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Marine Ecology & Stock Assessment (MESA) Program

Harbor Seal Foraging Ecology Study in Glacier Bay National Park

Glacier Bay National Park historically has supported one of the largest breeding populations of harbor seals in Alaska. Harbor seals are an important apex predator and are the most numerous marine mammal in the park; however, harbor seal numbers have declined by 63%-75% in the park since 1992. The magnitude and rate of decline exceed all reported declines of harbor seals in Alaska, with the exception of Tugidak Island.

Little is known with respect to the foraging ecology, life-history, movements, behavior, and trends in available prey of harbor seals in Glacier Bay, so it is difficult to discern the causal factors contributing to the decline. Therefore, a multi-agency collaborative study began in 2005 between the National Park Service, ADF&G, and ABL aimed at addressing harbor seal declines in Glacier Bay.

Central to understanding the foraging ecology of harbor seals is identifying the spatial and temporal distribution of available prey, and which areas of the park represent critical foraging habitat. The objectives of our study are to 1) determine the location of important foraging habitat of seals, 2) determine foraging areas of seals relative to boat traffic and protected waters, 3) determine prey availability in areas where individual seals forage, and 4) determine prey availability near the two primary haul-out areas, a glacial ice haulout in Johns Hopkins Inlet and a terrestrial haulout in the Beardslee Islands.

To assess prey availability, annual hydroacoustic surveys of waters in the vicinity of haulouts and foraging areas have been conducted. Auke Bay Laboratory has played a primary role in these surveys by supplying acoustic equipment, expertise, and analysis. Standardized large-scale acoustic surveys were conducted in 2005 and 2006 using a box pattern.

These surveys were replicated three times every other week from May through August in Johns Hopkins Inlet and around the Beardslee Islands to determine seasonal dynamics of available prey near these haulouts. Random adaptive fine-scale acoustic surveys were also conducted to estimate prey availability in foraging areas of individually tagged harbor seals.

The acoustic hardware used to disperse and collect acoustic data consisted of a 38-kHz split beam system towed in a tow body at a depth of 2 m and a speed of 6-7 knots. Nautical area scattering coefficient (NASC), average, maximum, and minimum depths, and total area surveyed will be calculated in order to analyze prey concentrations. Echoview is the acoustic analytical software used to manipulate the data, tabulate depths and survey areas, and calculate the NASC values used. Field collection of hydroacoustic data will be continued in 2007.

Preliminary hydroacoustic analysis of the 2005-06 data for the Beardslee Islands shows dense aggregations of small schooling fish from the surface to 70 m at irregular intervals throughout the area. In contrast, Johns Hopkins Inlet had diffuse layers of prey available at 80-100 m and from 250 m to the bottom with a maximum of depth of 320 m. Preliminary data for tagged animals show seals that haul out in the Beardslee Islands with its higher density of prey, travel shorter distances, and rarely leave the island complex. However, seals that haul out in Johns Hopkins Inlet, where the prey concentrations are less dense and more scattered, have to travel greater distances to forage. In some cases, seals tagged in Johns Hopkins Inlet traveled over 100 km to forage.

Acoustic surveys will provide future information related to seasonal prey availability and density, which are central to understanding the variability in movement of seals between haulouts and throughout the park. Ultimately, the locations of harbor seal foraging and haul-out areas will be integrated with data from time depth recorders attached to the animals, hydroacoustic data, and tagged animal data. The integrated data will provide information on foraging behavior in relation to prey availability and may shed light as to why harbor seals are declining in Glacier Bay National Park.

By David Csepp

2006 Sablefish Longline Survey

The AFSC has conducted an annual longline survey of sablefish and other groundfish in Alaska since 1987. The survey is a joint effort involving two divisions of the AFSC: ABL and the Resource Assessment and Conservation Engineering (RACE) Division. It replicates as closely as practical the Japan-U.S. cooperative longline survey conducted from 1978 to 1994 and also samples gullies not sampled during the cooperative longline survey.

In 2006, the 28th longline survey of the upper continental slope of the Gulf of Alaska and eastern Aleutian Islands was conducted. One hundred-forty-eight longline hauls (sets) were completed between 4 June and 1 September 2006 by the chartered fishing vessel Alaskan Leader. Sixteen kilometers of groundline were set each day, containing 7,200 hooks baited with squid.

Sablefish was the most frequently caught species, followed by giant grenadier, shortspine thornyhead, and Pacific cod. A total of 87,032 sablefish were caught during the survey. Sablefish, shortspine thornyhead, and Greenland turbot were tagged and released during the survey. Length-weight data and otoliths were collected from approximately 2,400 sablefish.

Killer whales took fish from the longline at several stations in the Aleutian Islands and in the western Gulf of Alaska near Dutch Harbor, where these whales have frequently impacted survey operations in past years. Sperm whales were often present during haul back and were observed depredating on the longline at multiple stations in the eastern and central Gulf of Alaska.

Several special projects were conducted during the 2006 longline survey. Corals caught on the line were collected for identification and sample preservation. A seabird occurrence study was conducted for the fifth year, which helps to address where and when certain seabird species occur in Alaska waters. Spiny dogfish were sampled during the west Yakutat and central Gulf legs for biological studies conducted by graduate students from the University of Alaska Fairbanks and the University of Washington. A giant grenadier reproductive biology study was conducted during the Southeast leg, and maturity samples of these fish were taken for histological analysis.

A marine mammal observer was on board during the first two survey legs in the Aleutian Islands and the western Gulf of Alaska to collect photo identification of resident killer whales that were observed depredating on the gear. A second marine mammal observer studied sperm whale depredation in the eastern and central Gulf of Alaska. Photo identification, dive behavior observations, and biopsy samples were collected. Finally, a 2-day experiment was conducted off Yakutat to collect genetic tissues of rougheye rockfish and to investigate depth distribution patterns of “light” and “dark” color phases of rougheye rockfish.

By Chris Lunsford

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