Shima et a\ Spatial dislributlon of Theragm chalcogramma in the Gulf of Alaska 



309 





Ji Kadiak Island 



Kodiak  area 630 



Shumagin - area 610 



Gulf of Alaska 



Figure 2 



Example of the spatial coverage of the triennial bottom trawl survey for a sample year. Dots 

 indicate station locations. 



and area boundaries defined by the North Pacific Fisheries 

 Management Council (http://www.fakr.noaa.gov/rr/figures/ 

 fig3.pdf). Allocation of sampling effort within each stra- 

 tum was based on coefficients of variation, mean CPUE, 

 and sampling densities for all fish species from data col- 

 lected on previous triennial surveys (see Cochran, 1977). 

 Sampling density within a stratum depended on the antic- 

 ipated fish density of that particular stratum (Martin and 

 Clausen, 1995; Stark and Clausen, 1995; Martin, 1997). 

 Stations were prioritized within each stratum so that in 

 the event of complications, sampling density would remain 

 controlled. The total survey area was reduced by 7% after 

 1987 when stations deeper than 500 m were eliminated 

 (Stark and Clausen, 1995). In the interest of consistency, 

 only data from stations less than 500 m in bottom depth 

 were used in our analyses. Station density was spread 

 over a wide range; thus the survey data were most suit- 

 able for analyses of geographic distribution and commu- 

 nity composition on a large scale (Fig. 2). 



For each survey a Nor'eastern trawl (NT) with a 1.25 

 inch codend liner was used. The liner helped retain juve- 

 nile pollock (Brown, 1986). The nylon and polyethylene 

 NTs had net dimensions of 18.3 m wide by 4.7 m high 

 and 18.3 m by 5.5 m, respectively. In 1984 and 1987 a Jap- 

 anese bottom trawl (JBT) was also used. The horizontal 

 opening of the JBT ranged from 19 to 30 m, the vertical 

 opening from 3.2 to 3.3 m (Brown, 1986). CPUE from the 

 JBT were corrected (using Tables 28 and 31 in Munro and 



Hoff, 1995) to account for differences between the catch- 

 ability of the NTs and the JBT. 



Data analyses and statistical considerations 



Analyses were conducted on total pollock catch as well as 

 on four size groups of pollock: <150 mm (age-0 or young- 

 of-the-year); 150-230 mm (age-1 juveniles); 230-330 mm 

 (age-2 juveniles); and >330 mm (adults). The length-at-age 

 categories were based on a histogram of pollock lengths 

 from a historical database of all NMFS research surveys 

 conducted during the summer. The catchability of age-0 

 pollock may not have been as high as that for the other 

 juvenile age classes and thus supports the interpretation of 

 all juvenile data as index values (Bailey and Spring, 1992; 

 Guttormsen and Wilson, 1999). Comparable numbers of 

 length measurements were taken every triennial year but 

 measurements were not taken at every station. Thus, anal- 

 yses of different age groups were based on subsets of the 

 data. The abundance offish in a specified age category was 

 influenced by year-class strength (Table 1; Megrey et al., 

 1996; Hollowed et al.^). Strong year classes were present in 

 1984 (age 0), 1990 (age 2), and 1996 (age 2). 



Hollowed, A. B., E. Brown, B. Megrey, and C. Wilson. 1996. 

 Walleye pollock. In Stock assessment and fishery evaluation 

 report for the 1997 Gulf of Alaska groundfish fishery, 64 p. 

 North Pacific Fishery Management Council, 605 W 4"' A. venue, 

 Suite 306, Anchorage, AK 99501. 



