Proc.lnt. Bymp. Biol. Mgmt. Walleye Pollock Nov. 1888,Anchorage, Alaska

Delineation of Walleye Pollock Theragra chalcogramma!Stocks off the Pacific Coast of Canada Mark W. Saunders, Gordon A. McFarlane, and William Shaw Pacific Biological Station Nanaimo, British Columbia, Canada

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Walleye pollock are found in the inlets and open waters along the British Columbia coast. Age and growth information from commercialand research catches collected from 1976 to 1987 indicate that several discrete stocks of pollock are present. Strait of Georgia pollock were significantly smaller at age than fish from northern areas. Length frequencies indicated that pollock in Dixon Entrance are part of southeastern Alaskastocks and suggest that there is little intermingling north and south of Queen Charlotte Sound. The implication of these findings to managementof pollock are examined.

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W ll y po ll k tTh ~hl,,~l d 1 species that inhabits the continental shelf and slope along the northern rim of the Pacific ocean Bakkala et al. 1986!. In the Canadian zone, concentrations of pollock have been found in Dixon Entrance/Hecate Strait, Queen Charlotte Sound, Strait of Georgia and off the west coast of Vancouver Island Figure 1!.

379 """"",".'~///I/',~~ IsidNpsKKA I f. L //////j61'Q ' QI sooALLvcHAA'NEL

SELIVVNINLET- VdfH~fO VINL AVSON CHANNEL tL

CNARLDTTE SDIDALD CHd OUEEIVRLOT TE STRAIT~

ILLALO NOOTKASOUNO SVONEVINLET RARKLIVSOONO dPFRQUSF Q STRAITOFJOAN OERLFCA

Figure 1. The location of spawning walleye pollock in the northern coastal waters off the west coast of Canada.

380 The fishery for pollock in Canadian waters peaked in 1979 when 3,385 t were landed. Landings continue to be low, a result of poor markets for roe and fillets as well as decreased abundance. The peak of the Canadian fishery appearsto have coincided with a peak in pollock abundance noted throughout the northeast Pacific. Despite the enormity of stocks in the north Pacific, and the importance of the fishery, little it known about the discreteness of pollock stocks throughout its range. Evidence presented to date has been conflicting and inconclusive. Bakkala et al. 986! note that while significant genetic differences have been found between eastern and western Pacific pollock, the differences within eastern and western areas are weak, and that the possibility of multiple stocks exists based on regional differences in age and growth parameters,morphometric and meristic characteristics and isolated spawning grounds. The strongest evidence to date for stock separation within Canadian waters is based on parasitological data. Arthur 983! found significant differences between west coast of Vancouver Island, the Strait of Georgia, and northern waters. No differences were found between Queen Charlotte Sound and Dixon Entrance. Shaw and NcFarlane 986! reported that age analysis indicates that pollock in the Strait of Georgia are smaller for a given age than those in the north coast area and that length frequency data suggest little intermingling north and south of QueenCharlotte Sound. Thompson981! suggested that pollock in Dixon Entrance appear to be part of a larger stock found off BED Alaska. The purpose of this paper is to review regional differences in age and growth, maturity and spawning behavior to assess the discreteness of pollock stocks in Canadian waters.

Biological data used in this report were assembled from trawl and hydroacoustic surveys conducted by the Canadian Department of Fisheries and Oceans, and from commercial trawl catches sampled by port samplers or by onboard vessel observers. Pollock were measured for fork length and random lots of tubs, full of fish were sampled for sex, gonad condition and pectoral fins were collected for age determination. Age was determined from cross-sections of pectoral fins Beamish

381 1981;Chilton and Beamish1982! with the exception of WashingtonState ages which were surface readingsfrom otoliths~ Length and ageat 50'tmaturity were calculated usingProbit analysis SASInstitute Inc. 1985!. In a pilot tagging studyFloy tags FD-68!were applied to trawl caught fish. A Diamond-5midwater trawl wasused to capturepollock and the fish wereimmediately transferred to a holding tank. Only fish in goodcondition no scale loss or abrasion!were tagged andreleased. Tenpercent of the fish wereinjected with OTC oxytetracyline!, using a dosage rate of 25 mg/kg.

