Assessing Population-Level Effects of Anthropogenic Disturbance on a Marine Mammal Population

Abstract

The Population Consequences of Disturbance (PCoD) model is a conceptual framework used to assess the potential for population-level consequences following exposure of animals to a disturbance activity or stressor. This framework is a four-step process, progressing from changes in individual behavior and/or physiology, to changes in individual health, then vital rates, and finally to population-level effects. Despite its simplicity, there are few complete PCoD models available for any marine mammal species due to a lack of data available to parameterize many of the steps. Here, we present an application of the PCoD framework for migrating humpback whales exposed to a simulated commercial seismic survey scenario. We approached the framework in two ways; first, progressing sequentially forwards through the steps and basing our assessment on lactating females. This cohort was considered to be the most vulnerable in terms of energetic costs of disturbance, and most likely to influence any change in population growth due to future breeding success. Field measurements of behavioral responses of migrating humpback whales to seismic air guns from a previous study were used to parameterize an agent-based model (ABM). This ABM was used to estimate the probability of response, where a response was defined as a change in the migratory movement of female-calf pairs, and the duration of any resulting delay in migration. We then estimated the energetic consequences of any delay in migration for the lactating females and created population growth models with which to assess any population-level effects. The results of the forwards approach suggested a low potential for population consequences of seismic surveys on migrating humpbacks. Working backwards through the framework, we investigated “worst case” scenarios that could potentially lead to a population-level effect. Here, we started with increasing calf mortality and assumed that an exposure time greater than 48 h would increase mortality risk. We determined the most likely context in which this exposure would occur (resting area) and then tested this context within an ABM. This backwards approach illustrates how the PCoD model can be used to make management decisions regarding animal populations and exposure to anthropogenic stressors.

A review of bioenergetic modelling for marine mammal populations

“…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 ).…”

Emerging themes in Population Consequences of Disturbance models

“…Because humpback whales rely on finite energy stores during migration, these behavioural changes could alter a female's energy budget, thereby reducing calf growth rates, and delay arrival at the foraging grounds [ 40 ]. When incorporated into a PCoD model, Dunlop et al [ 41 ] found that similar behavioural responses to a simulated 10-day seismic survey during peak migration had negligible effects on female body condition and population growth. However, the costs of repeated exposures may accumulate over the long migration, particularly for populations migrating along coastlines with high levels of human activity.…”

Biologically Important Areas II for cetaceans within U.S. and adjacent waters - Updates and the application of a new scoring system

“…An understanding of where, when, how, and why marine mammals congregate and move is important when assessing direct interactions with human activities that can result in injurious or lethal impacts, such as vessel strike or fishing gear entanglement. Further, the growing body of evidence clearly indicates that an animal's behaviors, activities, or states when exposed to the pervasive array of nonlethal stressors, such as underwater noise, can affect their immediate response (e.g., McHuron et al, 2018;Pirotta et al, 2021), with cumulative exposures potentially affecting individual fitness, which may ultimately result in population-level impacts (e.g., New et al, 2014;Dunlop et al, 2021;Pirotta et al, 2022).…”