A million-strong we cling
to avoid the searching maw
Do seabirds smell our fear?
Why are we sampling?
Changes in the abundance of forage fish (small, schooling fish that are the prey species of larger fish, marine birds, and marine mammals) can have dramatic effects in marine ecosystems because much of the energy transferred from lower to upper food web levels passes through these species. Forage fish typically produce a large number of offspring and have short lifespans, meaning their abundance and distribution can fluctuate dramatically in response to environmental variation. The response of animals that prey on forage fish to fluctuations in forage fish abundance depends on the predator’s ability to shift its foraging behaviors and the relative nutritional value of each species of forage fish and other types of prey in their diet.
Where are we sampling?
Our study includes surveys throughout Prince William Sound.
How are we sampling?
We conduct hydroacoustic and trawl surveys in Prince William Sound during July of each year. We use a combination of aerial spotting surveys, hydroacoustics, and various fishing techniques (i.e., midwater trawl, dip net, cast net, jig, gill net, beach seine, purse seine, video) to collect target species to measure age, length, and weight. We also ground truth the echoes in the water column received back from hydroacoustic equipment, termed “backscatter,” to make estimates of the biomass of individual species detected.
What are we finding?
While our forage fish monitoring study is young and we continue refining our methods each year, we observed differences in marine habitat and forage fish distribution between 2012-13 (cooler) and 2014-15 (warmer) years. Age-1 capelin made up a smaller proportion of trawl catches in warmer years compared to cooler years. Our findings are consistent with long-term monitoring on Middleton Island that shows a lower proportion of capelin in seabird diets in recent warm years. Capelin is a cold-water species that responds quickly to climate change and uses glaciated fjords as cold water refuge habitat. All of the capelin we caught in 2015 in Prince William Sound were near glaciers in Columbia and Unakwik bays.
Also, based on our aerial surveys, acoustic sampling, and fishing, we are developing a general understanding of the distribution of common forage fish species throughout Prince William Sound. Walleye pollock and Pacific herring are more widely distributed in Prince William Sound compared to capelin, Pacific sand lance, and eulachon.