This PhD will examine African penguin – fish school interactions using a combination of acoustic survey and biologging off Robben and Dassen Islands, South Africa. It will provide training in various aspects of field data collection, and progress towards ecosystem-based fishery management and conservation of an IUCN Red-Listed species.
Many pelagic fish species are obligate schoolers. Schooling on the one hand lends protection to individual school members from predators that target individuals, but on the other makes the fish collectively attractive and viable targets to predators, including fishing vessels, that need to obtain more than just single individuals (to satisfy energetic and economic [catch per unit effort] demands). Our previous work has shown that, across a number of fish and pelagic crustaceans, school sizes remain more or less constant as overall stock abundance fluctuates. The alternative behavioural possibility, where school numbers remain constant but their sizes change, is not supported by available data. This group behaviour has important implications for fishers and predators targeting pelagic prey. As stock size diminishes, fewer schools will remain and the probability of predators locating prey will diminish.
Ecosystem-based fishery management approaches need to consider the consequences to dependant predators of declining fish availability. But, crucially, changes in fish stock biomass may be caused by natural variability, by fishing, or by both. Moreover, the act of fishing itself could modify the school sizes-stock abundance relationship in non-linear ways. Fishing removes biomass from the environment, which could in turn reduce the probability of predators finding prey schools. But, fishing can also attract predators, induce sub-lethal effects on fish that escape the nets, or fragment schools, making them easier for predators to catch. Which mechanism should dominate, however, is unclear because there have been no concerted at-sea efforts to study 100+km-scale impacts of fishing on schools, or the impacts on predators.
South Africa has a long-running fishery experiment to look at the impacts of fishing on African penguins, which rely upon sardines and anchovy for food. In alternate years fishing is closed around Robben and Dassen Islands, off Cape Town, and foraging effort and reproductive output of African penguins breeding at each island is monitored. Previous work on this system indicates that penguin chick survival and condition improve in the years when fishing is banned around each colony, and that penguin foraging effort responds in a non-linear fashion to the local abundance of pelagic fish (measured with conventional echosounders). However, the foraging effort data have not shown a clear response to the fishery closures experiment. In short, we still lack the mechanistic understanding of how fishing affects fish shoaling behaviour and how that in turn affects the probability with which penguins can find prey. This missing link fundamentally limits the broader applicability of the results coming from this long-running fishery experiment. This PhD will make that crucial link.
We will make multibeam echosounder observations of schools of fish off Robben and Dassen islands and simultaneously equip breeding penguins with biologging technology from 2020. We have conducted preliminary observations already, and know that the approach can work. We will measure sizes and numbers of schools in fished and unfished waters, and examine penguin foraging effort (using GPS devices and accelerometers) and prey capture rates (using animal-borne video cameras) against school metrics and overall fish stock abundance in both the presence and absence of fishing.
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