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Midwater Assessment & Conservation Engineering (MACE)

Bering Sea Summer 2004 Pollock EIT Survey

MACE Program scientists conducted an echo integration-trawl (EIT) survey between 5 June and 1 August 2004 on the U.S. and Russian Bering Sea shelf near Cape Navarin aboard the NOAA ship Miller Freeman. This was the first year since 1994 that permission was granted to survey in Russian waters. The principal objective of the survey was to collect echo integration and trawl data to estimate midwater pollock abundance and distribution. Scientists from the AFSC’s National Marine Mammal Laboratory (NMML) were aboard to census marine mammals along the EIT survey track during Leg 1. Four Russian scientists participated in the survey.

The planned survey design, including Russian waters, consisted of 31 north-south transects spaced 20 nmi apart over the Bering Sea shelf from Port Moller, Alaska, to Cape Navarin, Russia. Due to the ship’s mechanical difficulties, only the southern portion of transect 30 was completed, and transect 31 was not completed (Fig. 8 below). Echo integration and trawl data were collected during daylight hours. Nighttime operations included additional trawling, target strength data collection, field-testing of a midwater trawl with a multiple-opening-closing codend device, net selectivity experiments, and acoustic system testing.

Figure 8, see caption
Figure 8.  Relative backscattering (NASC) attributed to pollock observed between 12 m from the surface and 3 m off bottom along tracklines during the summer 2004 echo integration-trawl survey on the Bering Sea shelf, MF2004-08. Transect numbers are underlined.

Biological data and specimens were collected from 154 trawl hauls, which included: 117 hauls with a large midwater trawl; 20 with a bottom trawl; 9 with a Marinovich trawl, and 8 with a Methot trawl. Walleye pollock was the most abundant and jellyfish (Cnidaria) was the second most abundant taxon captured by weight in both midwater and bottom trawl hauls. Jellyfish were the dominant species group by weight for both the Marinovich and Methot trawls. Numerically, age-0 pollock dominated Marinovich catches, and euphausiids dominated Methot catches. Trawl catches indicated that east of 170°W, pollock 38 cm to 50 cm in length were on average 8% heavier than pollock west of 170°W. Fewer than 1% of the pollock larger than 29 cm fork length (approximately age 3 and older) were actively spawning. Most pollock were either in the developing or post-spawning maturity stage.

Pollock were observed on all but the first transect (Fig. 8 above). In the U.S. EEZ most pollock were observed west and southwest of St. Matthew Island (transects 20-26). In Russia’s EEZ, most were observed near the north ends of transects 26 and 29. Preliminary abundance estimates for pollock between 12 m from the surface and 3 m off the bottom indicated that approximately 90% of the total biomass was found in the United States and about 10% was observed in Russia, off Cape Navarin. Of the U.S. pollock abundance, about one-third of the population was located east of 170°W and about two-thirds was observed west of 170°W. The largest pollock were observed east of 170°W, where the predominant length mode was 44 cm and relatively few juveniles were observed (Fig. 9 below). As the survey continued west, the predominant length mode became 39 cm in the U.S.-west of 170°W region. Off Cape Navarin, the predominant length mode was 31 cm, and relatively few adult pollock larger than 40 cm were observed.

Figure 9, see caption
Figure 9.  Estimated length distribution of pollock observed between 12 m from the surface and 3 m off bottom in three regions during the summer 2004 Bering Sea shelf echo integration-trawl survey, MF2004-08.

By Taina Honkalehto

Trawl Avoidance Study

As part of the 2004 eastern Bering Sea Bottom Trawl survey, scientists from the MACE and the Groundfish Assessment Programs conducted a collaborative experiment designed to evaluate the behavior of walleye pollock in response to cues from oncoming vessels actively engaged in trawling operations. The study was conducted aboard the chartered fishing vessel Aldebaran in the vicinity (~58°30 N, 172°40 W) of Zemchug Canyon between 26 July and 5 August. A free-drifting buoy equipped with a calibrated 38-kHz scientific echosounder and associated electronics was used to observe pollock abundance and vertical distribution as the Aldebaran towed an 83/112 Eastern bottom trawl past the buoy. The acoustic buoy was deployed on six occasions, and the trawl was towed past the buoy a total of 24 times. Data from the acoustic buoy will be used to determine whether walleye pollock exhibit a behavioral response to the approaching vessel and trawl.

By Alex De Robertis

Fishery Interaction Study in Gulf of Alaska

Field work for the fourth year of a fishery interaction experiment was completed between 12 August and 6 September off the east side of Kodiak Island in the Gulf of Alaska as a collaborative effort between RACE and REFM scientists from the AFSC. The work is part of a larger program designed to evaluate the effect of commercial fishing activity on the prey availability of walleye pollock and other forage fish species to endangered Steller sea lions (Eumetopias jubatus).

Figure 10, see caption
Figure 10.  Trackline surveyed for each pass (see text) during the August-September 2004 echo integration-trawl survey of walleye pollock off the east side of Kodiak Island, Gulf of Alaska.

The principal objective of the experiment was to use standard acoustic survey methods to describe the spatio-temporal variability in pollock abundance and distribution patterns in two troughs over a period of several weeks before and during the commercial pollock fishery. The study area consisted of a treatment site, Barnabas Trough, where commercial fishing was allowed, and a control site, Chiniak Trough, where commercial fishing was prohibited. Repeated survey passes (Fig. 10 above) were conducted within each trough before and during the fishery to document if a fishery-induced perturbation occurred in the fish distribution. To characterize the physical environment, oceanographic data were collected using drifters, CTDs, XBTs, and a vessel-mounted thermosalinograph.

Most of the acoustic backscattering was generally attributed to two principal groups: adult pollock, and age-0 pollock with some capelin (Mallotus villosus). The adults were generally detected as near-bottom schools or as an on-bottom “carpet.” Adult pollock were generally distributed within the northern half of Barnabas Trough, although significant quantities extended farther south along the eastern side of the trough. Adult pollock were distributed throughout Chiniak Trough. Relatively large, dense aggregations of age-0 pollock/capelin, located in mid-water during daylight, were broadly distributed throughout Chiniak Trough and predominantly in the northern portion of Barnabas Trough. Unlike the deeper dwelling adults, the age-0 pollock/capelin mix often extended over relatively shallow bottom depths of less than 100 m. It appeared that fewer capelin and many more age-0 pollock were encountered during the survey compared to earlier surveys in 2000-02. However, additional data analyses need to be completed to verify this finding.

The size composition of adult pollock ranged between modes of 44-50 cm fork length. The size composition of age-0 pollock had prominent length modes at 6 or 7 cm standard length.

Analyses are currently under way to determine if significant changes in fish abundance and distribution patterns were detectable in response to the commercial fishing activities. Pollock and capelin echosign spatial patterns will be explored with variography and by comparison of other descriptive parameter estimates of the fish aggregations to evaluate if the scale of patchiness changed during the study period.

By Michael Guttormsen



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