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

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Forage fish are an important node in marine food webs because they link primary and secondary producers with higher trophic levels.

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A view of the mountains and the ocean in Prince William Sound

A scenic view of the mountains and the ocean in Prince William Sound.
(Credit: Sarah Schoen, USGS. Public domain.)

Fluctuations in forage fish abundance can have dramatic ecosystem effects because much of the energy transferred from lower to higher trophic levels passes through a small number of key forage species. Forage fish typically produce a large number of offspring and have short life spans, and these traits predispose populations to large fluctuations in abundance, with associated impacts on predators. In response to a lack of recovery of wildlife populations following the Exxon Valdez Oil Spill (EVOS), and evidence of natural background changes in forage fish abundance, there was a significant effort to document forage fish distribution, abundance, and variability in Prince William Sound (PWS) in the 1990’s. Since then, ongoing research has focused on commercially valuable Pacific herring, and walleye pollock, whereas less has been done in recent years to monitor other ecologically important forage species such as capelin, Pacific sand lance, eulachon, and euphausiids.

Project Overview: Beginning in summer 2012 we implemented a program to: 1) measure forage fish abundance, distribution, and community composition using a combination of hydroacoustic surveys and net-sampling methods that  are cost effective and  adequate for analyses of  long-term trends;  2) measure indices of forage fish biology that are important in determining population dynamics, including growth and age-at-length; and, 3) measure physical and biological variables in the environment that may affect fish distribution and population dynamics.

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.)

In 2013 we explored the use of adaptive cluster sampling, and tested combined aerial and acoustic surveys with validation (“aerial-acoustic surveys”) as means to increase our encounter rate with target species. Adaptive cluster sampling (i.e., intensive sampling right over schools we found during surveys or by chance) generally involved a high degree of effort and did not facilitate a quantitative means of assessing abundance and distribution at the sound-wide scale. An experienced spotting pilot directed the ship or a skiff to forage fish schools visible from the plane. Schools were captured with nets, jigs, video, and hydroacoustics whenever possible. The ground crew recorded, and relayed to the pilot, information about fish species, fish size, and depth of the schools. After the pilot left, we conducted hydroacoustic surveys of the area, and we used midwater trawls, gill nets, cast nets, dip nets, jigs, or video to confirm the species composition and fish size for conversion of acoustic backscatter to biomass.

Although this work facilitated a better way to target near-surface forage fish schools available for observation from a plane, our sampling efforts resulted in relatively low-encounter rate with forage schools below the depth visible to the spotter pilot (> 10-15 meters), We know, however, that humpback whales are efficient predators of forage species (fish and euphausiids), and whale distribution may be a key indicator of high density prey patches at depths that are not visible to observers in a plane. Therefore we have collaboration with GWA humpback whale PIs and have developed a study to quantify prey aggregations near foraging humpback whales (2017-2021).

A Pacific sand lance, Pacific herrring and a capelin in Prince William Sound, Alaska

A Pacific sand lance, Pacific herrring and a capelin in Prince William Sound, Alaska.
(Credit: Mayumi Arimitsu, USGS. Public domain.)

The objective of this study is to detect changes in populations of forage fish in Prince William Sound, we will gather new data on the distribution, abundance, community structure, habitat, and body condition of key species. This forage fish project has two main components, these include support for the longest time series of seabird diets in the Gulf of Alaska at Middleton Island in collaboration with Scott Hatch (Institute for Seabird Research and Conservation, ISRC), and the Integrated Predator Prey survey in Prince William Sound in collaboration with the humpback whale (John Moran, NOAA, and Jan Straley, UAS), and marine bird (Mary Anne Bishop).

  1. Estimate an index of forage fish availability, species composition and biomass within persistent predator foraging areas
  2. Assess population trends of key forage fish species such as capelin, Pacific sand lance, Pacific herring, and euphausiids over time
  3. Detect changes in body condition of capelin and sand lance from size at age relationships determined from otoliths.
  4. Detect changes in biotic and abiotic features of the marine environment that may influence forage fish population dynamics and distribution over time.
  5. Support annual field and laboratory efforts to continue the Middleton Island long-term seabird diet index



Graphic showing variation between years of kittiwake diet at different stages of breeding on Middleton Island, Alaska

Interannual variation in kittiwake diet composition at three stages of breeding on Middleton Island, 1978 to 2017.
(Public domain.)

Middleton Island: The Institute for Seabird Research and Conservation (ISRC) entered into a cooperative agreement with the U.S. Geological Survey, Alaska Science Center, to continue field studies of seabirds on Middleton Island as part of Gulf Watch Alaska.  The latter program, a 20-year effort (2012-2031) funded by the Exxon-Valdez Oil Spill Trustee Council, is intended to uncover and monitor natural and anthropogenic factors affecting ecosystem functioning in the Gulf of Alaska and Prince William Sound.  The particular contribution of Middleton seabird monitoring will be to quantify dietary shifts in predator species, especially black-legged kittiwakes and rhinoceros auklets, as indicators of forage fish dynamics in the region.

Acoustic-trawl surveys: We will conduct coupled hydroacoustic and trawl surveys using a sample design that includes ancillary environmental sampling in areas of historically high whale concentrations. A 38-120 kHz SIMRAD EK60 echosounder to estimate prey depth distribution and density. Prey composition will be evaluated using a midwater trawl in deep water, or other means of sampling such as video, jig, cast net or dipnet as necessary. We will evaluate mean depth of acoustic backscatter (NASC) and mean density (fish m-2) by species on transects and transects sampled near feeding whales.

The crew of the R/V Alaskan Gyre lowering a hydroacoustics instrument in to the water

The crew of the R/V Alaskan Gyre lowering a hydroacoustics instrument in to the water.
(Public domain.)

Habitat Sampling: We also collect ancillary habitat data to explain any observed differences in forage fish distribution among treatment groups. At the beginning or end of each transect we will sample oceanography (water column temperature, conductivity, fluorescence, turbidity, bean transmission, oxygen, photosynthetically active radiation, and nutrients) with a CTD/water sampler, and zooplankton with a 50 meter vertical haul of a 150µ mesh ring net. Habitat data will provide a means of describing spatial and temporal changes in abiotic and biotic features across the study area. Linear and non-linear methods may be used when appropriate to relate habitat to species-specific biomass.


EVOS Reports:
Monitoring Long-Term Changes in Forage Fish Distribution

Scientist lowering CTD equipment into the water

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