Factors influencing population trends of some abundant vertebrates in sardine-rich coastal ecosystems

Abstract

Many abundant populations of vertebrates in sardine-rich coastal ecosystems have experienced large changes in population sizes. This has led to wide fluctuations of some fish harvests and concern over the conservation status of some top predators. Research over the past 20 years into the causes of population changes of some abundant fish species, some southern pinnipeds and the most important guano-producing seabirds is briefly reviewed. Particular emphasis is given to populations off south-western Africa. Fish populations have been influenced in a complex manner by a variety of factors, including fishing, biological interactions and environmental change. Climate, operating at a basin scale, is probably an important cause of large and long-term changes in the abundance of some sardine populations. The impact of anticipated climate change on sardine-rich ecosystems is expected to receive emphasis in the future. The populations of some pinnipeds and seabirds were drastically reduced by former excessive harvests. Whereas pinnipeds have tended to recover under regimes of controlled exploitation, the numbers of some seabirds have continued to decrease even in the absence of exploitation. Various factors have contributed to the ongoing decreases, including competition with fisheries for prey resources and competition with pinnipeds for breeding space. The 1980s saw the introduction of legislation specifying conservation of marine ecosystems as a management objective in Antarctica and South Africa. Means of achieving this objective, given the biological interactions at the upper end of the trophic pathway, are expected to provide another focus for future research. © 1991 Taylor & Francis Group, LLC.