Defining ecologically relevant scales for spatial protection with long‐term data on an endangered seabird and local prey availability

Abstract

Human activities are important drivers of marine ecosystem functioning. However, separating the synergistic effects of fishing and environmental variability on the prey base of nontarget predators is difficult, often because prey availability estimates on appropriate scales are lacking. Understanding how prey abundance at different spatial scales links to population change can help integrate the needs of nontarget predators into fisheries management by defining ecologically relevant areas for spatial protection. We investigated the local population response (number of breeders) of the Bank Cormorant (Phalacrocorax neglectus), a range-restricted endangered seabird, to the availability of its prey, the heavily fished west coast rock lobster (Jasus lalandii). Using Bayesian state-space modeled cormorant counts at 3 colonies, 22 years of fisheries-independent data on local lobster abundance, and generalized additive modeling, we determined the spatial scale pertinent to these relationships in areas with different lobster availability. Cormorant numbers responded positively to lobster availability in the regions with intermediate and high abundance but not where regime shifts and fishing pressure had depleted lobster stocks. The relationships were strongest when lobsters 20-30 km offshore of the colony were considered, a distance greater than the Bank Cormorant's foraging range when breeding, and may have been influenced by prey availability for nonbreeding birds, prey switching, or prey ecology. Our results highlight the importance of considering the scale of ecological relationships in marine spatial planning and suggest that designing spatial protection around focal species can benefit marine predators across their full life cycle. We propose the precautionary implementation of small-scale marine protected areas, followed by robust assessment and adaptive-management, to confirm population-level benefits for the cormorants, their prey, and the wider ecosystem, without negative impacts on local fisheries.

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

“…However, if prey availability is not accounted for adequately, a few extreme years or temporal trends could easily confound environmental variability and fishing impacts when experimental periods are short [ 8 , 9 ], even with a BACI design. Future experimental closures, both in South Africa and elsewhere, would benefit from fisheries-independent assessments of prey availability on a scale relevant to the focal predator [ 38 ]. The above notwithstanding, the magnitude of improvement in chick condition at Robben Island, and the consistently higher chick survival during closed years at both Western Cape islands provides strong evidence for a fisheries effect over and above that of a common environmental driver.…”

“…Although continuing the closures will affect the South African purse-seine industry, estimates vary widely from less than 1% to approximately 9% of total annual catches for closures at both Western Cape colonies [ 39 ]. Any costs also need to be weighed against the high socio-economic value of penguin-based ecotourism [ 40 ] (our study colonies hold approximately 60% of South Africa's breeding penguins) and the likelihood that spatial protection around these islands would benefit wider marine biodiversity, including other threatened marine predators [ 38 ]. Conservation actions are sometimes deferred because of doubt or fear of failure, but delay can increase the risk of extirpation or extinction [ 41 , 42 ].…”

Predicting the distribution of foraging seabirds during a period of heightened environmental variability

“…In addition, the use of SDMs with integrated data sets can improve our general understanding of the system, and broaden the ecological inference that can be gained from these modeling exercises (Croll et al 1998, Hunt et al 1999, Field and Francis 2006, Becker et al 2007, Ainley et al 2009, Santora et al 2009, Sydeman et al 2015, Watters et al 2020. Empirical relationships form the basis of management actions undertaken to protect seabirds and their foraging habitats, such as marine spatial planning (MSP) and marine protected area designations (MPAs; Montevecchi et al 2012, Sherley et al 2017. The ability to accurately predict where these regions will occur is an important first step, and depends on a detailed knowledge of the environmental conditions leading to lower and higher biomass of prey, the foraging behavior of the seabird, and the ability to connect these two variables at the scale at which correlations occur (e.g., Piatt 1990, Hunt et al 1992, 1999, Mehlum et al 1996, Logerwell et al 1998.…”

Factors affecting the seasonal distribution and biomass of E. pacifica and T. spinifera along the Pacific coast of Canada: A spatiotemporal modelling approach

“…This work is the first step towards quantifying the physical and climate variables that lead to the development and persistence of euphausiid hotspots, with the ultimate goal of delineating important foraging regions for euphausiid vertebrate predators along the BC coast. This type of information is critical, but often lacking, for the development of conservation management strategies such as marine spatial planning [62][63][64]. However, as with any modelling exercise, some caution should be exercised when interpreting the results.…”