SCIENTIFIC COUNCIL MEETING - JUNE Yellowtail flounder in Divisions 3LNO - an assessment update

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1 NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR(S) Northwest Atlantic Fisheries Organization Seria No,N835 NAFO SCR Doc. 8/VI/9 SCIENTIFIC COUNCIL MEETING - JUNE 198 Yellowtail flounder in Divisions 3LNO - an assessment update by W. B. Brodie and T. K. Pitt Fisheries Research Branch, Department of Fisheries and Oceans, P. 0. Box 5667, St. John's, Newfoundland A1C 5X1 INTRODUCTION TAC regulation This stock has been under quota regulation since 1973, when a precautionary TAC of 50,000 t was set. In 1976, the TAC was set at 9000 t following a drop in stock biomass when anticipated recruitment did not materialize. The TAC since then increased gradually to 23,000 t in 1982 and has declined to 17,000 t for 198 (Table 1). Catch trends The nominal catch peaked in 1972 at 39,259 t and catches have averaged about 1,000 t since 1978 (Table 1). Catches have declined gradually since 1981 and the 1983 catch of 9,279 t is the second lowest since Since 1975, the fishery has been conducted almost exclusively by Canadian trawlers, with the USSR taking significant catches between 1966 and 1975 (Table 1). In virtually all years, the majority of the catch has been taken in Div. 3N (Table 2). Catch/effort Catch rates of yellowtail by Can(N) TC 5 otter trawlers (Table 3) declined from an average level of approximately 0.6 t/hr in the period to an average level of approximately 0. t/hr in the period and have increased to a level between 0.53 and 0.6 t/hr in the period (Fig. 1). Total fishing effort has shown a decreasing trend in recent years with the 1983 value being the lowest in the series (Table 3). ASSESSMENT PARAMETERS Sam lin : The length measurements and otolith samples used (Table ) were collected by the ana Ian Commercial Groundfish Sampling Section in St. John's. Numbers at age: These were obtained in the usual way by applying quarterly age-length keys (sexes separate) to monthly length frequencies for each NAFO division. Total catch at age for 1983 (Table 5) was obtained by combining male and female numbers at age from all three divisions (3L, 3N, and 30). Table 6 contains the catch matrix for the years , and Table 7 shows the corresponding percent numbers at age. Weights at age: Average weights at age for 1983 were obtained by applying a length-weight equation to monthly average lengths at age (weighted by numbers caught at age). The 1983 weights are shown in Table 5 while Table 8 contains the average weights at age for Table 9 shows the calculated catch biomass (numbers at age X weights at age) and all yearly totals are within +11% of the corresponding nominal catch. Natural mortality: Since there is no evidence to suggest anything to the contrary, the value of 0.3 used in previous assessments was used for all ages. Research vessel survey data: Tables give the results of surveys conducted by Canadian research vessels in selected strata in Div. 3LN (Fig. 2) over the period Survey coverage of Div. 3V was incomplete in many years and is not presented. Also, there was no spring survey conducted in Div. 3LNO in The surveys were conducted by the A. T. Cameron while the 198 survey was carried out by the A. Needler.

