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 and a seasonal monitoring station on Middleton Island in northern Gulf of Alaska. More information about ongoing seabird monitoring and research is available at: https://middletonisland.org/.
How are we sampling?
The primary objectives of the forage fish monitoring project are to: 1) monitor the status and trends of forage fish in areas with known persistent aggregations of predators and prey during fall, and 2) support annual field and laboratory efforts to continue the Middleton Island long-term seabird diet index in spring/summer, 3) assess changes in forage fish abundance indices on acoustic-trawl surveys during summer, and 4) support Herring Research and Monitoring program aerial survey validation efforts in summer.
To meet our first objective, we integrate directly with the humpback whale and marine bird predation studies to provide estimates of forage biomass in the immediate vicinity of predator aggregations. To meet our second objective, in a collaboration with the Institute for Seabird Research and Conservation, we use seabirds as samplers of forage fish at Middleton Island to continue the long-term seabird diet data collection program as a cost-effective means to monitor forage fish stocks in the northern Gulf of Alaska. To meet our third objective, we conduct multi-frequency hydroacoustic transects and trawls to identify species composition and size distributions of ensonified targets. To meet our fourth objective, we sample fish schools from a vessel on the water under the direction of the aerial survey team to validate their observations.
What are we finding?
Plots of acoustic fish and macrozooplankton indices reflect interannual variability in depth distribution and magnitude of acoustic backscatterance among regions of Prince William Sound. In general, backscatter due to fish was patchier, with large swaths of nearly empty water column in 2018 than was observed on the same transects in 2017. We encountered juvenile herring near the Needle in Montague Strait, in Hanning Bay, and in Port Gravina in 2018. Walleye pollock, which typically form a diffuse, weaker scattering layer were notably missing from Bainbridge and Montague Strait subareas in 2018.
Acoustic indices of macrozooplankton also show changes in depth distribution and density of key forage taxa. Trawl samples revealed that this included primarily euphausiids, but also a mix of mysiids, amphipods and other large-bodied zooplankton taxa. In 2018, macrozooplankton layers were nearly absent from Bainbridge and parts of Montague Strait (particularly on the LaTouche Island side). The macrozooplankton scattering layer was denser and occupied a higher position in the water column in 2017 than in 2018 at Montague and Gravina subregions.
Although fixed transects were not established until 2017, and survey effort was lower in 2014 (especially in Montague Strait) there was a striking difference in the macrozooplankton scattering layer in 2014 when greatest densities of macrozooplankton were observed over large portions of the water column. This change in macrozooplankton as the basis for an important change in productivity of the system may help to explain the abrupt decline in humpback whale use in these areas since 2014.
Black-legged kittiwake diets in April and May 2018 showed a relatively high proportion of myctophids, perhaps reflecting those species’ resumption of near-surface migration at night). During incubation and chick-rearing in 2018, the kittiwake diet favored herring, sablefish, and sand lance. Consistent with results since 2014, a notable scarcity of capelin continued in 2018, and juvenile pink and chum salmon had a poor showing in the kittiwake chick diets. During summer, kittiwake diets (Jun-Aug) and rhinoceros auklet chick diets (Jul-Aug) indicate greater composition of sand lance in 2018 compared to recent years.
The foraging range of Middleton kittiwakes in 2018 was, on average, more extensive perhaps than in any prior year known from deployments of GPS tracking devices. During incubation, birds used deep-ocean waters regularly, including one record-setting pelagic trip reaching more than 200 km southeast of the island. But most of the foraging occurred north of the island, particularly along the outer coasts of Hinchinbrook and Montague Islands, and extending to inside waters of Prince William Sound to a greater degree than was previously known. Chick-rearing kittiwakes accessed much the same area. The foraging area of rhinoceros auklets overlapped that of kittiwakes, but was largely confined to neritic waters north and northwest of Middleton during both incubation and chick-rearing phases of breeding. Auklets foraged regularly along the southeast coast of Montague Island, up to 100 km from their nest sites on Middleton.