Scientists from the SSMA Program’s Fishery Interaction Team (FIT) conducted one of the first wintertime surveys of walleye pollock in the central Aleutian Islands in February 2008. The cruise was conducted from 16 February to 3 March on board the NOAA ship Oscar Dyson. The primary objective of the cruise was to collect acoustic and trawl data necessary to determine the distribution, biomass, and biological composition of walleye pollock in the central Aleutian Islands region between Seguam Pass and Tanaga Island. The cruise was part of a larger effort funded by the North Pacific Research Board to assess the spatial and temporal patterns in pollock distribution near Steller sea lion rookeries and haulouts.
The cruise was also an ecosystem survey, so several other objectives were included in the cruise plan. Physical oceanographic data (temperature, salinity, chlorophyll, and nutrient profiles) were collected at selected sites with conductivity-temperature-depth (CTD) profilers and Niskin bottle water samples. Sea surface temperature, salinity, nitrate, and fluorescence data were collected continuously throughout the cruise. Bongo nets were deployed at selected sites to sample ichthyoplankton and zooplankton. Stomach specimens and gonads from selected species were collected to elucidate key predator-prey relationships and spawning behavior in the Aleutian Islands region.
Seabird and marine mammal surveys were conducted by scientists from the U.S. Fish and Wildlife Service (USFWS) and the AFSC’s National Marine Mammal Laboratory, respectively. There were also several special projects completed, such as the collection of gadoid fish livers, genetic samples, and samples for microchemical analysis of fish eye lenses and otoliths. Additional details of this survey are provided below.
- Echo-Integration Trawl
The Oscar Dyson deck crew bringing in a survey trawl.
The Oscar Dyson crew deploying the CTD rosette.
Male killer whale with distinctly white coloration. (Photo taken under NMFS Scientific Research Permit No. 782-1719). Photo by Holly Fearnbach.
Survey operations were conducted 24 hours per day. The primary echo integration-trawl (EIT) survey operations were conducted during nighttime hours (approximately 12 hours per day). Acoustic data were collected continuously along a series of parallel transects. Transect spacing was 2.5 nautical miles (nmi), except in areas of anticipated high pollock biomass where transect spacing was smaller, at 1.25 nmi. See Figure 9 for transect locations. Daytime activities included making CTD casts, deploying the Bongo net, and conducting additional trawls for biological samples.
Trawl hauls were conducted to identify echosign and to provide biological samples (a.k.a. "verification tows"). Individual pollock from the verification tows were sampled to determine sex, fork length, body weight, age, and maturity. Maturity was determined by visual inspection and categorized as immature, developing, prespawning, spawning, or postspawning. Other acoustic fish targets, primarily Pacific ocean perch (POP), were sampled for sex and fork length. Trawl locations are shown in Figure 10. A total of 14 trawls were conducted during this cruise.
- Oceanographic Sampling
Conductivity-temperature-depth data and Niskin bottle samples were collected with the vessel’s CTD/rosette system. CTD profilers were deployed opportunistically throughout the survey, primarily during the day, at water depths from 100 to 200 m (Fig. 11). Additional CTD casts were made at selected areas, shown in Figure 11. In these areas, CTDs were deployed at water depths around 100 to 200 m and around 500 m. A total of 26 CTD casts were made during this cruise. In addition to the CTD oceanographic data, the ship’s Scientific Computing System was configured to log data from temperature, salinity, nitrate, and fluorescence sensors.
- Zooplankton Sampling
Bongo tows were conducted to 400 m, or 10 m off the bottom, whichever was shallowest. Tows were conducted during the day, at three different depths, if time allowed: shallower than 200 m
water depth, between 200 and 400 m, and offshore at 400 m. Bongo tows and CTD casts were made at the same stations whenever possible. Figure 11 shows the location of Bongo tows and CTD casts. A total of nine Bongo tows were made during the cruise.
- Seabird Operations
A USFWS seabird observer conducted observations when the vessel was under way. The observer operated from the bridge (on the port side) for several hours at a time during daylight hours. The observer surveyed a total of 1,198 km during 14 survey days. Weather and sea conditions prohibited surveys during portions of most days. On transect, a total of 2,900 marine birds, and 17 marine mammals were recorded. Marine mammals were sperm whale, killer whale, and Dall’s porpoise. Twenty-one species of birds were recorded, with five species accounting for 85% of the total. The most abundant birds were whiskered auklets (Aethia pygmaea; 35% of total birds), northern fulmar (Fulmarus glacialis; 33%), Laysan albatross (Phoebastria immutabilis; 5 %), glaucous-winged gull (Larus glaucescens; 6%), and common murre (Uria aalge; 7%).
- Marine Mammal Operations
Marine mammal observations were conducted over a total of 2,080 nmi. "On effort" marine mammal observations were conducted over 500.4 nmi, resulting in 18 sightings of three different cetacean species: killer whales (5), sperm whales (2), and Dall’s porpoise (11). Four of the killer whale sightings were confirmed from photographs to comprise the fish-eating "resident" lineage. One of these groups contained an adult male with distinctly white coloration, and identification photographs revealed a match to a 2001 sighting of a white whale in the central Aleutian Islands. An additional sighting of a single killer whale was recorded, but was not resighted or photographed. Photographs were also obtained from one group of sperm whales, which was notable for containing relatively small animals of varying sizes, indicating that this may have represented a nursery group of females and their young.
