DRYFOOS, CHEEK, and KROGER: ATLANTIC MENHADEN MIGRATIONS 



hypothetical northern subpopulation. Tagging 

 of known sexually active fish in the north in 

 the spring or summer followed by their recap- 

 ture in the North Carolina fall fishery, where 

 they could be assumed to be sexually active 

 again, would confirm this conclusion. For the 

 present, with such significant mixing of fish 

 from the various areas, the Atlantic menhaden 

 resource should be considered as one population. 



SURVIVAL AND 

 EXPLOITATION RATES 



Tag recoveries can provide estimates of sur- 

 vival and exploitation rates that are indepen- 

 dent of those obtained from catch and effort 

 data (Widrig, 1954), and with such data can 

 provide measures of changes in resource avail- 

 ability. Estimates of annual sui-\'ival rates were 

 calculated for releases of each season by deter- 

 mining the ratio of recoveries in successive 

 years. These estimates vary considerably because 

 of fluctuations in availability. Survival rates 

 also were obtained using the Robson-Chapman 

 catch curve analysis (Paulik, 1962). The latter 

 rates for the entire recovery period were the 

 most consistent (Table 14). Better estimates of 

 survival rates will be possible when we have a 

 longer series of recovery data. Survival rates 

 based on analyses of the age composition of the 

 catch for 1966 to 1968 average 0.22 and are in 

 good agreement with the average estimate of 

 0.24 from tag returns. Using an average value 

 of 0.23 corresponds to a total instantaneous 

 mortality rate of Z = 1.47. 



Exploitation rates can be obtained from the 

 estimated percentage of tags recaptured from 

 early season releases after corrections are made 

 for tagging mortality and tag shedding (Table 

 15). Tagging mortality and shedding losses for 

 1967-69 are thought to be about 10% for New 

 York and New Jersey, 20% for Virginia, 25% 

 for North Carolina, and 40% for Florida releases. 

 The differences in percentage result primarily 

 from differences in fish size. The average ex- 

 ploitation rate along the Atlantic coast, weight- 

 ed by the catch in numbers of fish and prelim- 

 inary estimates of tagging mortality, varied 

 between 42 and 54% . Estimates for 1967 and 

 1968 from North Carolina fall fishery releases 

 agree with values obtained from the weighted 

 average summer releases. This indicates that 

 most menhaden occurring in the fall fishery 

 also occur within the fishery during the sum- 

 mer. The best single estimate of exploitation 

 rate obtained from these tagging data is 50% . 



Fishing and natural mortality rates can be 

 obtained from exploitation rates and total mor- 

 tality rates. Using our estimates of the instan- 



Table 15. — Exploitation rate by area 

 and year of release. 



Table 14. — Survival rates for all releases by season and by year, 1966-69. 



' Survival rate is ratio of recoveries in either fall or summer fishery 1 yr to recoveries in the 

 same season the previous year. 



2 Robson-Chapman annual survival rate calculated from combined summer and fall recoveries. 

 For fall releases recoveries were only considered for subsequent summer and fall seasons. 



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