Targeted prey fields determine foraging effort thresholds of a marine diver: Important cues for the sustainable management of fisheries

Abstract

Understanding functional relationships between seabirds and prey can play an important role in the sustainable management of fisheries that compete for the same resources yet data linking the foraging performance of seabirds directly with concurrent information on prey supplies remain limited. We assess the influence of prey availability on the foraging performance of breeding African penguins Spheniscus demersus using data from instrumented birds during 14 pelagic fish surveys carried out over 3 years around their two largest breeding colonies, St Croix and Bird islands in Algoa Bay. Characteristics of prey fields targeted by African penguins were identified by assessing the response of foraging birds to the acoustically determined distribution of small pelagic fish. Path‐ and dive‐derived foraging performance metrics were modelled as a response to overall and targeted prey abundance estimates to assess potential nonlinear functional relationships between penguin foraging effort and prey availability. African penguins displayed marked spatial heterogeneity in their selection of foraging sites, but birds from both colonies invariably foraged on fish schools <40 m deep. Generalized additive model fits improved for all foraging performance responses when using targeted prey abundance (i.e. only fish in areas frequented by penguins and at depths <40 m, as opposed to estimates using all prey). Models for trip duration (r2 = 0.45) and foraging path length (r2 = 0.44) showed a negative nonlinear form in their response to targeted prey abundance, with significant improvement in penguin foraging effort when prey abundance was greater than 45% of the maximum prey biomass recorded. Synthesis and applications. Functional relationships between seabird foraging performance and prey supplies have the potential to inform the sustainable management of fisheries that compete for the same prey. Our findings highlight the importance of quantifying prey that is available to seabirds when assessing critical thresholds in prey abundance below which seabird foraging performance is adversely affected. These findings can be integrated into dynamic ocean management strategies to mitigate resource competition around seabird colonies. A framework for flexible, colony‐centred closures is proposed as a more dynamic alternative to time/area closures with the potential to respond to prevailing habitat conditions in near real‐time.

Passive acoustic monitoring of the endangered African Penguin (Spheniscus demersus) using autonomous recording units and ecoacoustic indices

“…This is in line with the current development of passive monitoring for this species (e.g. McInnes 2016, McInnes et al 2019. In particular, acoustic surveys could complement the currently trialled automated system comprising PIT (Passive Integrated Transponders) readers and weighbridges to assess foraging trip length and foraging success, especially in remote areas which cannot be visited regularly.…”

Changes in prey fields increase the potential for spatial overlap between gentoo penguins and a krill fishery within a marine protected area

“…These lines of evidence show that krill availability in 2001 was sufficient for gentoo penguins to meet their nutritional requirements during short‐range day trips, but low abundance and patchy distribution of krill in 2018 forced them to perform multi‐day trips that extended out to the shelf edge. Similarly, female macaroni penguins at South Georgia extended their foraging ranges and durations in a year of poor krill availability (Horswill et al., 2017) and African penguins Spheniscus demersus in South Africa performed longer duration trips as prey abundance within 40 m of the surface declined (McInnes et al., 2019).…”

“…One variant is the flexible closure management framework proposed by McInnes et al. (2019) for African penguin and anchovy–sardine ( Engraulis capensis–Sardinops sagax ) fisheries in South Africa. Its implementation at South Georgia would require real‐time collection of gentoo penguin satellite tracking data and rapid, automated krill density estimates from acoustic arrays installed on fishing or patrol vessels (Azira et al, 2019).…”

“…Its implementation at South Georgia would require real‐time collection of gentoo penguin satellite tracking data and rapid, automated krill density estimates from acoustic arrays installed on fishing or patrol vessels (Azira et al, 2019). Repetition of this across years would allow quantification of the relationship between penguin movements and krill prey fields, and the recognition of reference points at which NTZ2 would be extended (Hill et al., 2020; McInnes et al., 2019). Once the krill densities that lead to gentoo penguins leaving the NTZ are sufficiently well quantified, management can proceed using krill density alone, saving the expense of tracking penguins in perpetuity .…”

“…Dynamic Ocean Management may offer an alternative approach: closed area boundaries are adjusted through time in response to near real-time information on environmental and biological variability, which achieves equivalent protection to static boundaries within smaller areas (Dunn et al, 2016;Hazen et al, 2018;Lewison et al, 2015). One variant is the flexible closure management framework proposed by McInnes et al (2019) for African penguin and anchovy-sardine (Engraulis capensis-Sardinops sagax) fisheries in South Africa. Its implementation at South Georgia would require real-time collection of gentoo penguin satellite tracking data and rapid, automated krill density estimates from acoustic arrays installed on fishing or patrol vessels (Azira et al, 2019).…”