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Spawningdistribution

Throughoutthe northeastPacific, pollock form spawning aggregationswith peakspawning occurring during March and April' The largestspawning concentrations in Canadian watershave beenfound in DixonEntrance and the Strait of Georgia Shaw and NcFarlane 1986!. Smallerconcentrations havebeen found in manyof the coastal inlets including Dana andSelwyn Inlets on the east coast of Noresby Island Thompsonet al. 1981!,and in mainlandinlets Thompsonand Beamish1979! including FinlaysonChannel, Squally Channel, and Caamano Sound Figure 1!. The above findings were supported by the presenceof eggs and larvae in coastwide surveys conducted during 1980 Nason et al. 1981a-d!, and during 1981 in the Strait of Georgia Shawand NcFarlane1986!. Sampling was conducted at 77 stations on 18 transects located 30 minutes of latitude apart. During the period January 15-22, 1980 pollock eggs were foundonly at the entranceto Juande FucaStrait Figure 2!. During a second cruise conducted from February 13-20, 1980, eggs were found again off the southwest corner of Vancouver Island, and at one station centrally located in QueenCharlotte Sound. During March 12-20,1980 eggs were found off the southwest coast of Vancouver Island, in Queen Charlotte Sound and at one station on the southwest coast. of the Queen Charlotte Islands. At this time, larvae began to appear off the southwest coast Vancouver Island and at one station in eastern Dixon Entrance. During the last cruise conducted in April, only larvae and post-larval pollock were present off the west coast of VancouverIsland as far north as Brooks Peninsular Neanwhile in Dixon Entrance and northern Hecate Strait, eggs were found throughout the region with larvae and post-larval stages found in the most

382 Figure2. Thelocation of spawningwalleye pollock in the northern coastal waters off the west coast of Canada.

383 westerlysites andat onestation in northeasternHecate Strait. Overall, these data indicate the presence of four areas wherespawning takes place, DixonEntrance/northern Hecate Strait, QueenCharlotte Sound,off the west coast of VancouverIsland and in the Strait of Georgia Figures 1 and 2!. Eggsand larvae were noticeably absent from the northwest coastof VancouverIsland, HecateStrait, the westcoast of the Charlottes and Dixon Entrance. The fact that spawningtakes place simultaneously coastwideand in distinctly separate geographic regions, suggeststhat as larvae,pollock form discrete stocks. The degreeto which,if any, that fish from these spawning concentrationsdisperse and the area of eventual recruitment of their offspring to the adult stock is unknown,but differences in parasite loading, age and growthparameters and results of a pilot tagging study provide someinsight.

Parasite prevalence and abundance

Arthur 983! conducteda preliminary study of the discreteness of pollock stocks in Canadian waters as indicatedby the prevalenceand abundance of 13 speciesof parasites. He used Kruskal-Wallisand non-parametric simultaneoustest procedure STP! tests to look for heterogeneityin prevalenceand abundanceamong collections of female pollockfrom the west coast of VancouverIsland, the Strait of Georgia, Queen Charlotte Sound and Dixon Entrance. He concluded that the Strait of Georgia and the west coast of VancouverIsland are relatively discrete from each other and from the two northern areas. He cautions that while QueenCharlotte Soundand Dixon Entrance pollock mayconstitute a single stock, the results maysimply be an artifact of similar environmental conditions'

Age and growth

The major stocks as indicated by parasite and spawning locality data, also exhibit differences in a number of age and growth parameters. In Dixon-Entrance and Queen Charlotte Sound the maximum age in the population is 12 years, and samplesare dominated by ages 4 to 7 Figures 3 and 4!. With the exception of the 1980 Two Peaks sample, the proportion at each age remains consistent between the years 1978 and 1983. This sLay

384 Proportion

0,7 0,6 0.5 0,4 0.3 1363Twe Peaks 63 MelntyraSay 0.2 TewHill a Pooka 0,1 ~aks 0 2 3 i 5 6 7 8 0 1 AGE year! Figure3. Agefrequencies of walleyepollock in DixonEntrance from 1978 to 1983.

Proportion

0.35

0,3

0.25

0.2

0.15

0. 'I

0.05 0 2 3 4 5 6 7 8 tt 10 AGE years! Figure 6. Age frequencies of walleye pollock in QueenCharlotte Sound during 1978 Finn Ch=Finlayson Channel, Nar=March, i'= midwatertrawl, QCSQueen Charlotte Sound, BT=bottom trawl!.

385 indicate that recruitment was constant over the time period or that ageing error could be smoothing the age distribution. The latter is possible given Thompson's 981! report that between- reader agreement of samples ranged from 30 to 95 percent. Samplesof primarily younger fish resulted in greater agreement 9-Bit! than those of older fish 0- 61%!, The smoothing of the age structure precludes the ume of different year-class strengths in the population aa evidence for stock separation. As well, any future stock assessment, in particular if catch-at-age analysis is used, should address this problem of ageing error. The above discussion deals with precision; the issue of accuracy has not been addressed. While length frequencies provide reasonable validation of the first two growth zones Beamish 1981!, validation remains to be carried out for the older age classes. In the Strait of Georgia and off the west coast of Vancouver Island, the maximum ages are less than Dixon Entrance-Queen Charlotte areas, at 10 and 8 years, respectively, and age frequencies Figures 5 and 6! are comprised of predominantly younger age classes age 3 to 5 ! ~ The absence of older fish in samples collected off the wemt coast of Vancouver Island may be the result of fishing mortality incurred during the offshore Pacific hake fishery' or it may simply be that they have not been encountered. There have been no mid-water research hauls conducted over bottom depths greater than 200 m during winter months, the time and place where pollock are most abundant in Dixon Entrance. Research age frequencies indicate that within each of the ma]or areas somesegregation of the stock occurs, with juveniles remaining separate from the adults. Age frequency distributions collected from around the perimeter of Dixon Entrance Figure 7! are comparable, comprised mainly of ages 5 to 11 ' Younger pollock are found in the nearshore areas and occasionally in central and southern Hecate straits The recruitment of pollock from the nearshore areas, through Hecate Strait, and eventually to Dixon Entrance is implied by the mutually exclusive modes, predominant in the three areas ~ Age 3 predominates in the Selwyn and Dana Inlet.s, ages 4 and 5 in the South Flats region of Hecate Strait and as discussed above, ages 5 and older found in Dixon Entrance Figure 8!.

