) factors contributing to variations in fish growth with the assumption that otolith increment growth is a proxy for somatic growth. In this study, three otolith biochronologies were developed using archived sardine otoliths: a 48-year biochronology (1974- 2021) for sardine of northern Benguela, a 58-year sardine biochronology (1962-2019) for sardine of southern Benguela, and a 50-year Cape horse mackerel biochronology of northern Benguela. The predicted sardine biochronology in northern Benguela displayed short-term fluctuations but no significant change in patterns, due to the absence of otolith data from before the population collapse in the late 1960s. The predicted annual growth of sardine in the northern Benguela significantly correlated negatively with SST in spring, and positively correlated significantly with upwelling in summer. Sequential t-test analysis of regime shifts (STARS) performed on the biochronology of sardine growth in the southern Benguela that indicated four regimes with three alteration points in 1986, 2006, and 2015 that correspond with periods of low, high, average and low biomass, respectively; i.e. 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, iii prey availability and sardine biomass factors with the assumption that otolith increment growth is a proxy for somatic growth. Predicted sardine growth positively correlated with sardine biomass, sea surface temperature and zooplankton abundance in the southern Benguela. This observation suggests that sardine population dynamics exhibit a depensation mechanism, potentially destabilizing populations after the fishery collapse in the southern Benguela. Depensatory population dynamics have caused sea surface temperature and zooplankton abundance primary factors influencing sardine growth in the southern Benguela. 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 for sardine in the northern Benguela. 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 in northern Benguela. Sequential t-test analysis of regime shifts (STARS) performed on the Cape horse mackerel biochronology of fish growth indicated three regimes with two alteration points in 1980 and 1990 for Cape horse mackerel. Otolith biochronologies revealed decadal variability that indicated rapid regime shifts in the southern and northern Benguela marine ecosystem. Therefore, integrating otolith chronologies provides a broader understanding of how fish respond to environmental changes. The utilization of preserved otoliths in this study enabled the analysis to identify prolonged shifts in sardine growth, which are influenced by a range of biological and physical factorsenSardinops sagaxTrachurus capensisBenguela ecosystemRegime shiftNamibiaUniversity of NamibiaOtolith biochronologyThesis