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Midwater Assessment & Conservation Engineering Program

Miller Freeman and Oscar Dyson Intervessel Comparison

Scientists from the Midwater Assessment and Conservation Engineering (MACE) Program continued an intervessel comparison of acoustic-trawl survey data collected aboard the NOAA ships Miller Freeman and Oscar Dyson during the Bogoslof and Shelikof Strait surveys. The primary goal of this work is to investigate if walleye pollock differentially avoid the Oscar Dyson, which was designed to meet the ICES specification for underwater radiated noise to minimize vessel avoidance during fish abundance surveys, and the Miller Freeman, a conventionally built vessel which exceeds this specification.

It is possible that fish will respond differently to the vessels due to the disparate auditory stimuli produced by each vessel. If this is the case, differential vessel avoidance may influence the biomass estimates derived from standard survey methods with the two ships. This is of particular interest for resource management in Alaska waters as the Oscar Dyson is scheduled to become the primary vessel for walleye pollock acoustic-trawl surveys, which have historically been conducted aboard the Miller Freeman.

Both vessels continuously collected acoustic backscatter at 18, 38, 120 and 200 kHz while traveling in close proximity to one another. The two-part experimental design, which was developed and implemented by MACE staff, consisted of a component in which the vessels traveled side-by-side at a distance of 0.7 nautical miles (nmi) along survey tracklines, which allowed for standard survey operations without compromising the data for use in stock assessment, and a component in which one vessel followed the other at a distance of 1 nmi.

Acoustic data from both vessels were collected over a wide range of densities of adult walleye pollock and conditions typical of acoustic surveys in these areas. Analysis of these data is in progress. Further fieldwork comparing the two vessels is planned for the winter and summer of 2008.

Winter Surveys in Gulf of Alaska and Southeastern Bering Sea Near Bogoslof Island

MACE scientists conducted winter echo integration-trawl surveys aboard the NOAA ship Miller Freeman in the Gulf of Alaska (GOA) and southeastern Bering Sea near Bogoslof Island. The surveys provide data on the abundance, distribution, and biological composition of spawning walleye pollock (Theragra chalcogramma). Areas surveyed in the GOA between 6 and 14 February included the Shumagin Islands, Sanak Trough, and Morzhovoi Bay (Figure 1), and between 11 and 29 March included the Shelikof Strait area, Chirikof shelf-break area, and Marmot Bay (Figure 2).

The area in the vicinity of Bogoslof Island in the Bering Sea was surveyed from 1 to 10 March (Figure 3). All surveys were conducted 24 hours per day.

The GOA survey results indicated that walleye pollock abundance decreased in 2007 compared to 2006 in all areas. Nevertheless, localized dense adult walleye pollock aggregations were observed in some of the surveyed areas. The densest aggregations were located in the northern part of Sanak Trough and relatively high fish densities were detected in Shumagin Trough, along the eastern portion of the Chirikof shelf-break near the mouth of Barnabas Trough, and in the southern end of the Shelikof Strait proper. In Marmot Bay, which was surveyed for the first time this year, the highest densities were located in the western end of the bay.

Walleye pollock size compositions for Sanak Trough, Morzhovoi Bay, and Chirikof shelf-break GOA surveys were unimodal, with most fish between 50 and 60 cm fork length (FL). In the Shumagin Islands and Marmot Bay, age-1 (11 to 12 cm FL modes) and age-2 (19 to 22 cm FL modes) fish dominated the size compositions. In Shelikof Strait the size composition was broadly distributed, with age-1 (12 cm FL), age-2 (21 cm FL), age-3 (30 cm FL) and adult length modes (53 cm FL). Preliminary analysis of maturity stages indicated that survey timing was appropriate for the Shumagin, Shelikof Strait, Chirikof shelf-break, and Marmot Bay surveys but may be scheduled slightly earlier for future Sanak Trough and Morzhovoi Bay surveys.

The pollock abundance estimate this year for the southeastern Bering Sea survey near Bogoslof Island was similar to the 2006 estimate. Adult pollock were concentrated in two main areas as in recent years’ surveys: northeast of Umnak Island, and north of Samalga Pass between the Islands of Four Mountains and Umnak Island (Figure 3).

In 2007 most pollock were in the Samalga region, whereas in 2006 most were in the Umnak region. The pollock size composition ranged between about 39 cm and 72 cm FL in both areas and was characterized by a dominant mode at about 50 cm FL, with a weaker mode at about 61 cm FL. The majority of the dominant-mode pollock were most likely from the 7-year-old, 2000 year class. Preliminary analysis of maturity stages indicated that in 2007 the majority of females were prespawning in both Samalga and Umnak, in contrast to 2006 when most Samalga females were prespawning, but most Umnak females were postspawning.

By Chris Wilson


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