Length at 50% maturity was derived for pollock in each of the four areas Table 1!. In the Strait of Georgia the length at 50t maturity ranged from 26.0 to 32.1 cm. for males and from 30.2 to 35.1 cm for females. Dixon Entrance and Queen Charlotte Sound pollock matured at larger sizes of 37.4 to 41.4 cm for males and 38.8 to 43.5 cm for females. The west coast of Vancouver samples more closely resemble the north coast samples with the length at 50% maturity for males ranging from 37.1 to 39.8 and females a value of 40.1, although the 95't confidence limits are wide at 29 ' 4 to 39 ' 5 cm Table 1!.
Table 1. Length cm! at 50%maturity for walleye pollock by area, year, nmnth, and sex within Canadian waters. The 95 percent Fiducial limits are enclosed in brackets.
Jervis Inlet 1983 FEB 32.1 8.5-34.0! Jervis Inlet 1988 NAR 35.1 2.1-36.5! Central Str. 1983 FEB 27.1 4.8-29.3! 31.9 8.8-33.6! 1983 APR 28.9 5.1-30.7! 1984 NAR 26.0 0.4-28.4! 30.2 5.9-31.7! 1988 NAR 32.5 9.8-33.7! E. Nayne Isl 1983 FEB 29.5 8.5-30.3! 34.6 0.8-39.4!
West Coast of VarxxxxverIsland
U.S. waters 1983 AUG 39.3 8.1-43.4! Can. waters 1983 AUG 37.1 9.4-39.5! 40.1 8.9-41 7! Can. waters 1983 SEP 39.8 8.4-41.7!
1983 AUG 37.4 6 ' 5-37.9! 38.8 8 ' 0-39.4!
Dixon Entrarue
1979 38.7 40.5 1980 41.4 43.5
A number of other life history traits are compared between areas in Table 2. Differences between the Strait of Georgia and the north coast pollock are evident. Strait of Georgia
396 pollockspawn earlier, attain a smallermaximum length, are smallerat maturity and havedifferent Vonb parametersthan eachof the other areas. The growthparameters of f the westcoast of VancouverIsland pollock moreclosely resemble the north coast stocks. Table2. Summaryof life history traits of walleye pollock from the Strait of Georgia S. of G.!, west coast of VancouverIsland WCVI!, QueenCharlotte Sound QCS! and Dixon Entrance/Hecate Strait DE/HS! SST= mean sea surface temperaturea:
AREA
S.of G. WCVI QCS DE/HS
Age at first spawn 3
Naximum age 10 12
Naximum length cm! 66 61 74
Nin. length mature cm! Hales 25 36 32 Females 28 36 32 Hax. length immature cm! Hales 41 51 49 Females 43 50 50 Von b parameters to - Hales 0.590 -0.97 -0.455 -0.190 Females 0.559 -0.96 -0.485 -0.166 k Nales 0.997 0.32 0.363 0.426 - Females 0.835 0.30 0.289 0.414 L max Hales 42.5 50.5 57.3 55.4 Females 46.5 56 ' 5 67.3 57.9
Inst. Tot Nortality 0.7-0.9 0.6-0.9 8!
Nean SST Centigrade! 10 3c 9 5d 2e
British Columbia lighthouse temperature data time series, Pacific Biological Station, Nanaimo, B.C. Canada. b Entrance Island lighthouse 1936-86 mean. c Amphitrite Point lighthouse 1934-86 mean. d NcInnes Island lighthouse 1954-86 mean. mea Boonilla Island lighthouse 1960-86 mean.
The difference in a ge nd a growth parameters between pollock in Queen Charlotte Sound and Dixon Entrance an d t s oc k s in o eorgia is evidence for the discreteness of
397 these stocks, however the reason for the distinction is not clear. In southern waters, pollock are at the end of their range, and may be impacted by a number of physical or biological factors. For example, in the Strait of Georgia and off the west coast of VancouverIsland, the rangesof pollock and Pacific hake ci rodu~ct,s!, overlap. Hake is a gadidof similar habit, and perhapscompetition between andlor predation on pollock by hake, are significant. Whether or not the source of mortality is predation by hake, southern stocks may be experiencing a higher rate of natural mortality, which results in differing age and growth parameters.Dunbrack and Ware 987!, foundthat the rate of natural mortality N! for most pelagic species, is negativelycorrelated with asymptoticlength, age-at-first maturity andmaximum observed age, and positively correlated with ~ In the absence of estimates of N for each area, we utilized mean sea surface temperature in its place, given Pauly's 980! conclusion that there is a positive correlation between N and mean environmental temperature. Although the numberof degrees of freedom are too few to be statistically significant, correlations between mean sea surfacetemperature in the four areas, and values of age and growthparameters are suggested Table 2! ~ It wasfound that maximumage, maximumlength and length-at-maturity are negativelycorrelated with sea surface temperature,while K is positivelycorrelated. Theseresults are consistentwith Dunbrackand Ware's 987! findings and suggest that the differences between areas are a result of varying rates of natural mortality. The actual sourcesof mortality remain to be investigated.
Tagging A total of 942 pollock were tagged and released in the Strait of Georgia, east of Gabriola Island Figure 1!. As of September 1988 only two recoveries have been received, however the distances travelled make these recover ies noteworthy. The first recovery was December9, 1985 from the mouth of Jervis Inlet at a midwater depth of 110-150 m. The fish travelled a straight line distance of 40 nm in 266 days at large. The secondwas recovered February 2,1986 off Port Renfrew, using bottom trawl gear. The fish had travelled 110 nautical miles in 320 days at large. D!either of the two fish were OTC injected. These results while limited in scope, indicate that dispersion north and south of the central Strait of Georgia is taking place. 398 Thereare distinct spawningstocks in the Strait of Georgia, off the west coast of VancouverIsland, in QueenCharlotte Soundand in northernHecate Strait/Dixon Entrance. Pollock northof QueenCharlotte Sound are distinct frompollock in the Strait of Georgia and off the west coast of Vancouver Island. DixonEntrance, northern Hecate Strait and SE Alaskapollock appearto constituteone stock. Thepollock in the Strait of Georgiaincluding U.S. watersconstitute onestock. Pollock inhabit nearshoreinlet habitats prior to recruitment to offshore concentrations.
Implications to management
Nanagersmust be prepared to managepollock from Queen Charlotte Soundto Dixon Entranceseparately from pollock in the Strait of Georgia. In the extremesof its' rangeie. DixonEntrance and southernStrait of Georgia,pollock shouldbe jointly assessedand managed by Canadianand U.S. agencies.
A k o 1 dg * t Weappreciate the generosityof Nr. B. Brackenof the AlaskaDept. of Fish andGame and Nr. G. Bargmannof the Stateof WashingtonDepartment of Fisheries for providing data from pollock stocks in their respectiveregions. Weare gratefulfor the commentsof Dr. D. Warewho reviewed the manuscript.
Refgr~nc~
Arthur, J. R. 1983. A preliminary analysis of the discreteness of stocks of walleye pollock Th~eacCr~ hl~,! fo thoth t P f a
399 Beamish, R. J. 1981. Use of fin-ray sections to age walleye pollock, Pacific cod, and albacore, and the importance of this method. Trans. Am. Fish. Soc. 110: 287-299.
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401