Quantifying the projected impact of the South African sardine fishery on the Robben Island penguin colony

Quantitative methods are needed to evaluate the ecological effects of fishing forage species upon which predators depend. African penguin Spheniscus demersus numbers at the Robben Island colony rose during the 1990s co-incidental with a marked increase in sardine Sardinops sagax and anchovy Engraulis encrasicolus abundances, but decreased appreciably during the 2000s as sardine suffered a series of poor recruitments. A population dynamics model is developed which relates penguin adult annual mortality to local sardine biomass, and is fit to penguin moult counts and re-sightings of tagged penguins. The predator–prey interaction is best explained by a sardine–penguin mortality relationship with average penguin survival decreasing only when the local sardine biomass is less than approximately one-quarter of the maximum observed. Results suggest that the rapid growth of the colony during the 1990s was driven primarily by immigration. Penguin projections are generated by linking to future sardine abundances predicted under the operational management procedure used to set catch limits for these sardine and anchovy fisheries, and compared with equivalent scenarios without fishing. Results indicate that fishing is likely to have a relatively small impact on penguins, especially when compared with uncertainties that arise from the variable spatial distribution of the sardine population.