AHRENHOLZ ET AL.: ATLANTIC MENHADEN POPULATION AND FISHERY 



Ul 

 O 



< 



X 



a 



D 



o 

 a. 



X 



1200 



1000 



° 800 



c 

 o 



^ e 



< 

 I- 

 o 



600 



400 



200 



•6S 



•SS 



89 



S7 



\.71 



64 . 



s« \ '."eb 74 



.: 6r>2 " 



•7- 1" 

 ° 66 



J 1_ 



J I I I L 



10 



J I L 



12 



14 16 



J I L. 



18 



YEAR CLASS SIZE 



(numbers x 10') 

 Figure 21. — Estimated yield contribution of Atlantic menhaden in 

 thousands of metric tons, by the 1955-76 year classes through age 5, 

 plotted against year class size. 



Table 11. — Estimates of annual Fat age for the 1955 and 

 1976. and 1956 and 1975 year classes of Atlantic men- 

 haden, through age 5. 



ages of 0-2 than was the 1976 year class. In fact, 

 yield from the 1955 year class could have been 

 markedly increased with a reduced fishing rate 

 on these younger fish. The harvest rates of the 

 1956 and 1975 year classes were similar for ages 

 0-1, but markedly greater at age 2 for the 1975 

 cohort. While the high age-2 fishing mortality 

 rate probably contributed to the lower yield, it did 

 not fully explain the marked difference. The gen- 

 erally higher rates of F for ages 3-5 exhibited for 

 both the 1975 and 1976 year classes were proba- 

 bly inconsequential for this comparison of yield. 

 While an increase in the true value of natural 

 mortality could cause a decrease in total yield by 

 year class, differences in growth provide a more 

 obvious explanation for the differences within 

 these two pairs of year classes. Growth curves (in 

 length and weight) for the 1956 and 1975 year 

 classes and the 1955 and 1976 year classes show 

 that individuals were much smaller during the 

 dominant harvest ages (1-3) for the two most re- 

 cent year classes (Figs. 22, 23). These differences 



could be great enough to account for most of the 

 differences observed in yield. Slower relative 

 growth in post age-1 fish has been apparent for 

 year classes 1973-78. 



To determine if age of maximum theoretical 

 biomass had changed owing to the different (flat- 

 ter) shaped growth curves displayed by the year 

 classes in the later 1970's, the relative biomass at 

 age of an unfished hypothetical year class was 

 estimated with the growth equation for the 1975 

 year class with M equal to 0.45 and the annual 

 weight-length expression for 1972. The results 

 display an increase in maximum age (to about 

 3.25 years), a decrease in total biomass, and a 

 much slower ascent and even slower descent from 

 maximum biomass than results for the 1970 year 

 class growth curve (Fig. 24). These results indi- 

 cate that age of entry to the fishery could have 

 been greatly delayed in the later 1970's with little 

 chance of losing yield. 



Given the progressive decrease in average size 

 at age of fish in the age classes which dominate 

 landings (Fig. 25), the decline in yield per recruit 

 following 1971 (Fig. 17) is expected. However, the 

 rapid decline in size at age is not entirely ascrib- 

 able to density-dependent growth. More impor- 

 tantly, the potential for increased yield with re- 

 ductions in F is probably greater than the results 

 from the MAREA yield-per-recruit model indi- 

 cate owing to the likelihood of size selective fish- 

 ing and the potential for differential stock- 

 specific growth rates. Additionally, the relatively 

 high MSY estimate obtained from the PRODFIT 



593 



