Intrinsic and extrinsic controls on foraging effort in an iconic Benguela seabird

Abstract

Abstract Seabirds are upper trophic level predators and as such reflect bottom-up changes in the conditions of their prey species through behavioural or demographic changes. This is especially useful in fisheries management where seabirds can act as biomonitors of commercially-important prey species, potentially providing an attractive, albeit coarse, supplementation to expensive and labour-intensive conventional stock surveys. The Cape gannet Morus capensis is an important predator of both sardine Sardinops sagax and anchovy Engraulis encrasicolus within the Benguela upwelling region, and has been shown to be a useful biomonitor of these fish stocks. A recently-developed automated monitoring system that measures foraging trip durations (a reflection of foraging effort and prey availability) in Cape gannets using VHF technology was used in this study to explore a range of intrinsic and extrinsic drivers of foraging effort that potentially need to be accounted for when relating foraging effort to availability of prey resources. Data from three seasons (2011/2012, 2012/2013 and 2013/2014) representing 5470 foraging trips from 50 chick-provisioning pairs were used for this purpose. Using a linear mixed-effects modelling approach, Cape gannet foraging trip behaviour was shown to be influenced by chick age, parent sex, meteorological conditions (barometric pressure, rainfall and wind) and ambient light conditions. These factors ideally need to be accounted for when foraging effort in seabirds is used as a proxy for prey availability.