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Recovering Humpback Whales and the Future of Alaska’s Hatcheries, Fisheries and Coastal Communities

Investigator

Janice Straley Janice Straley
Sitka Sound Science Center

Student

Synopsis

The recovery of humpback whales in the North Pacific after the end of industrial whaling is a conservation success. However, it also means there are a lot more very big mouths to feed. Recently whales have started feeding on juvenile salmon as they are released from hatcheries. Since these fish were raised to supplement salmon fisheries in years to come, whale predation is in conflict with salmon fisheries. The goal of this research is to evaluate how changes to hatchery release strategies could reduce the rate of whale predation and if it would make financial sense to make those changes. Researchers will use echosounders to map the distribution of salmon in the release area following different types of releases (e.g., releasing 2 gram fish vs. releasing 4 gram fish). They will use a long pole to attach a tag to the whales’ backs using suction cups. The tags will record whale movement under water to determine how much energy the whale spent while foraging. Researchers can compare the costs of foraging at hatcheries to costs of foraging on other types of prey like krill and herring. With enough tags, they can start to understand how characteristics of prey groups, such as density and depth, affect the amount of effort a whale has to put in to feed on that group. If hatcheries can change the characteristics of their fish in the water—for example by releasing them at a larger size—whales may be less likely to feed there. Finally, there may be some costs associated with alternative release strategies. The scientists will help determine if changes are financially sensible for hatcheries to implement.

For project news, please see the 2015 news story ”Fast-food opportunity appeals to humpback whales.”

Overview

The issue

The increasing humpback whale population is an ecosystem change that is already affecting Alaska’s salmon hatcheries and fisheries. Identification of strategies that hatcheries can use to adapt to the increase will help preserve the economic and sociocultural benefits of sustainable salmon harvest for coastal communities.

Why is this an Alaska Sea Grant project?

This project is relevant to two Alaska Sea Grant themes. The project addresses impacts on and strategies for coastal ecosystems and coastal communities adapting to change. This is needed because an increasing population of humpback whales will change the seascape for local communities in many ways, including direct competition with fisheries and possibly indirectly by consuming lower trophic level prey. The project also addresses improvements to the sustainability of Alaska’s coastal communities. A continued increase in humpbacks could create a situation where salmon yields are insufficient to offset hatchery operation costs. The results of this study will give hatcheries valuable tools to decrease humpback whale predation and maximize early marine survival of juvenile salmon. Maintaining high and consistent yields of Alaska salmon will benefit hatcheries, fishers, processors, consumers, and residents of these coastal communities.

How will researchers conduct their study?

1. Researchers will use traditional echosounders and high-resolution imaging sonar to map the prey field in a hatchery release area following two different release strategies to document how prey group characteristics change over time.

2. Researchers will attach biologging tags to humpback whales for analysis of underwater behavior. Researchers can then use bioenergetics models to compare foraging efficiency among hatchery release strategies in the context of wild prey options. Hatchery staff will continue to provide predator observation and release strategy information, which will be used in concert with rates of returns of adult salmon to further inform this method.

3. Researchers will perform a cost-benefit analysis comparing the economic impact of individual release strategies on the rate of salmon return.

Research collaborators

Sitka Sound Science Center
Florida International University (FIU)
Armstrong-Keta, Inc.
Duke University
Northern Southeast Regional Aquaculture Association
Douglas Island Pink and Chum
Southern Southeast Regional Aquaculture Association
Prince William Sound Aquaculture Association
Valdez Fisheries Development Association

Results

What researchers learned

Since 2008, humpback whales have been reported feeding on juvenile salmon released by enhancement facilities in Southeast Alaska, and this feeding has been implicated in poor recruitment to the salmon fisheries. The goal of this study was to understand the energetic incentives motivating humpback whales to adapt to this novel prey source. We measured the prey field characteristics and conducted behavioral observations at Hidden Falls release sites in 2014. We found the prey field was relatively diffuse at a salmon hatchery release site with higher acoustic densities in shallow and nearshore areas. Release sites had few acoustically-detected schools, which were temporally and spatially unstable. Humpback whales fed on salmon close to shore and near potential barriers like docks, the surface, shoreline and frequently blew bubbles and bubble nets while feeding. We identified 20 individual whales within 20 km of the Hidden Falls release site, but only four were observed feeding on juvenile salmon. This feeding was observed on 14 out of 22 days, often multiple times per day. Staff at five release sites on Baranof Island conducted standardized 15-minute time-sampling observation periods during their release season from 2010–2015. The probability of occurrence of humpback whales varied across sites, increased after releases began, decreased after releases concluded and did not increase significantly over the six-year study period. Diffuse prey aggregations and salmon avoidance behaviors likely make release sites energetically challenging, perhaps explaining why few whales feed there, they frequently blow bubble nets, and there has been no increase in feeding over time. This is the first scientific study to document humpback whales actively feeding on salmon, but the potential economic effect on salmon fisheries remains unknown.

Research outcomes

Publications
Chenoweth, E.M., J.M. Straley, M.V. McPhee, S. Akinson, and S. Reifenstuhl. 2017. Humpback whales feed on hatchery-released juvenile salmon. Royal Society Open Science 4(7) https://doi.org/10.1098/rsos.170180

Anticipated benefits: This project will allow hatcheries to make informed decisions about optimal release strategies. It may lead to changes in release strategies and rearing practices to increase the early marine survival of juvenile salmon, resulting in larger rates of return to sport, commercial, and subsistence harvesters who rely on hatchery salmon. Understanding humpback whale prey selection is also important for other species, such as herring and krill.