A combined Bering Sea acoustic-trawl and bottom-trawl survey: Evaluating the feasibility of midwater trawl sampling for pollock size composition on the bottom trawl survey vesse...

BACKGROUND

Walleye pollock in the eastern Bering Sea (EBS) are assessed every summer with a bottom trawl (BT) survey for the demersal component of the stock and every other summer with both a BT and an acoustic-trawl (AT) survey for the demersal and pelagic (i.e., midwater) components of the stock. The BT survey is conducted from two commercial fishing charter vessels and the AT survey is conducted from the NOAA Ship Oscar Dyson. When both the BT and AT surveys are done during the same year, they are not completely synchronous because the vessels work independently within the survey area. Conducting two different surveys for the same stock is not only expensive and time consuming, but the asynchronous surveys also result in additional uncertainty associated with spatial and temporal changes in pollock vertical distribution in response to environmental stimuli (Kotwicki 2014). Recent research suggests that combining BT and acoustic survey data can produce the most reliable survey index of abundance for stock assessment purposes, when both BT and acoustic data are simultaneously collected from the same vessel (Kotwicki 2014). A combined BT-acoustic survey for semi-pelagic species is possible and can provide better accuracy and precision in abundance estimates than by either survey separately. However, there are challenges that need to be addressed before a combined AT-BT estimate of EBS pollock abundance becomes a reality.

The AFSC has been leading an effort to combine the AT and BT surveys for walleye pollock with the acoustic vessel-of-opportunity (AVO) index of midwater pollock biomass (Honkalehto et al., 2011), and more recently with the development of methodology for combining acoustic data collected in conjunction with the BT survey (Kotwicki et al., 2013). Acoustic data for the AVO index has been collected routinely during BT surveys since 2006, and the AVO index has been used in the EBS walleye pollock stock assessment since 2010 as an additional fishery-independent index of pollock abundance (Ianelli et al., 2013). The AVO index uses acoustic data collected opportunistically by the two BT survey charter vessels to provide an index of relative abundance of midwater pollock during those biennial years when AT abundance estimates of are not available; however, in contrast to the AT survey index, the AVO index does not provide estimates of abundance-at-age. To build upon the AVO index and work towards a combined BT-

acoustic survey index that preserves the Tier 1 status of the Bering Sea pollock assessment, it is necessary to evaluate the feasibility of conducting midwater tows to sample midwater backscatter layers for confirming species classification and to collect biological specimens for converting the backscatter data into pollock abundance-at-age.

APPROACH

We propose to continue the step-wise development of methods for a combined BT-acoustic survey index. Techniques for capturing midwater pollock aggregations with the BT survey trawl net were developed in 2013 and 2014; however, in assessing the time requirements, it was determined that additional vessel charter days are needed in 2015 to successfully incorporate routine midwater tows into the survey framework. Midwater tows on targets of suspected pollock need to be done whenever dense scattering layers are encountered during routine survey operations to confirm the identity of the acoustic targets and to obtain length information. Successful completion of the EBS shelf BT survey would be jeopardized without compensating for the time lost doing additional midwater tows because time requirements for a midwater and bottom trawl tow are the same. We propose to conduct a total of 30 midwater tows on targets of suspected pollock scattering layers during the 2015 EBS shelf BT survey. The additional charter days will be used to conduct midwater tows with the 83-112 EBS shelf BT for obtaining species composition and size and age samples from midwater pollock. Data from the 30 additional midwater tows should be sufficient for calculating the statistically-required levels of midwater trawling effort needed in future years to obtain acoustic estimates of total abundance-at-age that are comparable to those obtained by the AT survey on the NOAA Ship Oscar Dyson. Therefore, we request $64.76K in funds for five (5) additional charter vessel days and the associated fuel costs.

EXPECTED BENEFITS

A statistical, age-structured assessment model is used for the assessment of pollock in the EBS (Tier 1). Thus, survey estimates of total pollock abundance, as well as abundance-at-age are required as model inputs. The AVO index is currently based only on pollock backscatter because no pollock size composition data are available to derive abundance-at-age estimates. Size information from midwater tows made during the BT survey will provide an estimate of the pelagic pollock abundance-at-age and total abundance for the AVO index. Such an improvement to the AVO index provides a basis for evaluating the feasibility of collecting acoustic and size data in conjunction with BT surveys. A combined AT-BT survey can provide researchers with abundance estimates corrected for spatially- and temporally-variable catchability of each sampling method (e.g., vertical availability of pollock to BT or acoustic sounder changing in response to light level or to the size and extent of the EBS shelf cold pool). A combined estimate would also improve our understanding of spatial and temporal changes in pollock distribution and abundance, and provide more accurate estimates of uncertainty in abundance indices. Moreover, information on both the demersal and pelagic components of the pollock stock can be provided to assessment scientists annually rather than biennially, and once fully adopted by the Fishery Management Council, would also free-up the NOAA Ship Oscar Dyson to focus on other critical fisheries research needs in Alaska.

This project was in response to the Improve a Stock Assessment RFP.

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