Seabed mapping to develop a habitat-based catchability function for the Bristol Bay red king crab (Paralithodes camtschatica) stock assessment

Principal Investigator: Robert McConnaughey and William Stockhausen, Alaska Fisheries Science Center, Seattle, WA. Scott Goodman, Bering Sea Fisheries Research Foundation, Seattle, WA

Co-Principal Investigators:

External Collaborators:


Stock assessment models use simplifying assumptions to account for complex and poorly understood phenomena. For example, estimates of abundance are commonly based on bottom-trawl surveys where it is assumed that all fish and crabs in the path of the net are captured. However, this component of catchability (capture efficiency) is often less than 1.0 due to escapement under the footrope or may exceed 1.0 due to horizontal herding. The common practice of using a single value of catchability for an entire survey compounds the estimation problem, if environmental factors cause significant spatial or temporal variability in survey-trawl performance. If these effects are not removed, inaccurate data are used in the stock assessment thereby introducing error into the fishery management process.

Crab populations support extremely valuable fisheries on the Alaska continental shelf, but are characterized by sizable fluctuations in abundance and occasional stock collapses. The NMFS assessment survey in the eastern Bering Sea (EBS) is a generalized bottom trawl survey that provides abundance and biomass estimates for crab, demersal and semi-demersal fish species alike, and as such it is not optimized for assessment of crab stocks. Moreover, field studies have shown that the capture efficiency for the standard NMFS 83-112 survey-trawl may be low and spatially variable. In particular, the overall catchability for EBS snow crab (Chionoecetes opilio) in the NMFS survey can be as low as 60% of that for an alternative net. Catchability also varies for different sexes and size classes according to sediment texture and depth. The capture efficiency of the NMFS trawl reportedly decreases with depth and increases with grain size, likely due to differences in the height of the footrope above the substrate. Accounting for these effects in the stock assessment process using a trawl-efficiency model based on side-by- side comparative tows and interpolated sediment samples produced a 64% increase in the 2011 Over Fishing Limit (OFL) for snow crab.

In general, development of sediment-based catchability functions is limited by the availability of data collected with traditional (inefficient) point sampling gears, such as grabs and corers. An alternative is to use acoustic methods to map and quantify seafloor properties, as was successfully done for EBS species-distribution models using backscatter from various sonar types. These findings prompted a project to evaluate the use of single-beam- echosounder data to quantify seabed-related effects on survey-trawl performance and to develop new Bayesian priors for use in the EBS snow crab assessment models (HAIP study #13-016, McConnaughey and Somerton). The new trawl-efficiency models based on acoustic data outperformed the broadly interpolated sediment data used in the previous study, for both male (+6%) and female (+35%) snow crab in terms of explained deviance. It is anticipated that processed data from more sophisticated classes of sonars will be even more informative to this (and other) stock assessments utilizing data from the NMFS 83-112 survey trawl.

Red king crab catchability

The current study is designed to acquire new acoustic data and develop a habitat-specific catchability-at-size function for the Bristol Bay red king crab (BBRKC; Paralithodes camtschaticus) stock. It will also generate a consistent set of quantitative habitat data to characterize the seafloor and water quality on the eastern half of the EBS continental shelf. These foundational habitat data will be broadly applicable to other (shell)fish-assessment, scientific, and management needs in the region. In the process, new hydrographic data will be used to update nautical charts for areas with outdated or non-existent bathymetric information.

The project team of biologists, physical scientists, engineers, and technicians from the Alaska Fisheries Science Center, the NOAA Marine Operations Center – Pacific, the Pacific Hydrographic Branch and the Atlantic Hydrographic Branch, the US Navy, and private industry collected new acoustic data with three different sonars during a 19 day cruise on the NOAA Ship Fairweather (9-27 August 2016). Fairweather is a multi-mission survey vessel equipped with a hydrographic- quality multibeam echosounder (Kongsberg EM710, 73-97 kHz), as well as specialized equipment to support the project's high-speed broad-coverage sidescan sonar system (Klein 7180 LRSSS, 180 kHz) and the independent vertical-incidence single-beam echosounder (38 kHz) mounted on the LRSSS towfish. Quantitative backscatter and bathymetric data were simultaneously and continuously acquired with the three sonars along tracklines defined by the standard NMFS bottom-trawl survey stations (Fig. 1). Physical measurements and samples of the seafloor were collected periodically to aid the interpretation of backscatter data during post-processing. This project focused on the eastern half of the EBS survey grid, fully covering the Bristol Bay region where red king crabs are found.

The acoustic data will be processed with new generations of software (FMGT by QPS and IMPULSE by Maritime Way Scientific, Ltd. 1) that are designed to produce quantitative information about seabed characteristics and then combined with data from paired fishing experiments that estimate the relative efficiency of the 83-112 survey trawl. On three occasions (2013-2015), the Bering Sea Fishery Research Foundation 2 (BSFRF) simultaneously collected catch and effort data for the 83-112 and a specially designed Nephrops trawl along parallel courses ~400 m apart. Study findings ultimately will be used to adjust station-specific size compositions and the overall-abundance time series that are integral to the length-based, stock-assessment model currently used by the State of Alaska and NMFS to co-manage the BBRKC stock.


1 Reference to trade names does not imply endorsement by the National Marine Fisheries Service, NOAA.
2 The BSFRF is an industry partner that supports cooperative research to improve the sustainability and management of fishery resources of the Bering Sea, with emphasis on the commercially valuable crab stocks. http://www.bsfrf.org/index.htm

New and improved information for EBS crab stock assessments is particularly important given recent developments. For the 2017 fisheries, the Total Allowable Catch (TAC) for snow crab is the lowest in 45 years, the BBRKC TAC is reduced 15% (1.5 million pounds), and the Tanner crab fishery will be closed; all because of overly low estimates of stock abundance.



Figure 1. Summary of survey operations during the 2016 cruise on NOAA Ship Fairweather. The survey tracklines (green) deviated from the planned lines (red) because of poor weather conditions. LNM is the abbreviation for linear nautical miles.

Klein 7180 long-range sidescan sonar

Figure 2. The Klein 7180 long-range sidescan sonar collected quantitative information to characterize the seafloor and measure water column properties. Note the unusually calm seas and turquois-colored waters that are indicative of the coccolithophorid bloom that was observed over a broad expanse of the survey area.

Control room

Figure 3. Control room for operating the project's survey systems installed on NOAA Ship Fairweather. From left to right are the data archiving system that managed nearly 1 TB of acoustic data per day, the USBL acoustic positioning system, the integrated navigation computer, remote controls and closed-circuit video for the tow winch, and the top side acquisition and processing computers for the long-range sidescan sonar. The hull-mounted Kongsberg EM710 multibeam echosounder was operated by ship's personnel at a different location on the vessel.

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InPort ID#




Alaska Fisheries Science Center (AFSC)


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