Northern Fur Seals in the Bering Sea are Declining; Researchers Want to Know Why

Since the early 1990s, the population of northern fur seals in the eastern Bering Sea has declined by more than half for reasons that are still unknown. With a grant from the Lenfest Ocean Program, Dr. Jeremy Sterling of the Alaska Fisheries Science Center (AFSC) is leading a research team that aims to understand the role that food availability may play in this ongoing decline. At the end of year one, Sterling discussed what he’s learned so far with the Lenfest Ocean Program.

This interview has been edited for clarity and length.

Q: What challenges are northern fur seals facing and how can your project help?

A: In the Bering Sea, the eastern Pacific stock of northern fur seals breed and care for pups over three to five months in the summer on three islands -- the Pribilof Islands, St. George and St. Paul Islands, and Bogoslof Island. In the last three decades, the number of pups born each year has declined significantly, which could mean that young seals are not surviving to reach adulthood. Since the early 2000s, the decline has almost exclusively been driven by losses on St. Paul Island, home to the largest northern fur seal breeding colony across its entire range.

A pup’s condition at the time it is weaned and strikes out on its own is an important predictor of survival during the first two years of life.  Evidence shows that pup condition at St. Paul Island is poorer compared with other breeding colonies. As a result, we are interested in pup-provisioning mothers and if they may be having a difficult time finding food while in the Bering Sea. We are focusing on availability of their primary prey during summer, walleye pollock, which is by volume the largest U.S. domestic fishery.

To estimate how much prey fur seals need during this critical period of pup care and how much prey is being consumed by the entire population, we’re taking a bioenergetic approach. We are developing a model to characterize the dietary needs of fur seals, and will integrate this model with existing data and models for pollock. The results of this work will provide insight into whether food availability may be contributing to the ongoing decline and will develop a scientific foundation to incorporate predation by northern fur seals into existing stock assessment models for pollock.

Furthermore, it will expand understanding of how environmental conditions affect this predator-prey dynamic and the potential outcome under different climate scenarios. Addressing these needs could inform a more holistic management approach for fur seals, allowing for integration into ecosystem-based fishery management for the eastern Bering Sea.  

Q: What progress have you made on the project so far?

A: We’ve completed the northern fur seal bioenergetic modeling effort. The model takes into account the fact that pup growth rates and female foraging behavior vary considerably across the model years. Understanding how energy demands change across years and what factors drive births, deaths, and other population processes during summer is important given predicted climate change in the Bering Sea and related predicted declines in pollock.

We coupled the bioenergetic model with independent diet estimates, which allowed us to quantify how much prey fur seals are consuming in our five “high resolution” years. These are years for which we have considerable data on fur seal energetics and foraging behavior. Because of long-term research efforts by NOAA and others, our model is almost entirely based on field observations of fur seals from the Pribilof Islands. Our goal is to continually update and refine the model as new observations become available and old observations are revisited. We also revisited some old, previously unpublished data as part of this project on Factors affecting energy expenditure in a declining fur seal population” that is now available in the open access journal Conservation Physiology.

Our initial results have revealed that there is a lot of variability in how much prey of each species fur seals consume. This variability is in part due to the fact that foraging location depends on what island and what part of the island seals are departing from when they go to sea to forage. For example, seals that forage on the Bering Sea shelf predominantly consume pollock; those that venture close to shelf break or into the Bering Sea basin eat more squid and deep-sea smelts. Fluctuations in pollock energy content and the availability of different prey species also contribute to this observation, particularly as it relates to the different size classes of pollock that are consumed.

We found that fur seals consume as much pollock – if not more than – the three fish predators included in the Alaska Fisheries Science Center’s (AFSC) multispecies stock assessment model for pollock, Pacific cod, and arrowtooth flounder. However, we need to estimate how much pollock fur seals have consumed for the full 40 years for which we have historical fish survey and catch data. This is the period we model using multispecies and ecosystem models. So, we are now expanding our analyses to include additional years with medium resolution data. This inclusion will fill in gap years and make it possible to incorporate how much pollock fur seals eat into the multispecies stock assessment model.

Q: How are you accounting for climate change?

A: Our project is designed to complement the work of many other research efforts. The AFSC has several climate-driven prediction models for walleye pollock and some published papers give us an idea of how the pollock population may change in the future. We will couple our bioenergetic model with those other AFSC models to estimate past and present consumption and dietary needs of fur seals as well as under future climate and fishing scenarios. The key here is we’ll be able to provide well-informed, spatially explicit estimates of consumption and dietary needs of fur seals by rookery complex and by pollock age, now and in the future. This understanding can inform future management actions and contribute to climate research.

Q: How have you used outreach to support the work you’re doing? 

A: Part of our mission at the AFSC is to engage and communicate our research results to all stakeholders involved in the fishery management and fur seal co-management process. Since we began this project, we produced a two-page summary of the project for all stakeholders prior to holding a webinar in which all stakeholders and the public were invited to participate. We have provided regular updates to the Science and Statistical Committee, Ecosystem Committee, and Groundfish Plan Teams of the North Pacific Fishery Management Council. We have also engaged with native communities on the Pribilof Islands, the Marine Stewardship Council, and NOAA’s Alaska Scientific Review Group.

Q: What are the next steps?

A: This year – 2020 – will be a big year. First, we will publish two papers that include a tremendous amount of new information. The first paper focuses on the factors that influence at-sea metabolism in nursing mothers and was just published in Conservation Physiology. The second will describe the bioenergetic model and our initial results. We are still reviewing the latter manuscript and hope to submit it soon. 

We are also already coupling our bioenergetic model with other existing models that integrate climate information: FEAST, a spatially explicit food web model; and CEATTLE, the multispecies stock assessment model that includes walleye pollock. Later in 2020, we will also start developing forecasts under different climate and fishing scenarios and incorporating these into our foraging model of nursing mothers.

Research Team

Jeremy Sterling, NOAA
Kirstin Holsman, NOAA
Kerim Aydin, NOAA
Ivonne Ortiz, JISAO, University of Washington
Elizabeth McHuron, JISAO, University of Washington
Nicholas Bond, JISAO, University of Washington

You are also welcome to reach out the Lenfest Ocean Program outreach manager, Emily Knight, [email protected].