We were interested in characterizing the prey availability and ocean habitat for marine predators along the Alaska Peninsula and Aleutian Archipelago. Forage fish sampling can be expensive and logistically difficult to conduct, so we opportunistically fished from a U.S. Fish and Wildlife Service vessel that normally supports other research in the study area. Here we describe the results of our efforts.
Hydroacoustics and Thermosalinograph
Hydroacoustic measurements approximate the amount of biomass in the watercolumn by sending an acoustic signal into the water and “listening” for the signal to return. Sound is reflected off the seafloor or organisms such as zooplankton or fish that are in the path of the acoustic beam, and the reflected sound is translated as acoustic backscatter. During our study, acoustic backscatter was low to moderate and dispersed fairly evenly along the entire shelf off the Alaska Peninsula. The area of highest acoustic backscatter was detected in Shelikof Strait at depths between 75 and 100 m, indicating this region had the highest biomass during our sampling. Other areas with high acoustic backscatter were found at Umnak Island and Samalga Pass in association with cold surface waters and presumably resulting from upwelling of nutrient rich waters from the deep and turbulent mixing of the watercolumn over shallow regions.
June hydroacoustic data showing biomass estimates are shown in the top graphs. The graphs are separated by depth. The bottom graphs show the surface temperature and salinity along the Aleutian Archipelago. (click on image for a larger view)
We captured at least 32 species of nearshore fishes using a beach seine. Pacific sandlance were the most frequently encountered species, followed by pink salmon, and northern rock sole. More than 80% of beach seine tows were conducted in the Andreanof Islands, where the catch was dominated by Gadids and Pacific sandlance. We also collected several other nearshore forage fishes including several species in Salmonidae (salmon), Hexagrammidae (greenlings), Cottidae (sculpins), Pholidae (gunnels), Trichodontidae (sandfish), and Pleuronectidae (flatfishes) families.
This is a map of the Andreanof Islands, with the pie charts showing differences in beach seine species composition. The ‘Other’ group includes capelin, greenlings, pricklebacks, and gunnels. (click on image for a larger view)
These northern rock sole were collected in one haul of the beach seine on Adak Island, Alaska. (click on image for a larger view)
We captured at least 24 species of pelagic fishes with our midwater trawls. Pacific sandlance, walleye pollock and capelin were distributed throughout the study area. Young of the year walleye pollock made up 95 % of the catch. The remaining 5 % was composed of Pacific sandlance (74%), capelin (12 %), Pacific cod (6 %), Pacific sandfish ( 4 %) and other species (< 1% each). We also caught 6 species of euphausiids and at least 4 species of copepods. Other important forage species included eulachon, juvenile Pacific cod, daubed shanny , and prowfish.
These three maps show the distribution of Pacific sand lance, walleye pollock, and capelin, thre major forage species for seabirds and other marine predators. The red dots indicate that the forage fish were caught using a midwater trawl and the yellow showing beach seine caught fish. The bigger the circle the more fish caught in that area. (click on image for a larger view)
Spawning capelin are an important forage fish species that are sensitive to fluctuations in ocean temperature. These capelin were caught with a modified herring trawl in the Shumagin Islands on July 30, 2006. (click on image for a larger view)
Marine predators such as seabirds and marine mammals rely on small-schooling fish to survive and reproduce. Scientific research on the distribution and abundance of forage fish is generally expensive and logistically difficult to conduct in the Aleutian Islands because prey are patchy and move quickly. To overcome this problem, we opportunistically sampled prey resources with hydroacoustics and trawl surveys using a research vessel that routinely travels along the Alaska Peninsula and Aleutian Islands to support other research. Our conclusions from this relatively low-cost pilot study are briefly described below.
We documented over 30 species of nearshore fishes and 24 species of pelagic fishes throughout the 1700 km study area. Walleye pollock (Theragra chalcogramma), Pacific sand lance (Ammodytes hexapterus) and spawning capelin (Mallotus villosus) dominated the pelagic catch, and young of the year Gadids, Pacific sand lance and pink salmon (Oncorhynchus gorbuscha) dominated the nearshore catch. Cooler, more saline, and nutrient rich conditions were found in the central Aleutians compared to the Alaska Peninsula during our study. Great efficiency in spatial sampling was achieved because transit time and cost were reduced by the opportunistic use of the M/V Tiglax, which supports ongoing research throughout the Alaska National Maritime Refuge system. We conclude that this research platform has enormous potential for monitoring key ecosystem components in the Gulf of Alaska and Aleutian Archipelago on seasonal, annual and decadal time-scales.