Recruitment 



The estimated sizes of the 1961-73 year classes at age 1, 

 computed by cohort analysis, are given in Table 12 and 

 plotted in Figure 9. Sizes ranged between 430 million fish 

 (1962 and 1963 year classes) and 7,786 million fish (1967 

 year class) and averaged 2,108 million. Only 4 of these 13 

 year classes were above the mean. The median size was 

 1,658 million. 



SPAWNING STOCK 



Figure 9. — Atlantic mackerel spawning stock biomass (metric tons) 

 in 1962-77 and abundance at age 1 of the 1961-77 year classes from 

 cohort analysis. Open circles indicate estimated year-class sizes. 



The 1974 year class at age 1 was estimated to be 2,516 

 million fish based on the autumn survey age index 

 (Table 4, Fig. 6) and 2,104 million based on the spring 

 survey age 1 index (Table 5, Fig. 7). At age 2 this year 

 class was estimated to be 1,488 million fish based on the 

 spring survey age 2 index (Table 5, Fig. 8). The 

 corresponding catch of 349.5 million fish at age 2 in 1976 

 (Table 8) implies, using Equation (3), an F of 0.314. It 

 then follows that the size of this year class at age 1 (from 

 cohort analysis) would be 2,447 million fish. The mean of 

 the above three estimates at age 1 was 2,356 million fish. 

 The catch of 375.4 million fish at age 1 in 1975 (Table 8) 

 and the year-class estimates of 2,516 and 2,104 million 

 fish at age 1 result, using Equations (2) and (3), in year- 

 class estimates at age 2 of 1,543 and 1,238 million fish, 

 respectively. The mean of these two estimates and the 

 other estimate of 1,488 million at age 2 was 1,423 million 

 fish. The reported catch of 349.5 million at age 2 from a 

 year-class size of 1,423 million fish requires, from Equa- 

 tion (3), an F of 0.331. Cohort analysis, using this F at 

 age 2 in 1976, results in a year-class size of 2,358 million 

 fish at age 1 in 1975. Given this estimate and the mean 

 (2,356 million) of the three other estimates, the 1974 year 

 class at age 1 was set at 2,360 million fish. 



The 1975 year class at age 1 was estimated to be 614 

 million fish based on the autumn survey age index 

 (Table 4, Fig. 6) and 915 million based on the spring sur- 



vey age 1 index (Table 5, Fig. 7). At age 2 this year class 

 was estimated to be 652 million fish based on the spring 

 survey age 2 index (Table 5, Fig. 8). The catch of 33.0 

 million fish at age 2 in 1977 (Table 8) implies, from 

 Equation (3), an F of 0.060, which then infers (from 

 cohort analysis) a year-class size of 898 million fish at age 

 1 in 1976. The mean of the above three estimates at age 1 

 was 809 million fish. The catch of 12.3 million fish at age 

 1 in 1976 (Table 8) and the year-class estimates of 614 

 and 915 million fish at age 1 result, from Equations (2) 

 and (3), in year-class estimates at age 2 of 444 and 667 

 million fish, respectively. The mean of these two esti- 

 mates and the other estimate of 652 million at age 2 was 

 588 million fish. The reported catch of 12.3 million fish at 

 age 2 from a year-class size of 588 million fish requires an 

 F of 0.067. Cohort analysis, using this F at age 2 in 1977, 

 results in a year-class size of 809 million fish at age 1 in 

 1976. Given this estimate and the mean (809 million) of 

 the three other estimates, the estimated size of the 1975 

 year class at age 1 was considered to be 810 million fish. 



The 1976 year class at age 1 was estimated to be 417 

 million fish based on the spring survey age 1 index (Table 

 5, Fig. 7). There were no fish from this year class caught 

 at age (Table 4) during the 1976 autumn survey. The 

 survey catch-per-tow index for this year class at both 

 ages and 1 was lower than for any other year class dur- 

 ing 1963-77 (Tables 4, 5). This indicates that this year 

 class may be very weak. Previously the weakest year 

 classes since 1961 appeared in 1962 and 1963 (430 million 

 fish at age 1). Based on the single estimate from the 1977 

 spring survey data, the size of the 1976 year class at age 1 

 was set at 415 million fish. 



There are no estimates of the size of the 1977 year class 

 available. Since the contribution of age 1 fish to the 1978 

 catch is expected to be low, estimation of the size of the 

 1977 year class is not particularly critical to the results of 

 the assessment. However, the consequences of overesti- 

 mating the size of this year class are more detrimental to 

 conservation management than of underestimating it. If 

 the year class is underestimated, any unrealized catches 

 at age 1 can be regained in later years since yield per re- 

 cruit reaches a maximum at about age 4 (ICNAF 1973). 

 However, if the year class is overestimated, the 1978 

 allowable catch may be set too high to achieve manage- 

 ment objectives and the 1979 stock size would be less 

 than projected. The 1977 year class at age 1 was, there- 

 fore, set at the minimum level of the weak 1976 year 

 class. 



Partial Recruitment 



Atlantic mackerel appear to have been fully recruited 

 to the fishery at age 3 and older in recent years, based on 

 age-specific mortality rates (Table 3). Partial recruit- 

 ment at ages 1 and 2 varied considerably during 1963-77 

 (Table 13). Partial recruitment at age 1 ranged from 1 to 

 lOOTc and at age 2 from 16 to 90%. Values prior to 1968 

 are less precise than those since because the numbers-at- 

 age data for 1962-67 are based on very limited sampling 

 data and are not as reliable as later data (Anderson et al. 



10 



