Cumulative Effects of Multiple Stressors

Marine mammals are exposed to multiple stressors that are a consequence of human activities, including bycatch in fisheries, interactions with vessels, underwater noise, entanglement in or ingestion of marine debris, and exposure to pollutants. In addition, marine mammals are exposed to environmental stressors, many of which may increase with changing ocean temperatures and rising sea level; these include loss of habitat, decreased prey availability, and exposure to pathogens and harmful algal toxins. These multiple environmental and anthropogenic stressors can interact in unpredictable ways. For example, in 2014-2015, a marine heatwave in the Northeast Pacific led to a shift in prey that prompted humpback whales to alter their normal migration pattern and forage closer to the California coast. Simultaneously, a bloom of Pseudo-nitzschia diatoms, also driven by the warmer temperatures, resulted in increased levels of an algal toxin in seafood that delayed the opening of the Dungeness crab fishery. This delay resulted in an overlap of peak humpback whale presence off the California coast coincident with peak crab fishing activity, which in turn led to a spike in the rate of whale entanglements.

Developing approaches to identify and understand the complex ways in which marine mammals are exposed and respond to multiple stressors is essential to determine how to best manage stressors; or, in the case of stressors such as environmental change that cannot be managed, to develop alternative measures to support marine mammal conservation. The Commission was one of several federal sponsors for a National Academies study to develop Approaches to Understanding the Cumulative Effects of Stressors on Marine Mammals. Since that study was conducted, Commission scientists have continued to pursue and support research, and development of tools and technologies, to identify, understand, and mitigate the impacts of multiple stressors on marine mammals.

Hawaiian Monk Seals – A Story of Success

Hawaiian monk seals, the most endangered pinniped in U.S. waters, are a prime example of how multiple stressors can impact marine mammal populations.  Threats to the population include entanglement in marine debris, loss of island habitat from sea-level rise, infectious disease, shark predation, and limited prey availability related to variable ocean productivity. Although still highly endangered, diligent conservation work and implementation of a robust recovery plan that addresses multiple stressors has led to a positive trend for the Hawaiian monk seal population. After years of decline, this population is now growing at an average rate of 2% per year, owing at least in part to decades of intensive interventions to help individual monk seals survive.

Developing conceptual frameworks and analytical approaches to understand effects of multiple stressors

Understanding marine mammals’ exposures to multiple stressors, the behavior and physiological responses to such stressors, how these responses affect health and ultimately affect the vital rates (survival and reproduction) that drive population trends, is a complex problem. This is especially true when stressors, responses, health effects, or changes in vital rates interact.  Commission scientists and Scientific Advisors are working with collaborators to develop and refine conceptual frameworks and quantitative methods that can help to predict the consequences of multiple stressors on marine mammal populations.

What the Commission is Doing

Commission scientists are part of a Population Consequences of Multiple Stressors (PCOMS) Working Group that has applied the PCOMS framework and is implementing quantitative models for several case studies, including a Bayesian state-space model to assess the effects of multiple stressors on North Atlantic right whales (NARW).

Commission scientists also developed a conceptual model for how multiple climate-related environmental stressors are resulting in population-level consequences for marine mammals. The conceptual model, presented in a review article of climate change effects on marine mammals in US waters, outlines how physical ocean changes lead to abiotic (sea-level rise, altered storm activity) and biotic (food web) changes, and how those manifest as effects (negative or positive) on marine mammal populations.

Emerging tools and technologies to understand effects of multiple stressors

Commission scientists are also pursuing new technologies to help identify changes in marine mammal population health, which serve as an early warning before changes in vital rates can be detected. One such emerging approach is the application of epigenetics to detect acceleration in the aging process for bottlenose dolphins, which could increase population mortality rates. The advantage of such an approach is that it inherently captures the cumulative effects of multiple stressors on an animal’s aging and lifespan. The current limitation is that it is difficult to determine which of potentially many underlying stressors are driving acceleration of the aging process.