Garlich-Miller, J. L. | Search bioRxiv

bioRxiv2024

Assessing the Sustainability of Pacific Walrus Harvest in a Changing Environment

Harvest sustainability is a primary goal of wildlife management and conservation, and in a changing world it is increasingly important to consider environmental drivers of population dynamics alongside harvest in cohesive management plans. This is particularly pertinent for harvested species that are acutely experiencing effects of climate change. The Pacific walrus (Odobenus rosmarus divergens), a critical traditional subsistence resource for indigenous communities, is simultaneously subject to rapid habitat loss associated with diminishing sea ice and an increasing anthropogenic footprint in the Arctic. We developed a theta-logistic population modeling-management framework to evaluate various harvest scenarios combined with four potential climate/disturbance scenarios (ranging from optimistic-pessimistic) which simulates Pacific walrus population dynamics to the end of the 21st century. We considered two types of harvest strategies: (1) adaptive harvest scenarios wherein harvest is calculated as a percentage of the population and annual harvests are updated at set intervals as the population is reassessed, and (2) non-adaptive harvest scenarios wherein annual harvest remains constant. All climate/disturbance scenarios indicated declines of varying severity in Pacific walrus abundance to the end of the 21st century, even in the absence of harvest. However, we found that an adaptive annual harvest of 1.23% of the independent-aged female subset of the population (e.g., 1,280 independent-aged females harvested in 2020, representing contemporary harvest levels) met our criterion for sustainability (>70% probability of maintaining population abundance above maximum net productivity level) under all climate/disturbance scenarios, accepting a medium risk tolerance level of 25%. This suggests that the present rate of Pacific walrus harvest is sustainable and will continue to be--provided the harvest adapts to match changes in population dynamics. Our simulations suggest that a sustainable non-adaptive harvest is also possible, but only at low levels if the population declines as expected. Applying a constant annual harvest of 1,280 independent-aged females (equivalent to contemporary harvest levels of 1.23) exceeded our criterion for sustainability and resulted in a >5% chance of quasi-extinction by the end of the 21st century under three of the four climate/disturbance scenarios we evaluated. Our results highlight the importance of adaptive co-management strategies, and we suggest such modeling frameworks are useful for managing for harvest sustainability in a changing climate.

ecology↗

bioRxiv2023

Assessing the Population Consequences of Disturbance and Climate Change for the Pacific Walrus

Climate change and anthropogenic disturbance are increasingly affecting wildlife at a global scale. Predicting how varying types and degrees of disturbance may interact to influence population dynamics is a key management challenge. Population Consequences of Disturbance (PCoD) models provide a framework to link effects of anthropogenic disturbance on an individuals behavior and physiology to population-level changes. Bioenergetic models often constitute the basis of these frameworks, wherein an individuals daily energy balance is simulated over the course of its lifetime, allowing many individuals to be subjected to different environmental conditions and ultimately simulate population-level vital rates under varying degrees of disturbance. In the present study, we develop a Pacific walrus (Odobenus rosmarus divergens) PCoD model to encompass the population-level effects of both anthropogenic disturbance and climate change. Pacific walruses are an Arctic/subarctic ice-associated pinniped. As the Arctic has become increasingly ice-free with climate change, walruses spend more time on land-based (rather than ice-based) haulouts from which they must expend more energy to reach foraging areas, and where they have a greater risk of predation and disturbance-based mortalities. Concurrently, sea ice loss is increasing the anthropogenic footprint in Arctic regions (e.g., fisheries, shipping, energy exploration) which creates additional disturbance. We developed a bioenergetic Dynamic Energy Budget (DEB) model for the Pacific walrus and applied it to four scenarios (ranging from optimistic-pessimistic) which incorporate different global sea ice model projections along with varying degrees of anthropogenic disturbance. All scenarios indicated a decline in Pacific walrus carrying capacity and population growth rate (and thus overall abundance) to the end of the 21st century, but demonstrated that the intensity of that decline could be mitigated by global efforts to reduce carbon emissions (i.e., lessening the rate of sea ice loss) and local management and conservation efforts to protect sensitive habitat areas. In summary, we introduced a flexible PCoD modelling framework in a novel context which will prove useful to researchers studying walruses and other species similarly threatened by rapid environmental change.

ecology↗