Midwater Trawls

Acoustic Backscatter to Estimate Fish Abundance

Alex de Robertis and Chris Wilson

Objectives – Determine the abundance, vertical distribution, and environmental associations of the dominant midwater organisms over the eastern Chukchi Sea shelf.

To the best of our knowledge, no quantitative assessment of the vertical distribution of midwater fishes in the Chukchi Sea has been conducted to date. Hence the abundance of midwater fish, jellyfish, and other large zooplankton (e.g., euphausiids) will be measured using acoustics. Acoustic backscatter will be continuously measured along the vessel track from the vessel conducting the surface trawl survey (see previous section). The identity of acoustic scatterers will be confirmed using a pelagic trawl and optical measurements from a drop camera. The underway measurements will be supplemented with multi-frequency acoustic measurements to increase taxonomic information (for example zooplankton abundance) from the acoustic measurements. The acoustic results will be directly comparable to pelagic surveys conducted by NOAA’s Alaska Fisheries Science Center over the eastern Bering Sea shelf (e.g. Honkalehto et al. 2005). Preliminary analysis of opportunistically collected acoustic records indicates that there may be a large biomass of midwater fish in the study area, although identity of the organisms remains unknown. The abundance of midwater organisms will be classified using echosign characteristics, trawl catches, drop camera images, and the relative frequency response (Korneliussen and Ona 2003). This midwater/acoustic survey is supported through a subcontract to UAF with funds from CIAP and with equipment and salaries for AFSC personnel provided by NOAA. As noted above, this BOEM Interagency Agreement provides for ten additional ship days for the surface and midwater surveys to extend the sampling area to include higher latitudes of the Chukchi Sea in both 2012 and 2013.

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