Table 13. — Percentage of fishing mortality (F) of 

 Atlantic mackerel at ages 1 and 2 compared with mean 

 F at ages 3 and older (partial recruitment). 



Year 



Agel 



Age 2 



Year 



Agel 



Age 2 



1962 



78.9 



15.8 



1970 



41.6 



16.2 



1963 



9.5 



23.8 



1971 



20.9 



64.6 



1964 



100.0 



82.1 



1972 



4.7 



28.8 



1965 



46.2 



32.7 



1973 



37.3 



67.6 



1966 



46.7 



70.0 



1974 



11.3 



89.9 



1967 



0.9 



40.5 



1975 



38.0 



85.0 



1968 



17.4 



25.2 



1976 



2.4 



44.3 



1969 



2.1 



44.4 



1977 



1.5 



17.2 



see footnote 2). Partial recruitment at ages 1 (1.5%) and 2 

 (17.2%) in 1977 was near the low end of the range of 

 values. In view of the wide fluctuations evident in previ- 

 ous years, the 1977 partial recruitment coefficients may 

 not reflect the situation that would actually occur in 

 1978. An average of the 1968-77 values (except 1970, 

 1973, and 1975) was used for age 1 in 1978 (9%). The high 

 values in 1970 and 1975 were excluded from this average 

 because they occurred as a result of large catches taken 

 from strong incoming year classes which does not repre- 

 sent the expected situation in 1978. The high 1973 value 

 was also excluded because it resulted from a large 

 harvest of age 1 fish from a year class of below-averge 

 size. This catch reflected a shifting of intensive fishing 

 effort onto younger age groups in an attempt to maintain 

 high levels of catch at a time when older age groups ex- 

 hibited a sharp drop in abundance. 



An average of the 1968-77 values (except 1974-75) was 

 used for age 2 in 1978 (39%). The 1974-75 values were ex- 

 cluded because they were unusually high and do not 

 represent the expected situation for 1978. Such high 

 values may have resulted from: 1) large catches being 

 taken from strong year classes, and 2) apparent diversion 

 of fishing effort onto that age group from older age groups 

 in an attempt to maintain high levels of catch. Partial re- 

 cruitment in 1978 was, therefore, predicted to be 9% at 

 age 1, 39% at age 2, and 100% at ages 3 and older (Table 

 14). 



sisting of 50% of the age 2 fish and 100% of the age 3 and 

 older fish (Isakov 13 ; Moores M ), increased from around 

 500,000 t during 1962-67 to 1.8 million t in 1970-72 before 

 decreasing to 434,700 t in 1977 (Fig. 9). 



Catch and Stock Size Projections 



Under the assumption that the 1977 catch would be 

 92,000 t, the total stock biomass at the beginning of 1978 

 was estimated to be 468,600 t (11% decrease from 1977) 

 with a spawning stock biomass of 402,500 t (7% decrease 

 from 1977). Catch options for 1978 and resultant spawn- 

 ing stock biomass levels in 1979 were calculated for 

 values of F ranging from to 0.70 (Table 15). In the 

 absence of any catch in 1978 (F = 0.00), the spawning 

 stock biomass would increase about 6% from 1978 to 

 428,000 t in 1979. A catch of 23,500 t (F = 0.07) would 

 maintain the spawning stock in 1979 at the 1978 level. 



Table 15. — Projected Atlantic mackerel catch (metric tons) in ICNAF 

 Subareas 3, 4, and 5 and Statistical Area fi in 1978 with fishing 

 mortality ranging from 0.00 to 0.70, and the resulting spawning stock 

 in 1079 and its percentage change from 1978. 









Spawning 



' r change in 



Fishing 



Total 



Catch in 



stock 



spawning stock 



mortality 



mortality 



1978 



in 1979 



from 1978 



(F) 



(Z) 



(10 J tons) 



(10 3 tons) 



(by weight) 



0.00 



0.30 



0.0 



428.0 



+6.3 



0.05 



0.35 



16.9 



409.6 



+ 1.8 



0.07 



0.37 



23.5 



402.5 



0.0 



0.10 



0.40 



33.0 



392.6 



-2.5 



0.15 



0.45 



48.5 



376.3 



-6.5 



0.20 



0.50 



63.2 



360.8 



-10.4 



0.25 



0.55 



77.3 



346.0 



-14.0 



0.30 



0.60 



90.8 



331.9 



-17.5 



0.35 



0.65 



103.7 



318.5 



-20.9 



0.40 



0.70 



116.0 



305.6 



-24.1 



0.45 



0.75 



127.8 



293.4 



-27.1 



0.50 



0.80 



139.0 



281.7 



-30.0 



0.55 



0.85 



149.8 



270.6 



-32.8 



0.60 



0.90 



160.1 



260.0 



-35.4 



0.65 



0.95 



170.0 



249.8 



-37.9 



0.70 



1.00 



179.5 



240.1 



-40.3 



Table 14.— Summary of parameters used in the Atlantic 

 mackerel assessment. 



Parameter 



Value 



Fishing mortality in 1977 (age 4 + ) 



0.39 



Recruitment at age 1 : 1974 year class " 



2,360.0X10« 



1975 year class 



810.0X10 6 



1976 year class 



415.0X10 6 



1977 year class 



415.0X10 6 



Partial recruitment in 1978 (%): age 1 



9 



age 2 



39 



age 3 + 



100 



1978 spawning stock (10'tons) projection 



402.5 



Stock Size 



Age-specific stock size estimates generated from 

 cohort analysis and summed biomass values determined 

 by applying mean weights at age to these estimates are 

 given in Table 12. Total stock biomass (age 1 and older) 

 increased from about 600,000 1 in 1962-66 to a peak of 2.4 

 million t in 1969 and then declined steadily to an esti- 

 mated 524,400 t in 1977. Spawning stock biomass, con- 



Equilibrium yield calculations, assuming a constant 

 level of recruitment at age 1 and partial recruitment of 9, 

 39, and 100% at ages 1, 2, and 3+, respectively, indicate 

 that F i = 0.40. The F 01 level, defined as the level of Fat 

 which the change in yield per recruit with respect to the 

 change in F is only 10% of that which would occur if the 

 fishery began on the virgin stock (Gulland and Boerema 

 1973), has been used recently within ICNAF as a basis for 

 setting catch quotas. Fishing mortality at F _i = 0.40 

 would result in a 1978 catch of 116,000 t and would 

 reduce the spawning stock by 24% in 1979. 



An assessment assuming a total catch of 110,000 t in 

 1977 (TAC of 105,000 plus 5,000 for the U.S. recreational 

 catch) instead of 92,000 t has no significant effect on the 



,3 Isakov, V. I. 1976. On some results of biological studies on mack- 

 erel from the Northwest Atlantic. Int. Comm. Northwest Atl. Fish. Res. 

 Doc. 76/VI/52, Ser. No. 3838 (mimeogr.), 14 p. 



H Moores, J. A. 1976. Mackerel research in the Newfoundland area 

 during 1975. Int. Comm. Northwest Atl. Fish. Res. Doc. 76AT/18, Ser. 

 No. 3798 (mimeogr.), 10 p. 



11 