Thompson 981! suggested that pollock in Dana and Selsyn Inlets, given the truncated age frequency, their small size and proportion mature at age 3, const.ituted discrete stocks . It is unlikely that a source of mortality is present in the

386 Proportion

0.5

0.4

0.3

0.2 0.12 34 5 6 78 910 AGE years! Figure 5. hge frequencies of walleye pollock from three areas in the Strait of Georgia U.S.=United States portion of the Strait of Georgia, Central=Halibut Bank area, E. Mayne Isl.=East Mayne Island! .

Proportion

0,8

0.6

0,4 0.22 3 4 5 6 7 8 AGE yeors! Figure 6. Age frequencies of walleye pollock off the west coast of Vancouver Island WCVI! during 1976 and 1983 WASH=northern Washington State! .

387 Proportion

0,4

0.3

0.12 34 56 76 910 '11 AGE years! Figure 7. Agefrequencies of walleye pollock in three areas of Dixon Entrance during 1978.

Proportion

0.7

0.6

0.5

0.3

0.2 0.12 3 4 56 76 910 11 12 AGE years! F gure 8. Age frequencies of walleye pollock from the southeastern Ala ska portion of Dixon Entrance SE Alaska !, northern Hecate Stra i t and S~lwvn and Dana inlets combined Selwyn Inlet!.

388 inlets that could explain the absence of older pollock. A piece of corroborating evidence is that Pacific herring ~pFy ~haenggs ~a~lasi! resident in coastal inlets, while smaller in size-at-age, have a life span comparable to the offshore stocks pers. comm V. Haist!. Hence, it is more probable that pollock migrate from the inlets at age three with some remaining in the inlets to age 5. Shawand cFarlane 986! found that similar segregation took place in the Strait of Georgia. Length frequency data presented below indicate that this also occurs off the west coast of Vancouver Island and in QueenCharlotte Sound. The growth rate of juveniles age 0+ and 1+! is similar coastwide Shaw and NcFarlane 1986!. Beyond age 2 growth differs between northern and southern areas with the mean length-at-age greater in Dixon Entrance/HecateStrait and Queen Charlotte Sound Figure 9!. Research samples collected from three areas in Dixon Entrance indicate that the mean length at age is comparablethroughout the area Figure 10! . In the Strait of Georgia, there are some differences between areas Fig 9!. The time series of mean size-at-age from the fishery in the U.S. portion of the Strait of Georgia Figure 11! indicates that the relationship is stable over time. Length frequency information indicates similar trends to that denoted by the age information and provides insights into areas where no age data are available. For example off the west coast of Vancouver Island Figure 12 presents length frequencies for pollock in Barkley Sound and for the adjacent offshore area. Within Barkley Sound,modes at 11 and 23 cm in research hauls indicate that age 1 and age 2 fish inhabit the mid-water. Bottom trawl hauls indicate that the 25-35 cm segment of the population inhabits the demersal areas of Barkley Sound. The remainder of the length frequency is comprised of the offshore componentof the stock. Length frequencies from each of the major areas area presented in Figure 13. The west coast of VancouverIsland and the Strait of Georgia are comparable with modesat 38 and 41 cm, respectively and the majority of fish between 25 and 50 cm. Dixon Entrance and Queen Charlotte Sound have modes at 55 and 53 cm, respectively and with most fish between 45 and 60 cm. The length frequency of fish from SE Alaska fisheries is comparable to the nearshore catches of pollock in HecateStrait and to bottom trawl catches made throughout the slope regions of SE Alaska during a Pacific Ocean perch Sebastes alutus! survey conducted in 1965 Westrheim 1967! Figure 14!. All hauls were conducted at depths ranging from 183 m 00 fm! to 296 m 62 fm!.

389 70

E 60 ZzW 50

30

202 3 4 5 6 7 8 9 IO

70 B

60

50 Uz 40

i!J 30

20 4 5 6 7 8 9 10 AGE years !

~ WCVI DE QCS SG- US 0-- SC CENTRAL SG - EMI SG- SH Figure 9, Meanlength-at-age of walleye pollock by area IVI= wear coast Vancouver Island, DE~DixonEntrance, qCS=queenCharlotte Sound, SG~Strait of Georgia, EMI~EastMayne Island, SH=Sandheads!and sex a~ales, B~females!.

390