Bayesian inference reveals positive but subtle effects of experimental fishery closures on marine predator demographics

Abstract

Global forage-fish landings are increasing, with potentially grave consequences for marine ecosystems. Predators of forage fish may be influenced by this harvest, but the nature of these effects is contentious. Experimental fishery manipulations offer the best solution to quantify population-level impacts, but are rare. We used Bayesian inference to examine changes in chick survival, body condition and population growth rate of endangered African penguins Spheniscus demersus in response to 8 years of alternating time–area closures around two pairs of colonies. Our results demonstrate that fishing closures improved chick survival and condition, after controlling for changing prey availability. However, this effect was inconsistent across sites and years, highlighting the difficultly of assessing management interventions in marine ecosystems. Nevertheless, modelled increases in population growth rates exceeded 1% at one colony; i.e. the threshold considered biologically meaningful by fisheries management in South Africa. Fishing closures evidently can improve the population trend of a forage-fish-dependent predator—we therefore recommend they continue in South Africa and support their application elsewhere. However, detecting demographic gains for mobile marine predators from small no-take zones requires experimental time frames and scales that will often exceed those desired by decision makers.

Sex‐biased survival contributes to population decline in a long‐lived seabird, the Magellanic Penguin

“…Juvenile survival for this population was considerably lower than an average across 32 diverse seabird species (e.g., eiders, gulls, albatrosses, penguins), based on data collated by Horswill and Robinson (2015) and Saether and Bakke (2000) (0.47 AE 0.27 SD), though Horswill and Robinson (2015) noted that many of the estimates they included were of poor quality. Juvenile survival rates for this population are, however, comparable to those seen for highly perturbed populations of African Penguins (e.g., <0.25 at the Robben Island colony, South Africa in 2004-2010 ], average 0.19 AE 0.12 across years/colonies [Sherley et al 2018]).…”

“…Their effectiveness remains uncertain, however, due in part to the small area protected and potential lag times of protected area benefits in long‐lived species (Sherley et al. ). Pichegru et al.…”

“…) and/or than is complemented by other conservation measures, such as bycatch reduction or lower catch limits for forage fish (Sherley et al. , , Weller et al. ).…”

“…These closures have focused on protecting foraging habitat near breeding colonies of the species. Their effectiveness remains uncertain, however, due in part to the small area protected and potential lag times of protected area benefits in long-lived species (Sherley et al 2018). Pichegru et al (2012) found that fisheries closures near the St. Croix colony temporarily reduced foraging effort of African Penguins and that juvenile survival increased with an increase in fish biomass.…”

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

“…Understanding the functional relationship between predators and their prey is essential for ecosystem-based management of the oceans (Sainsbury, Punt, & Smith, 2000), particularly where commercial fisheries target the same forage resources. Although the effect of forage fisheries on higher trophic levels is debated, fishing can at least impact central-place foragers via localised prey depletion (Conn, Johnson, Fritz, Fadely, & Rose, 2014;Sherley et al, 2015Sherley et al, , 2018. Truly understanding these impacts, however, depends on knowing how predators respond to natural changes in prey availability (Boyd et al, 2016;Sydeman et al, 2017).…”