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Approaches to Understanding the Cumulative

Efects of Stressors on Marine Mammals

Marine mammals face a number of natural and human stressors that can harm populations, such as noise and other pollutants, loss of habitat, and bycatch in ishing. Much progress has been made in understanding the response of marine mammals to some speciic stressors such as noise, but estimating risks to marine mammal populations requires understanding how the full array of stressors interact. This report offers a conceptual framework, research strategies, and adaptive management suggestions to improve the ability to assess the cumulative effects of stressors on marine mammal populations and identify which stressors could be reduced to bring the population into a more favorable state.

Years of research and assessments, including four National Academies’ reports, have doc-umented effects of human-induced sound on marine mammals. Based on this research, environmental reviews now routinely assess the number of animals that may be injured or disturbed by human-induced noise in the ocean.

There is growing recognition, however, that the effects of a single stressor such as noise must be considered in the context of all other stressors in order to predict poten-tial impacts on marine mammal populations. Advances in science are needed to better assess the impact of repeated exposures to the same stressor, the cumulative effects from all human stressors (e.g., noise and chemical pollution, marine debris, ishing), and the complex interactions of human and natural stressors that alter ecological drivers such as climate, prey, competitors, pathogens, and predators.

In recognition of this challenge, the Ofice of Naval Research, the National Marine Fisheries Service (NMFS), the Bureau of Ocean Energy Management (BOEM), and the U.S. Marine Mammal Commission funded this study to review the understanding of cumula-tive effects of human stressors on marine mammals and to identify new approaches that may improve the ability to estimate cumulative effects.

SCIENTIFIC PROGRESS IN UNDERSTANDING SOUND

Efforts to understand the effects of sound on marine mammals are illustrative of the types of research conducted to assess a speciic stressor. A main goal is to characterize the

rela-Repor t

HIGHLIGHTS

October 2016

There is growing

recog-nition that the efects

of a single stressor such

as noise must be

con-sidered in the context

of all other stressors in

order to predict

poten-tial impacts on marine

mammal populations.

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tionship between acoustic exposure level (the dosage) and a behavioral or physiological response, such as a temporary hearing loss. Dose-dependent functions have been determined for physiological and behavioral responses. Because of variation between individuals and across species, the physiological effects of sound cannot be generalized based on the amount of testing conducted to date.

The variation in behavioral response is even greater than that for physiological response. Location, activity patterns, and social structure, among other variables, affect the behavioral responsive of individ-uals. Generally, severity of response increases with strength of stimulus. However, severe responses have been observed at signal levels that are just dectectable, and signals that are expected to exceed known thresh-olds have resulted in no observable response.

THE CHALLENGE OF ASSESSING

CUMULATIVE STRESSORS

Although the relationship between the dose and the response of an individual animal can be determined for some stressors, the addition of a second stressor can add considerable complexity due to the potential for interactions between the stressors or between their effects. Stressors may interact in a synergistic or antag-onistic manner, where the resulting response is larger or smaller, respectively, than the sum of the individual stressor responses.

The need to understand the cumulative effects of multiple stressors has been illustrated in cases of mammal populations that have suffered an unexplained decline, or have failed to recover following the removal of a stressor. One example is the Cook Inlet beluga whale population, which is separated by the Alaska

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insula from other beluga populations in Alaskan waters. Its population declined from around 1,300 whales in 1979 to 367 in 1999. It was assumed that the Alaskan Native subsistence harvest was the major cause of the observed decline over this period. However, the popu-lation has shown no sign of recovery since 1999 when Alaskan Natives imposed a voluntary moratorium that drastically reduced subsistence harvest.

A rigorous approach for testing interactive effects of multiple stressors involves experiments that use different intensities of each stressor coupled with tests of some combinations of stressors. Both for prac-tical and ethical reasons, however, such experimental approaches are often not possible for marine mammals. In those cases, inferences must be based on quasi-exper-iments that reveal patterns associated with variation in the levels of stressors over space or time.

