Spatial overlap alone downplays the level of interaction between a central place forager and the local fishery

1. Quantifying interactions between fisheries and marine predators is fundamental to effective ecosystem-based fisheries management. However, methods to study fine scale overlap between pelagic fisheries and predator foraging behaviour were limited by spatial resolution until the mid-2010s when Automatic Identification Systems (AIS) started to become commonplace, enabling fishing activity to be monitored at finer resolutions.
2. We assess fine scale overlap between the foraging grounds of Critically Endangered African penguins Spheniscus demersus rearing chicks at two colonies in South Africa and the adjacent purse-seine fishery. Competition with the fishery is thought to be an important contributor to the penguin population decline (~78% over in the last 30 years). During a fishery-closure experiment, we established the spatial overlap between fishing events and GPS-tracked penguins at a population level. We also introduce a novel metric, overlap intensity, to quantify the number of individuals impacted by fishery overlap.
3. Both spatial overlap and overlap intensity varied between years, with 2016—the year of lowest recorded anchovy Engraulis capensis recruit biomass—having the highest degree of overlap (population level spatial overlap: 4%—equivalent to 4% of cells used by penguins overlapping with a vessel—and overlap intensity: 74%—equivalent to ~20% of penguin locations occurring in the same cells as fishing events). While spatial overlap between penguins and the fishery over the 4 years was generally low (<5% at a population level), the intensity of this overlap was consistently quite high (~50%–70% in 3 of the 4 years).
4. Synthesis and applications. While spatial overlap is commonly used to quantify interactions, it may underestimate the true degree of competition between foraging predators and fishing vessels. Our alternative population-level measure of ‘overlap intensity’ accounts for the number of individuals involved in the areas of overlap, and may provide a more comprehensive understanding of fisheries impacts. By incorporating measures of overlap intensity into fishery management strategies, more effective and better targeted protected areas could be designated, minimising both losses to fisheries and impacts on predator populations