Changes in prey availability impact the foraging behaviour and fitness of Cape gannets over a decade

Abstract

Seabirds respond to environmental changes by adjusting their breeding and forag- ing strategies, but this behavioural flexibility has limits. Cape gannets Morus capensis breeding in the southern Benguela on Malgas Island off South Africa's west coast have experienced large fluc- tuations in natural prey availability over the past decade, linked to environmental change and localised overfishing. When small pelagic fish are unavailable, breeding gannets increase their consumption of low-quality fishery discards (primarily hake Merluccius spp.). To investigate the limits of foraging flexibility of breeding gannets facing variable prey availability, we monitored foraging behaviour, nest attendance, adult body condition and chick growth between 2002 and 2012, along with diet composition and prey abundance (through annual hydroacoustic assess- ments) during the birds' breeding season. The combined biomass of sardine Sardinops sagax and anchovy Engraulis encrasicolus within the Malgas gannet colony's foraging range varied tenfold across the study period and was positively correlated with the proportion of these high quality fish in the gannets' diet (17 to 90%). Foraging effort increased and nest attendance decreased with decreasing sardine/anchovy consumption. Adult body condition was negatively impacted by increases in hake in the diet. Chick growth was lowest when low sardine and anchovy composi- tion was coupled with an increase in adult foraging effort, suggesting a limit to behavioural com- pensation for food shortages. This long-term study demonstrates the consequences of variable prey levels for Cape gannet behaviour and fitness. These results highlight the need for detailed investigations of seabird-fishery interactions, and the necessity to limit fishing within Cape gan- net foraging ranges during years of low natural prey abundance.