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Midwater Assessment & Conservation Engineering (MACE) Program

Spring Surveys

Since 1980, scientists from the Midwater Assessment and Conservation Engineering (MACE) Program have conducted echo integration-trawl (EIT) surveys in the Gulf of Alaska and Bering Sea to assess the distribution and abundance of walleye pollock. Spring 2004 surveys were conducted in the Shelikof Strait area and along the shelf break east of Chirikof Island (Fig. 1). Instrument calibration activities for the two acoustic systems were conducted during the winter field season: one during gear trials in Port Susan, Washington, and another in Izhut Bay, Afognak Island, Alaska. The primary objectives of the surveys were to estimate the biomass, numerical abundance, and various biological characteristics of the off-bottom component of the walleye pollock populations in these areas. A secondary cruise objective was to obtain biological information for other research projects as well as conduct several survey-related research activities.

Figure 1, see caption

Figure 1.  The distribution of combined juvenile walleye pollock and pollock-eulachon mix echosign in the Shelikof Strait and Gulf of Alaska shelf break near Chirikof Island based on data from EIT Survey, 25 March 1 April 2004. Transect locations are indicated by lines across the mapped distribution.

Acoustic data were collected with a Simrad EK500 echo sounder using 38-and 120-kHz split beam transducers and with a Simrad ER60 echo sounder using 18-, 38-, 120-, and 200-kHz split beam transducers. Data were recorded using SonarData EchoLog and analyzed using the PC-based SonarData Echoview post-processing software. Data for the four frequencies were also logged using ER60 software. Results presented here are based on EK500 38-kHz data.

For the first time since the 2000 survey, significant amounts of prespawning pollock were detected on the west side of Shelikof Strait, although the abundance was lower than in the mid- to late 1990s (Fig. 1). Significant quantities of adults (mostly assumed to be age-5 pollock) were also detected just outside the mouth of the strait. Few age-1 pollock were detected in the Shelikof Strait area. Along the shelf break near Chirikof Island, most of the pollock echosign was detected in the bight between 154W and 155W at 275-500 m water depths over bottom depths of 350-800 m. No pollock shorter than 37 cm fork length (FL) were caught in trawl hauls conducted in this area.

In the Shelikof Strait area, pollock were the most abundant species by weight in the 13 midwater trawl hauls, comprising 88% of the total catch. Pollock comprised 60% of the catch in the two bottom trawl hauls. Pollock comprised 99% of the catch in the four midwater trawl hauls conducted along the shelf break east of Chirikof Island.

For females greater than 40 cm FL in the Shelikof Strait area, 3% were immature,13% were developing, 77% were mature prespawning, 5% were spawning,and 1% were spent. The low percentage of spawning and spent fish (6%) indicated that survey timing was appropriate. For females greater than 40 cm FL along the shelf break near Chirikof Island, 0% were immature, 65% were developing, 32% were mature prespawning, 2% were spawning, and 1% were spent. The low percentage of spawners and postspawners (3%) indicated that peak spawning had not yet occurred. The high ratio of developing to mature pre-spawning females is consistent with previous survey results where the majority of fish are in the 40-45 cm FL range.

By P. Tyler Yasenak

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