2 Table 11 shows the results on a stratum by stratum basis. Overall, the numbers and weights per tow remained relatively stable over-the period , given that stratum 361, which historically has had a high abundance of yellcmtail, was not surveyed in The values for 198 must be regarded with caution as preliminary analysis has shown that the fishing gear used by the A. Needler may be 1.5 to 1.8 times as efficient in catching yellowtail than the gear use y - A. T. Cameron. Table 12 shows the average number per set at age for the same surveys (except 198) and Table 13 shows the corresponding total abundance values. From these tables it can be seen that the surveys apparently do not adequately sample young yellowtail and thus cannot be used to estimate recruitment. Also, noticeable from Table 13 is the apparent increase in the abundance of fish aged 7 and older in the period (average 61 x 10 6) over the level observed in the period (average 38 x 10 6 ). Again, however, these results must be interpreted with some caution as the surveys were somewhat incomplete in 1975, 76, and 81. Partial recruitment: The PR used to estimate population size in 1983 in the cohort analysis was calculated from average selectivity coefficients from a preliminary cohort run. The averages were normalized at age 8 and the resulting PR vector, as well as the PR used in the 1983 assessment (Brodie and Pitt, 1983) is shown in Table 10. Terminal fishing mortality in 1983: Several methods were used in attempting to calibrate the cohort analysis. ATillinia77675T results from several of these methods is given in Table 1. 1) Regressions of 5+ mid year population biomass from cohort vs CPUE ( Table 3) over a range of FT from 0.30 to 0.50 were barely significant at cc = The correlation coefficient (r) reached a peak of at F T = 0.35, although there was only a slight change in r over the range of FT. The 1983 point came closest to the line at F T = 0.5 while the line best fit both the 1982 and 1983 points at a value for F T of 0.0 (Fig. 3). Regressions of + mid year population biomass from cohort vs CPUE (Table 3) were performed at values of F T = 0.35, 0.0, and 0.5. For these calculations, the population size at age in 1983 was adjusted (by changing the PR) to equal approximately the geometric mean of the population size at age in the cohort analysis from These results show r to be decreasing as FT increases from 0.35 and that the 1983 point is closest to the line at F ` 0.0. As well, the best fit of the regression line to both the 1982 and 1983 points is at FT = 0.0 (Fig. ). Regressions of + fishing mortality weighted by population numbers from cohort against fishing effort (Table 3) were carried out under the same conditions described in 2) above. These results also show r to be decreasing over the range of F T used and although the 1983 residual is relatively large' for all three levels of F T, the line from the run at F T = 0.5 best fits both the 1982 and 1983 points combined (Fig. 5). It is also interesting to note that the intercept of the calculated line is very close to zero for the run at F T ) Regressions of + midyear population biomass from cohort versus average weight per tow from Canadian research vessel surveys, only (Table 11), were performed for a range of F T from 0.35 to The correlation coefficient increased from to over this range, and both the 1982 and the combined residuals were minimized at a level of FT between 0.5 and 0.50 (Fig. 6). However, these results should be interpreted with caution as a) the surveys were not complete in all years, and b) the regressions are essentially "two-point regressions", driven mainly by a cluster of points near the origin and the high 1971 and 1972 values. Regressions of exploitable biomass from cohort (calculated from yearly selectivity coefficients) on CPUE (Fig. 7) as well as average exploitable biomass (calculated from average selectivity coefficients at age) on CPUE (Fig. 8) over a range of F T were not significant. From the calibrations, it was determined that a value of F T for 1983 = 0.0 was appropriate. The cohort run at this level of F T is shown in Table 15. Recruitment for stock projections: The geometric mean ( ) of age numbers from the cohort run at FT = 0.0 was used to estimate recruitment at age for This value was 90 x 10 6 fish, slightly lower than the geometric mean (99 x 10 6 ) and 19% lower than the value used in the 1983 assessment (111 x 106). F 0.1 : The value of 0.52 calculated by Brodie and Pitt, 1981, and used in the last three vssessments is retained.

3 3 Catch projections: Projections were carried out to 1985, using the population size in 1983 from the cohort run at FT = 0.0 (age replaced by geometric mean), the PR used to calculate the 1983 population, and the long-term ( ) average weights. Table 16 shows that assuming the catch in 198 equals the TAC of 17,000 t, the projected F_ u.1 catch in 1985 would be 15,000 t. It should be pointed out that over 25% of this projected catch is comprised of fish from year-classes for whose size the geometric mean was used as an estimate. REFERENCES Brodie, W. B. and T. K. Pitt An assessment of the yellowtail stock in Div. 3LNO. NAFO SCR Doc. 81/VI/5, Ser. No N338. Brodie, W. B., and T. K. Pitt A stock assessment for yellowtail flounder in NAFO Div. 3L, 3N, and 30. NAFO. SCR Doc. 83/VI/57, Ser. No. N715. Table 1. Nominal catches by country and TACs (tons) of yellowtail-nafo Divisions 3LNO. Year. Canada France USSR Other Total TAC 1966,185 d 2,83 7 7, ,122 6, , , , , ,9 1 5, , , , ,26 197:1 2, , , , , , , , ,815 50, , , ,313 0, ,58 15, ,89 35, , ,057 9, , ,638 12, , ,66 15, , ,351 18, , ,377 18, , ,680 21, a11, ,26 23, a9, ,279 19, ,000 a :prel imi nary Table. 2. Breakdown of nominal catches 3L, N and O. tons) of yellowtail by NAFO Division Year. 3L 3N 30 Total a 1983a ,815 5,287 7,19 6,632 9,292,856 1,5 2, ,651 2,57 2,595 1,898 2,35 2,510 2,672 2,958 6,2 6,117 8,59 7,215 18,668 25,17 25,788 23,693 19,329 16,156 5,023 7,381 11,079 1,556 9,805 11,733 7,890 5, ,308 2,066 3, ,536,179,266 3,0,100 2,518 1,606 1,80 1, , ,130 7,026 8,878 13,3U 15,708 26,26 37,32 39,259 32,815 2,313 22,89 8,057 11,638 15,66 18,351 12,377 1,680 12,26 9,279 a : preliminary

