Detecting Long-term Changes in Forage Fish Populations in Prince William Sound, Alaska

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Forage fish are small pelagic schooling fish that play a key role in transferring energy between plankton and top marine predators. Many seabirds, marine mammals, and commercial fish species depend on forage fish to grow and survive.

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Project Overview

Top to bottom: Pacific capelin, Pacific herring, Pacific sand lance, and juvenile walleye pollock

Key forage fish in Alaska (from top to bottom): Pacific capelin, Pacific herring, Pacific sand lance, and juvenile walleye pollock. Cook Inlet, Alaska.
(Credit: Mayumi Arimitsu, USGS. Public domain.)

As part of the Gulf Watch Alaska long-term ecological monitoring program, we are collecting data on forage fish abundance and condition in order to better understand how the abundance and nutritional quality of key prey species changes over time. This information is helpful for understanding predator-prey interactions, especially in light of recent seabird die-offs in Alaska.


Why are we studying forage fish?

Forage fish are the link between plankton and predators. In this way they are a key to healthy marine bird populations. As the U.S. Department of Interior (DOI) has Trust responsibility for monitoring and conservation of all seabirds, our studies on forage fish and impacts of environmental change on seabird prey populations are vital to our research role in USGS and DOI.

Where are we working?

Northern Gulf of Alaska, including Prince William Sound, Cook Inlet, and the continental shelf region.

How do we monitor forage fish?

Since we define forage fish as a group of species that play a functional role in ecosystems, and because each species has a different seasonality and habitat due to differing life history strategies, we use a variety of methods to count, catch, and measure forage fish populations. For example:

Black-legged Kittiwakes diving into the water to catch small fish in Cook Inlet, Alaska

Black-legged Kittiwakes foraging for fish in Cook Inlet, Alaska.
(Credit: Sarah Schoen, USGS. Public domain.)

  • Collect forage fish in seabird diets: Seabird diets provide some of the longest timeseries data on forage fish in the Northern Gulf of Alaska. Middleton Island is located at the continental shelf break in the Gulf of Alaska. Seabirds catch their food in the ocean around their colony and bring it back to their nests, making it easy to collect information on their food.
  • Sample fish from a boat during aerial surveys: We sample forage fish that are surveyed by airplanes in coastal areas of Prince William Sound. Skilled observers in the plane have a better view than we do in the boat and they can direct us to the fish schools so we can find them more easily.
  • Measure fish density in the water column with hydroacoustics: From our research vessel USGS R/V Alaskan Gyre we conduct acoustic-trawl surveys in summer and fall. This method uses sonar technology that transmits and recieves sound in the water. When we find acoustic fish sign in the water column we send a net down to the correct depth to identify the fish species and size.
  • Sample forage fish habitat: We also collect information on the type of habitat forage fish occur in by measuring ocean temperature, salinity, turbidity, light availability, nutrients, phytoplankton, and the abundance of zooplankton (forage fish food) in the water.
Scientist lowering CTD equipment into the water

Mayumi Arimitsu lowering the CTD equipment into the water.
(Public domain.)


WhaleFest 2019 Podcast

To learn more, listen to USGS Research Ecologist Mayumi Arimitsu's interview for a podcast about whale prey, forage fish, and seabird die-offs. Arimitsu talked about research on changes in forage fish populations that were first signaled by large die-offs of seabirds during the 2014-2016 North Pacific marine heatwave – aka “The Blob.”