Estimates of availability and catchability for select rockfish species based on acoustic-optic surveys in the Gulf of Alaska
Introduction
Statistical catch-at-age assessments of marine fishes are the backbone of modern fishery stock assessments (Hilborn, 1992; Quinn and Deriso, 1999; Deriso, 1980). These assessments typically rely on fisheries-independent time series of stock biomass from sources such as standard bottom trawl surveys (von Szalay and Raring, 2016), acoustic-trawl surveys (Stienessen et al., 2017), longline surveys (Sigler, 2000), and/or underwater camera surveys (O’Connell and Carlile, 1993; Yoklavich et al., 2000). For some species such as rockfishes, which can be difficult to age, biomass estimates from fisheries-independent surveys can be used directly to set total allowable catch using methodologies such as Kalman filters (Spencer and Ianelli, 2005) or multi-year running averages (NPFMC, 2017). Stock assessments are thus largely dependent on unbiased estimates of stock size from fisheries-independent surveys.
For fisheries-independent surveys, the estimated abundance is a function of the catchability (q) of the trawl survey, which scales the area-swept survey estimates to population abundance. Catchability for a species and gear is composed of two primary components, the effectiveness of the gear in capturing the species (i.e., species vulnerability to capture) and the species availability in terms of the survey area and in relation to the gear (Arreguín-Sánchez, 1996; Cordue, 2007; Somerton et al., 1999). When components of the catchability coefficient are uncertain, error may be introduced into the assessment to bias resulting population estimates. Gear effectiveness could be related to length (age) selectivity based on such factors as gear mesh size, and could also be related to behavioral responses of the species to the gear such as herding or diving. The availability component of catchability is the probability of the fishing gear encountering individuals in the population (Marr, 1951). If a population is not uniformly distributed in time and space, or a fraction of the population is in habitat that is inaccessible to the sampling gear, a constant value or prior distribution on q based on auxiliary information is informative to account for availability, and thus catchability, of individuals of a species.
Population assessments for fisheries in the Gulf of Alaska (GOA) rely heavily on the biennial bottom trawl survey conducted by the Alaska Fisheries Science Center (AFSC). The bottom trawl survey routinely encounters areas deemed untrawlable due to high vertical relief. Many fish species, especially rockfishes, associate with, and find refuge in, high-relief substrate, where bottom trawl surveys are ineffective (O’Connell and Carlile, 1993; Yoklavich et al., 2000; Zimmermann, 2003). Abundance of many rockfishes can vary considerably between trawlable and untrawlable areas (Jagielo et al., 2003; Jones et al., 2012; Krieger and Sigler, 1996; Rooper et al., 2007, 2010; Stein et al., 1992). Nevertheless, mean estimates of species catch-per-unit-effort (CPUE) from locations sampled by the bottom trawl are expanded across the entire survey area, including untrawlable areas, to estimate the population biomass. This extrapolation of abundance from trawlable to untrawlable areas thus represents a substantial source of uncertainty (and potential bias) in absolute survey abundance estimates. Species that inhabit both habitat types to differing degrees should have corrections applied to their catchability coefficient to account for the proportion of the stock that is unavailable to the sampling gear (Cordue, 2007).
Acoustic-trawl surveys can effectively assess pelagic rockfish populations in areas of relatively low relief (Wilkins, 1986; Richards et al., 1991; Stanley et al., 2000; Krieger et al., 2001), to permit water-column estimates of abundance in untrawlable habitat. However, fishes on the seabed or in close proximity to the seabed within the acoustic dead zone will be largely undetected (Ona and Mitson, 1996), particularly in areas where the bottom terrain is rough or variable (Demer et al., 2009). In areas of high topographical relief, where abundance estimates are not possible near the seabed using acoustics and trawling, stereo camera surveys can provide information on species composition and abundance (Jones et al., 2012; Rooper et al., 2010).
The primary objective of this study was to use a combination of acoustic backscatter measurements and underwater imaging to improve estimates of availability of four rockfish species in the GOA; Pacific ocean perch (POP; Sebastes alutus), northern rockfish (S. polyspinis), dusky rockfish (S. variabilis), and harlequin rockfish (S. variegatus) to the NMFS GOA bottom trawl survey. This objective was accomplished by conducting acoustic surveys and deploying a lowered stereo camera (LSC) in grid cells designated as trawlable or untrawlable to determine substrate trawlability and fish species abundance. These data were used to develop estimates of density in the different habitat types and ultimately determine the availability, and thus catchability, of specific rockfish species to a survey bottom trawl.
Section snippets
Methods
This study was conducted across the GOA shelf during the biennial GOA acoustic-trawl surveys that took place June 8 - Aug. 9, 2013 (Jones et al., 2014), June 11 - Aug. 16, 2015 (Jones et al., 2017), and June 12 - Aug. 14, 2017 (Jones et al., 2019). All of the surveys were conducted aboard the NOAA ship, Oscar Dyson. At the completion of daily survey activities, sampling areas were selected from nearby “trawlable” or “untrawlable” grid cells as designated by the AFSC GOA bottom trawl survey
Results
A total of 110 BTS grid cells were surveyed in all years combined, of which 52 were designated as T, 43 were designated as UT, and 15 were not yet classified as T or UT (Fig. 1, Supplemental Table 1). A total of 228 lowered stereo camera deployments were conducted (Supplemental Table 1). The majority of grid cells (82 %) had at least two camera deployments completed within them. LSC images were not collected within 6 acoustically surveyed grid cells due to connectivity issues or weather
Discussion
Trawlability designation of an area is determined by the scale at which it is evaluated. The AFSC bottom trawl survey determines trawlability based on topography and whether or not a 15-minute bottom trawl haul at 3.0 knots can be successfully placed in any portion of a 25 km2 grid cell. This means that each trawlable grid cell could potentially have some portion of untrawlable habitat contained within it and vice-versa. In contrast, the determination of trawlability using the LSC was based on
CRediT authorship contribution statement
Darin T. Jones: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Christopher N. Rooper: Conceptualization, Methodology, Funding acquisition, Resources, Formal analysis, Writing - review & editing. Christopher D. Wilson: Conceptualization, Methodology, Funding acquisition, Resources, Supervision, Writing - review & editing. Paul D. Spencer: Validation, Writing - review & editing. Dana H. Hanselman: Validation, Writing - review & editing. Rachel E. Wilborn:
Declaration of Competing Interest
None.
Acknowledgements
The research was funded by the Alaska Fisheries Science Center and Essential Fish Habitat project #2015-07. We thank Pete Hulson, Cindy Tribuzio, Curry Cunningham, and Kresimir Williams for suggestions and comments on the manuscript. We also thank Rick Towler and Kresimir Williams for design and expertise in stereo camera and analysis software development and construction. The constructive comments and suggestions provided by anonymous reviewers and the Editor are greatly appreciated.
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Present address: Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, British Columbia, Canada V9T 6N7.