4 Table 3. Nominal catch and effort data for yellowtail in NAFO Divisions 3LNO. Column 2 refers to reported "directed" catch by Canada (N) Tonnage Class 5 Otter trawlers. Year Total cal cul ated Directed catch CPUE Total catch effort (tons) (tons/hr) (tons) (hours) , , , , , , , , , , , , , , a5, a, , , , , , , , , , , , , , , , ,279 18,922 25,751,191 62,237 6,677 50,876 57,751 56,950 2,268 27,513 31,181 35,95 19,339 23,909 23,326 16,689 Table. List of commercial samples available, by quarter and division, available for 1983 yellowtail, Division 3LNO, as provided by the St. John's Commercial Sampling Section. uarter Div 3 Total L Can(N) Catch (t) 692 1, ,520 (t) Samples Measured 2,131 3,078 1,212 6,21 Otoliths N Can(N) Catch (t) 717 1,178 2,305 1,30 5,50 (t) Samples Measured 2,88,02,057 10,569 Otoliths , Can(N) Catch (t) (t) Samples Measured ,612 Otoliths

5 Table 5. Table 6. Table 5. Table 6. Table 7. Table 8. Table 9. Average lengths, weights, and numbers at age, 3LNO yellowtai, Catch matrix (n. at age) for Percent numbers at age, Average weights, Catch biomass (numbers x weights), AGE WEIGHT st A k * * * 9 0,9 * AVERAGE CATCH LENGTH MEAN STD, ERR, v , , , CATCH NATRIX(X10-3) AGE I Q Table 7. PERCENT AT AGE AGE. I , , Table 8. AVERAGE WEIGHTS(KG) I i I I I I , Table 9. CATCH BIONASq0 I I I I I I I

6 -6- Table 10. Partial recruitment vectors, Div. 3LNO yellowtail, used in 1983 and 198 assessments. Age PR in 1983 assessment PR in 198 assessment Table 11. Average numbers and weights of yellowtail per 30 minute set for selected strata in Divisions 3LN. Surveys from were conducted by the research vessel A. T. Cameron and the 198 survey was conducted by the R.V. A. Needier. Year Stratum No. sets Av. No./set Av. wt./set No. sets 2 3 Av. No./set Av. wt./set No. sets 2 Av. No./set Av. wt./set No. sets 3 3 Av. No./set Av. wt./set No. sets 3 Av. No./set, Av. wt./set No. sets Av. No./set Av. wt./set No. sets 3 3 Av. No./set Av. wt./set Total No. sets Av. No./set Av. wt./set ,

7 -7- Table 12. Average number per set at age and average numbers for totals for yellowtail from Canadian research vessel surveys in NAFO Divisions 3L and 3N (see Fig. 1). A key stratum, number 361, was not fished in 1981, and in 1975 and 1976 strata 373 and 375 respectively were missed. Age Average number (3+) per set Table 13. Abundance of yellowtail (x10-3 ) from Canadian research vessel surveys for selected strata in Divisions 3L and 3N. Age ,599 3, , , ,525 18,797 29,756 3,8 7,966 3,015 5,13 1,383,111 1,889 3, ,30 58,60 25,09 25,576 15,10 22,921 8,383 15,788 3,957 11, ,562 67,380 32,789 3,865 21,79 31,35 20,25 29,167 15,737 29, ,076 36,31 33,51 22,13 25,186 28,750 5,76 30,258 0,589 2,5 8 9,691 11,556 12,80 2,663 6,17 5,82,686 15,786 19,33 13, ,090 1,222, ,37 1,60 2, , , ,116 2,71 1,61 10,623 21,217 27,166 33,033 28,951 21,8 5,55 5,268 1, Total 227, , , ,578 72,181 95,322 13,936 98,023 8,37 103,277 77, ,208 Age 7 and older 8,899 9,190 51,060 25,188 32,09 3, ,652 7,701 62,53 57,222 63,051 60,622 1Survey coverage incomplete.

