Local forage fish abundance influences foraging effort and offspring condition in an endangered marine predator

Abstract

Understanding the functional relationship between marine predators and theirprey is vital to inform ecosystem-based management. However, collecting concurrent data on predator behaviour and their prey at relevant scales is challenging.Moreover, opportunities to study these relationships in the absence of industrial fishing are extremely rare.2. We took advantage of an experimental fisheries closure to study how local prey abundance influences foraging success and chick condition of Endangered African penguins Spheniscus demersus in the Benguela ecosystem.3. We tracked 75 chick-provisioning penguins with GPS-time-depth devices, measured body condition of 569 chicks, quantified the diet of 83 breeding penguins and conducted 12 forage fish hydro-acoustic surveys within a 20 km radius of Robben Island, South Africa, over three years (2011)(2012)(2013). Commercial fishing for the penguins' main prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus, was prohibited within this 20 km radius during the study period. 4. Local forage fish abundance explained 60% of the variation in time spent diving for 14 penguins at sea within 2 days of a hydro-acoustic survey. Penguin foraging effort (time spent diving, number of wiggles per trip, number of foraging dives and the maximum distance travelled) increased and offspring body condition decreased as forage fish abundance declined. In addition, quantile regression revealed that variation in foraging effort increased as prey abundance around the colony declined. Policy implications.Our results demonstrate that local forage fish abundance influences seabird foraging and offspring fitness. They also highlight the potential for offspring condition and the mean-variance relationship in foraging behaviour to act as leading indicators of poor prey abundance. By rapidly indicating periods where forage resources are scarce, these metrics could help limit seabird fisheries competition and aid the implementation of dynamic ocean management.

Urofaecal glucocorticoid metabolite concentrations in African penguin (Spheniscus demersus) chick populations experiencing different levels of human disturbance

“…An inability of the Robben Island African penguin adults to meet the energy requirements of their growing chicks might have caused HPA axis hyperactivation. However, we found no significant difference in CCI between chicks of the three different study sites, even though CCI is responsive to food availability ( Campbell et al, 2019 ). As such, food intake might not be a primary driver of GC production in this study, but this requires further research to confirm.…”

“…Aside from direct human activity, several other factors might be responsible for the difference in baseline ufGCM levels of chicks from the two islands. The Robben Island penguin colony has been shown to be impacted by food availability ( Campbell et al, 2019 ), with adult and juvenile survival declining along with prey availability ( Sherley et al, 2014b , 2017 ). A decrease in resource availability and intake can lead to altered behaviour, as well as elevated energy expenditure and physiological stress ( Bauer et al, 2011 , Cote et al, 2010 , Fokidis et al, , 2012 ).…”

Important marine areas for the conservation of northern rockhopper penguins within the Tristan da Cunha Exclusive Economic Zone

“…The greatest threats to penguins globally that can be managed by marine spatial planning are overfishing, bycatch in fisheries, and marine pollution (Trathan et al 2014, Ropert-Coudert et al 2019, Boersma et al 2020. Fisheries for sardines and anchovies off South Africa are believed to compete with African penguins Spheniscus demersus for food (Campbell et al 2019) but the small zooplankton and fish species that comprise the diet of northern rockhopper penguins in Tristan (Booth & McQuaid 2013) are not yet of commercial value to fisheries. Penguins' diving behaviour, however, renders them vulnerable to entanglement in fishing gear and bycatch in gillnets (Crawford et al 2017).…”

A Cursory Look at the Fishmeal/Oil Industry From an Ecosystem Perspective

“…With respect to sustaining ecosystem functioning, there is high variability in both direction and magnitude of change in biomass of predatory fish in response to changes in catch of forage fish, dependent on ecosystem, local predator-prey relationships, and the spatial and temporal scales examined (e.g., Smith et al, 2011;Koehn et al, 2017). Seabirds and marine mammals often show biomass (and also foraging and breeding) responses closely reflecting forage fish availability (Koehn et al, 2017;Campbell et al, 2019;Saraux et al, 2020). Further, it is no simple matter to compare economic trade-offs between forage fish and their predators (Koehn et al, 2017;Konar et al, 2019), and a full socio-economic analysis is warranted at the local scale.…”