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Groundfish Assessment Program

Acoustic Backscatter Measurements of Walleye Pollock: Comparisons Between a Trawling and a Free-running Vessel

Figure 1, see caption
Figure 1.  Regression of acoustic area scattering values associated with trawl hauls (y-axis) against those values associated with free-runs (x-axis) in the layer below the headrope (0-2.4 m above the seabed; n = 133).  The acoustic area scattering values are on a logarithmic scale.

This study examined the potential to combine acoustic backscattering measurements collected during transits between established trawl stations to those made on-station aboard chartered fishing vessels during the eastern Bering Sea (EBS) bottom trawl survey. Scientists with the Resource Assessment and Conservation Engineering (RACE) Divisionís Groundfish Assessment Program were motivated to conduct this study by the prospect of the ability to collect additional walleye pollock abundance and distribution data to improve the precision of the time-series index of walleye pollock abundance in the Bering Sea. A previous study established a strong correlation between concurrent acoustic backscatter and trawl catch estimates of abundance of walleye pollock.

In this study, walleye pollock acoustic backscatter collected by a stern trawler at towing speeds of 3 knots was compared with that collected while the vessel was free-running at 10 knots. Acoustic backscatter was measured during the 2006 and 2007 EBS bottom trawl surveys in a pair-wise fashion at 134 trawl stations. The free-running transects were conducted either immediately before or immediately after the trawl hauls and were parallel to the trawl paths but offset by approximately 250 m. The backscatter during trawling was significantly greater in the near-bottom layer below the headrope, but the difference between trawling and free-running declined with increasing fish density (Fig. 1 above). This was also true in the layer immediately above the headrope, indicating that fish did not dive from mid-water into the path of the net.

A comparison of the seabed echo between trawling and free-running did not show a significant difference, suggesting that the difference in backscattering levels is due to difference in fish behavior and not to a difference in vessel attitude between the two operational modes. The greater backscatter values associated with trawling may be due to reactions of fish within a school that produce a coordinated swimming response, which has the potential to increase the average target strength.

By Paul G. von Szalay and David Somerton

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