Taking a deeper dive into fishery effects on predator prey relationship

Jane Williamson
Thursday 20 October 2022

Taking a deeper dive into fishery effects on predator prey relationshipJacqui GlencrossUniversity of St Andrews

Abstract: Seabird-fisheries conflict has been evident since the 1970s. While there has been much research into the interaction between seabirds and fisheries, it is often difficult to disentangle the true impact of fishing from other factors, such as climate change. A globally-unique fishery-closure experiment off South Africa has provided an opportunity to observe African penguin Spheniscus demursus populations and their fish prey (anchovy Engraulis capensis and sardine Sardinops sagax) with and without the pressure of localised fishing.

The experiment, started in 2008, involved two pairs of islands: Robben and Dassen Islands and Bird and St Croix Islands. Fishing within a 20 km radius of one of the islands within the pair is prohibited on a 3-year cycle, while the other continued to be fished. GPS-time-depth loggers were deployed on penguins from Robben and Dassen Islands throughout the fishery closures. Small-scale prey surveys were also conducted within the experimental closure area.

Some aspects of diving behaviour (notably maximum depth) were significantly different between open and closed years. Penguins made deeper dives when fishing was permitted. These results were consistent with data from the small-scale surveys, which showed that fish schools were deeper in the water column when an area was open to fishing. This study shows that pelagic fishing off Cape Town alters the behaviour of prey, and results in changes in penguin foraging behaviour. Repeated deeper dives will likely have physiological and energetic consequences for the penguins, which in turn could impact breeding success and survival. In addition, changes to prey schooling behaviour may influence the foraging success of penguins. Localised fishing pressure appears to be one of multiple stressors contributing to the decline of this endangered seabird.

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