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Broadband acoustic scattering techniques are not widely used in fisheries acoustics, but this may change due to the recent commercial availability of a broadband echosounder system operating at frequencies commonly used in fisheries surveys. A four-channel (15–150 kHz) broadband echosounder was used to investigate the potential of broadband methods to improve species discrimination during a walleye pollock (Gadus chalcogrammus) survey in the Gulf of Alaska. Narrowband echosounders combined with mid-water and bottom trawls were used to identify aggregations of interest for broadband measurements. Broadband frequency responses were measured for multiple pelagic and semi-demersal fishes as well as euphausiids. No clear patterns in the broadband frequency responses were identified that would aid in discrimination among the commonly encountered swimbladder-bearing species. The results are consistent with narrowband observations and suggest that both techniques face the same challenges when attempting to discriminate among acoustically similar species as frequency responses overlap within the measured bandwidth. However, examples are presented in which broadband frequency responses provide additional information about near-resonant scatterers. The benefits of broadband operations have not been fully realized and widespread adoption of broadband techniques and improved processing algorithms may yield improved acoustic-based species discrimination for use during fisheries surveys.

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Acoustic trawl surveys were conducted in 2000 and 2001 in two troughs located off the eastern coast of Kodiak Island in the Gulf of Alaska as part of a multiyear, multidisciplinary experiment to examine the influence of environmental conditions on the spatial distribution of adult and juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Continuous underway sea surface temperature samples and water column profiles collected in 2000 and 2001 showed the presence of a sharp shelf-break front in Chiniak Trough and a mid-trough front in Barnabas Trough. At distances <22 km from shore, the water column was well mixed, whereas a welldefined mixed layer was present beyond approximately 22 km from shore. Satellite drifter tracks in Barnabas Trough entered along the upstream edge of the trough and appeared to follow the frontal boundary across the middle portion of the trough. A storm in 2001 weakened stratification and cooled surface water temperature by 1.6-2.1°C. Wind mixing associated with the storm event mixed subsurface chlorophyll a to the surface and enhanced nutrients in the surface waters. The storm event revealed spatial partitioning of summer production in Barnabas Trough, with production concentrated in regions inside the mid-trough front. In contrast, post-storm summer production was distributed throughout Chiniak Trough. The spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat characteristics. Acoustic survey data identified four acoustic sign types: age-1 pollock, adult pollock, capelin, capelin-age-0 pollock mix. The spatial distribution of these four sign types appears to be influenced by the oceanographic and topographic features of the two troughs. Adult pollock were broadly distributed throughout Chiniak Trough, whereas adult pollock were aggregated on the coastal side of the frontal system in Barnabas Trough. In 2000, capelin occurred with age-0 pollock. In Chiniak Trough, capelin were most abundant along steep topographic gradients at the edges of the trough and in a deep region near Cape Chiniak, whereas the capelin-age-0 mix (2000) or capelin (2001) concentrations were observed in slope water intrusions over the outer shelf in Barnabas Trough. Results suggest that habitat selection of walleye pollock and capelin are controlled by different processes. Capelin distributions appear to be limited by oceanographic conditions while other factors appear to be more important for pollock.