8 - 8 - Table 1. Results of cohort calibration, 3LNO yellowtail. Regression Parameter pop. biomass (midyr.) vs CPUE r intercept slope 1983 resid resid ,333 87,37 +20,856 +6, ,286 8, ,019 +1, ,266 2,220 83,037 81,619 +5, ,68-5, ,195 80,79 3,889 7,889 + pop. biomass (midyr.) vs CPUE (pop. size at age in 1983 equal approx. to G.M.) r intercept slope 1983 resid resid ,36 139, , ,92-5, ,00 132,358 +2,226 -,377-5,05-9,13 + fishing mortality (wtd. by pop. nos.) vs effort (age in 1983 equal approx. to G.M.) r intercept slope 1983 resid resid pop. biomass (midyr.) vs Av. wt. per r.v. tow ( ) r intercept slope 1982 resid resid , ,81 +7, ,337 21, ,266 +1,90 +,328 +1, , , Table 15. Cohort analysis for 3LNO Yellowtail. Terminal F in 1983 = 0.0. POPULATION NUMBERS AGE ' I I , POPULATION BIOMASS (MID-YEAR) AGE I I I I AGE I , , , FISHING MORTALITY , , _ , ,

9 ;e Table 16. Results of catch projections to 1985, assuming the TAC Of 17,000t is taken in 198. CATCH NUMBERS I I i 5 I I I , CATCH BIOMASS POPULATION NUMBERS I I , POPULATION BIOMASS (AVERAGE) I 1983 ' I i , I ' ' ' + +, i , FISHING MORTALITY; ,-.---,,-.., --f-, D r-! m mi m -- rq LO U0 I-- Ul ; cil mi 1:1-1 o-l hi -.L rn a r. LI Years 1t3 r- 3-1 r-. m r- N LT) Qn in a-.a/ IDM 01 in al Fi g. 1. Catch rate of yellowtail in Divisions 3LNO by Can N) TC 5 otter trawls,

10 Fi. 2. Stratum Map for NAFQ Divisions 3LNO. Asterisks denote strata of prime importance i n determining yellowtail abundance.

11 c midyr biomass (t rn CPUE (t/hr) Fig. 3. Regression of 5+ midyear biomass from cohort (FT=. ) on CPUE, WI? ' POPULATION, BIOMASS (MXID-YR) VS CPUE ,00 U L A T ,13 I c t IL go ft A la I M 1 A A I S (t) I , +, CPUE (t/hr) Fig.. Regression of + midyear biomass from cohort (FT = 0.) on CPUE. Value for age in 1983 is equal approximatley to the geometric mean. 93

12 WEIGHTED. FISHING MORTALITY. VS EFFORT I. I L 11 is EFFORT,, k 000 nrs) Fig, 5. Regression of weighted F from cohort. (F T =.) on fishing effort. Population size in 1983 is equal approximatley to the goemetric mean A 1 T A 1 s (t ,0 1 1 *lb ls IV WI PER TOW (kg/30 min.) Fig. 6. Regression of + midyear biomass from cohort (FT= 0.) on mean weight per tow from selected strata in Divisions 3LN from Canadian research vessel surveys,

13 EXPk.0ITABLE BIOMASS MID-IR VS CPUE 1 it A lb 0 I T A L I i 16 C= 0-25 A A 9.2 ( t ) 111 A Q, cave (t/hr) Fig. 7. Regression of midyear exploitable bi0mass.from cohort analysis (Fpr. 0.) on CPUE,. 31WrZH2123 T I 1, -11 Ell. t 1 CO lo 36s ;a ra RA' twcariargtri- 3 ca 15 ra ran -5 So 13 III 3- M LI CPUE (t/hr) Fig. 8. Regression of midyear average exploitable biomass from cohort (F T = 0.) on CPUE.

14 B