A NEW CONCEPTUAL FRAMEWORK FOR

ASSESSING CUMULATIVE RISKS

Recognizing the dificulty of quantitative prediction of cumulative effects of stressors on marine mammals, the report’s authoring committee developed the “Pop-ulation Consequences of Multiple Stressors” (PCoMS) framework (Figure 1). The framework documents the pathways that are possible from exposures to stressors to their effects on physiology, behavior and health and from there to their effects on vital rates (e.g., survival) and population dynamics. The framework considers all of this in the context of ecological drivers, such as climate change.

The PCoMS framework does not provide an algorithm for predicting cumulative effects but instead serves to identify the most important components for evaluating cumulative effects. A key to the framework is that the health of an individual marine mammal, deined as the ability to adapt and self-manage, is used as the summation point for the cumulative effects of multiple stressors. The report discusses how a variety of health indices, including allostatic load, energy stores, immune status, organ status, stress levels, contaminant burden and parasite load, play into the framework.

MANAGEMENT OF CUMULATIVE EFFECTS

Once populations or ecosystems are found to be at risk of adverse impacts, the critical issue for selecting management actions is to decide what combinations of stressors could be reduced to bring the population or ecosystem into a more favorable state. Even though exposure to multiple stressors is an unquestioned reality for marine mammals, the best current approach

for management and conservation is to identify which stressor combinations could present the greatest risk to populations. The Committee developed a decision tree that could be used to identify situations where a detailed study of potential cumulative effects should be given a high priority. The decision tree was applied to three case studies to demonstrate its utility.

Research Recommendations

The report pro-vides a number of research recommendations that are designed to address the current challenges facing researchers and managers involved in monitoring, man-aging, and protecting populations of marine mammals, a subset of which are listed here.

Recommendation: Future research initiatives should include efforts to develop case studies that apply the PCoMS framework to actual marine mammal populations. These studies will need to esti-mate exposure to multiple stressors, predict changes in behavior and physiology from those stressors, assess health, and measure vital rates in order to parameterize the functional relationships between these components of the framework. Where possible, the data on changes in demography, population size, and the health of individuals collected in these studies should be used to improve estimates of the parameters of the PCoMS model and reduce uncertainty.

Recommendation: Future research initiatives should support evaluation of the range of emerging technologies for sampling and assessing individual health in marine mammals, and identiication of a suite of health indices that can be measured for diverse taxa and that best serves to predict future changes in vital rates. Potentially relevant measures include hormones, immune function, body condition, oxidative damage, and indicators of organ status, as well as contaminant burden and parasite load. New technology for remotely obtaining respiratory, blood and other tissue samples and for remote assessment (e.g., visual assessment of body condition) should also be pursued.

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COMMITTEE ON ASSESSMENT OF THE CUMULATIVE EFFECTS ON ANTHROPOGENIC

STRESSORS ON MARINE MAMMALS

Peter Tyack (Chair), University of St. Andrews, UK; Helen Bailey, University of Maryland, Solomans; Daniel Crocker, Sonoma State University, CA; James Estes, University of California, Santa Cruz; Clinton Francis, California Polytech-nic State University; John Harwood, University of St. Andrews, Ceres Cupar, UK; Lori Schwacke, NOAA Hollings Marine Laboratory, Charleston, SC; Len Thomas; University of St. Andrews, UK; Douglas Wartzok, Florida Inter-national University; Kim Waddell (Study Director, Gulf Research Program); Stacee Karras (Associate Program Oficer, Ocean Studies Board), Payton Kulina (Senior Program Assistant, Ocean Studies Board)

For More Information . . . This Report Highlights was prepared by the Ocean Studies Board based on the report

Approaches to Understanding the Cumulative Effects of Stressors on Marine Mammals. The study was sponsored by the Bureau of Ocean Energy Management, Marine Mammal Commission, National Oceanic and Atmospheric Admin-istration, and Ofice of Naval Research. Any opinions, indings, conclusions, or recommendations expressed in this publication are those of the authoring committee and do not necessarily relect those of the sponsors. Copies of the report are available from the National Academies Press, (800) 624-6242; http://www.nap.edu.

lines and contextual variables are critically important to interpreting responses.

Recommendation: Responsible agencies should develop relatively inexpensive surveillance systems that can provide early detection of major changes in population status. Surveillance systems should be developed irst for populations that currently lack adequate stock assessments. To be most effective in providing an early warning, the variables monitored will depend on the species and situation, and may change over time with development of new technology and increasing ecological knowledge.