Potential Biological Removal Levels: A Tool to Conserve Marine Mammals

Human activities can pose a significant threat to marine mammal populations. Recognizing these threats, Congress enacted the Marine Mammal Protection Act in 1972 to ensure that marine mammal populations continue to act as healthy, functioning elements of their ecosystems. In 1994, Congress amended the MMPA to reinforce this conservation goal by adding a management tool called the Potential Biological Removal level.

What is the Potential Biological Removal level?

The Potential Biological Removal level is a reference point defined by the Marine Mammal Protection Act. It is the maximum number of animals, not including natural deaths, that may be removed from a marine mammal stock (i.e., population) while allowing the population to maintain or recover to its optimum sustainable population size.

Human activities can result in the removal-death or serious injury -of marine mammals from their populations. Examples of such activities include fisheries bycatch, vessel strikes, military sonar, entanglement in marine debris, scientific research, and subsistence harvest. When human-caused mortality and serious injury exceeds a stock's Potential Biological Removal (PBR) level, it signals that we may need to take measures to mitigate human threats. Enacting additional protection measures will help the at-risk stock recover and maintain a healthy population size.

Origins of the Potential Biological Removal Level

Without a benchmark like the PBR level, it can be difficult for scientists and managers to determine when to intervene or how much action is needed to help recover a declining stock. Prior to the introduction of the PBR framework, we relied on detecting a population's decline before taking conservation action. This process can be costly and take a long time.

For example, in the 1960s and 1970s, it took experts 17 years to collect enough data on dolphin bycatch in tuna fishing operations to determine that eastern tropical Pacific dolphin populations had declined to a level that warranted a "depleted" designation under the MMPA. Establishing a reference point, such as the PBR level, allows scientists to detect potentially unsustainable threats earlier. This process lets us take management action in time to allow for the eventual recovery of the declining population.

In the early 1990s, NOAA Fisheries and other stakeholders proposed the PBR management framework (PDF, 2 pages) as a more effective alternative than relying on trend data. This new tool would allow managers to better assess the impacts of human activities on marine mammals even in data poor situations. After further development and some modification, the PBR approach was added to the MMPA in 1994 through an amendment.

Potential Biological Removal in Action

Under the MMPA, NOAA Fisheries must periodically assess the status of marine mammal stocks in U.S. waters and the effects of human activities on them. This information is published in our marine mammal stock assessment reports. In these reports, a stock's PBR level is compared to estimates of human-caused mortality and serious injury as part of determining the stock's status. When the level of human-caused mortality and serious injury exceeds a stock's PBR, it is considered strategic. This indicates human activities may be affecting the stock's ability to recover or sustain a healthy population size and action may be warranted.

For stocks where U.S. commercial fisheries-related incidental mortality and serious injury exceed the PBR level, the MMPA prescribes a framework for reducing them. First, we classify commercial fisheries by the level of mortality and serious injury of each stock compared to its PBR. Then, for those stocks of greatest concern, we develop take reduction plans to reduce incidental U.S. commercial fishery-related mortality and serious injury to levels below a stock's PBR.

The PBR framework also helps us manage other human causes of marine mammal mortality and serious injury. For example, it helps us monitor the impacts of vessel strikes, particularly for populations of large whales. For marine mammals that are hunted for subsistence purposes, the PBR framework helps us-and our co-management partners, such as Alaska Native Organizations-ensure marine mammal populations remain healthy for both the ecosystem and the people who rely on them for food.

Since being added to the MMPA in 1994, the PBR framework has drastically improved our ability to monitor and manage marine mammal populations. It has helped us meet our objectives under the MMPA and our mission of science, service, and stewardship. PBR is an effective conservation and management tool that helps us identify populations of conservation concern and efficiently implement a mitigation plan to stem population declines. It has been recommended by scientists generally for managing exploited species, and has been widely used, including to assess bycatch of seabirds in commercial fisheries and levels of hunting of wildlife in tropical forests. The PBR framework is a great example of how our scientists have helped further wildlife conservation and management globally.

How We Calculate a Potential Biological Removal Level

We calculate a PBR level for each marine mammal stock in U.S. waters and publish these levels in stock assessment reports.

As established by the MMPA, a stock's PBR level is determined by multiplying three fundamental elements:

• An estimate of the population's minimum abundance or size
• One-half of the estimated or theoretical maximum rate of population growth for the stock if it were small
• A recovery factor, with a value between 0.1 to 1, that helps ensure timely recovery

We select the values for each element of a stock's PBR level following NOAA Fisheries' Guidelines for Preparing Stock Assessment Reports to ensure the goals of the MMPA are met, even when there is uncertainty in the estimates of abundance or human-caused mortality and serious injury.

For example, it is difficult to estimate the exact abundance of marine mammal populations, so scientists use a reasonable range estimation. The PBR calculation then uses the lower end of the range because scientists are confident the true population size is at least as large as this minimum estimate.

It is also often difficult to estimate a marine mammal population's maximum growth rate. Because of this, PBR is often calculated using default values based on information from other stocks and species.

Finally, a recovery factor is incorporated into the PBR calculation to allow a portion of the population to be prioritized for recovery. This helps it reach its optimum sustainable population size even when there is uncertainty in the other PBR elements. Smaller recovery factors are applied to more at-risk species. This process is analogous to the approach for rebuilding fish stocks where quotas are reduced to allocate some of the population growth to rebuilding the stock.

Public link

Please use the above public link if you want to share this noodl on another website.