Predicting the population consequences of acoustic disturbance, with application to an endangered gray whale population

Abstract

Acoustic disturbance is a growing conservation concern for wildlife populations because it can elicit physiological and behavioral responses that can have cascading impacts on population dynamics. State‐dependent behavioral and life history models implemented via Stochastic Dynamic Programming (SDP) provide a natural framework for quantifying biologically meaningful population changes resulting from disturbance by linking environment, physiology, and metrics of fitness. We developed an SDP model using the endangered western gray whale (Eschrichtius robustus) as a case study because they experience acoustic disturbance on their summer foraging grounds. We modeled the behavior and physiological dynamics of pregnant females as they arrived on the feeding grounds and predicted the probability of female and offspring survival, with and without acoustic disturbance and in the presence/absence of high prey availability. Upon arrival in mid‐May, pregnant females initially exhibited relatively random behavior before they transitioned to intensive feeding that resulted in continual fat mass gain until departure. This shift in behavior co‐occurred with a change in spatial distribution; early in the season, whales were more equally distributed among foraging areas with moderate to high energy availability, whereas by mid‐July whales transitioned to predominate use of the location that had the highest energy availability. Exclusion from energy‐rich offshore areas led to reproductive failure and in extreme cases, mortality of adult females that had lasting impacts on population dynamics. Simulated disturbances in nearshore foraging areas had little to no impact on female survival or reproductive success at the population level. At the individual level, the impact of disturbance was unequally distributed across females of different lengths, both with respect to the number of times an individual was disturbed and the impact of disturbance on vital rates. Our results highlight the susceptibility of large capital breeders to reductions in prey availability, and indicate that who, where, and when individuals are disturbed are likely to be important considerations when assessing the impacts of acoustic activities. This model provides a framework to inform planned acoustic disturbances and assess the effectiveness of mitigation strategies for large capital breeders.

A review of bioenergetic modelling for marine mammal populations

“…Human activities might also modify an individual’s energy budget by interfering with its behaviour, for example if they cause expensive avoidance responses or disturbance during feeding, reducing energy acquisition (e.g. McHuron et al, 2021 ). These external drivers will interact with internal motivations (e.g.…”

“…The most widespread use of dynamic bioenergetic models in the past decade has been in the context (either implicit or explicit) of assessing the long-term consequences of sublethal anthropogenic disturbance on individuals or populations under the PCoD framework, which has included applications to pinnipeds ( Goedegebuure et al, 2018 ; McHuron et al, 2017a , 2018 ), harbour porpoises Phocoena phocoena ( Gallagher et al, 2021a ; Harwood et al, 2020 ; Nabe-Nielsen et al, 2014 ; Nabe-Nielsen et al, 2018 ), delphinids ( Hin et al, 2019 ; New et al, 2013b ; Pirotta et al, 2014 ; Pirotta et al, 2015 , 2020 ; Reed et al, 2020 ; Williams et al, 2006 ), beaked whales ( New et al, 2013a ), baleen whales ( Braithwaite et al, 2015 ; Christiansen and Lusseau, 2015 ; Dunlop et al, 2021 ; Guilpin et al, 2020 ; McHuron et al, 2021 ; Pirotta et al, 2018a , 2019 , 2021 ; Riekkola et al, 2020 ; van der Hoop et al, 2017 ; Villegas-Amtmann et al, 2015 ; Villegas-Amtmann et al, 2017 ), small- to medium-sized odontocetes ( Noren et al, 2017 ) and sperm whales ( Farmer et al, 2018b , 2018a ).…”

“…Moreover, empirical data suggest that tissue deposition (i.e. foetus growth) can be regulated based on female condition ( Christiansen et al, 2014 ), but to date only one study has modelled foetal size dynamically ( McHuron et al, 2021 ). Some studies have also explicitly considered the additional drag costs imposed by pregnancy during locomotion ( Pirotta et al, 2019 ).…”

“…Explicitly modelling the variation in individual decisions requires an understanding of these trade-offs from a fitness perspective ( Houston and McNamara, 1999 ; Mangel and Clark, 1988 ). Dynamic state variable models, implemented via stochastic dynamic programming, have been used to explore the multidimensional matrix of optimal behavioural and reproductive decisions at any moment in the life of an individual, given the combination of internal and external state variables ( McHuron et al, 2017a , 2018 , 2021 ; Pirotta et al, 2018a , 2019 , 2022 ). As an alternative, fixed rules (e.g.…”

Variation in body mass and food intake of northern fur seals (Callorhinus ursinus)

“…In migratory species like northern fur seals, this means they can have differing impacts on the ecosystems they inhabit depending on how their use of a given ecosystem overlaps with seasonal patterns in prey intake. The estimates of maximum prey intake rates presented here provide context for whether estimates of prey intake from bioenergetic models are realistically achievable and can be useful in parameterizing behavioral models to avoid unrealistic mass gains when simulated individuals encounter abundant prey resources (e.g., McHuron et al, 2021).…”