FISHERY BULLETIN: VOL. 86, NO. 3 



density-independent mortality after the juvenile 

 stage. 



In this study we used stock-recruitment, pre- and 

 postrecruitment mortality data for American shad 

 in the Connecticut River from 1967 through 1987 

 to 1) estimate egg mortality rates for the 1979-87 

 year classes; 2) measure the contribution of density- 

 dependent and density-independent mortality before 

 and after the juvenile stage; and 3) determine the 

 life stage(s) at which most of the density-dependent 

 mortality takes place. 



METHODS 



Data Source 



Estimates of adult recruitment and parent stock 

 size (± SE) from 1967 to 1982 (Table 1) were based 

 on annual population estimates derived by earlier 

 mark-recapture studies (Leggett 1976; Crecco and 

 Savoy 1987^) and by the annual mean number of 

 American shad lifted over the Holyoke Dam (Crecco 

 and Savoy 1985a). The parent stock size (PAR J 

 from 1967 through 1987 was the annual population 

 estimate of female shad minus that year's commer- 



cial catch of female shad. Parent stock (PAR^) was 

 separated into the parent stock that spawned above 

 the Holyoke Dam (PJ (Moffitt et al. 1982; O'Leary 

 and Booke 1986^, 1987^) and the spawning stock 

 that spawned below the dam (P,, = PAR, - P„). 

 Since female American shad mature between ages 

 four and six, female shad recruitment from the 

 1967-82 year classes was the sum of virgin 4-, 5-, 

 and 6-yr-old female shad in the 1970-87 runs based 

 on the age-class structure from previous studies 

 (Jones et al. 1976; Leggett 1976; Crecco et al. 

 1984^). Since the sex ratio of mature progeny from 

 each year class is close to 1:1 (Leggett 1976), total 

 recruitment (Rf) from the 1967-82 year classes was 

 estimated by doubling the female shad recruitment 

 estimates. Direct estimates of male shad recruit- 

 ment are biased by gill net selectivity and differen- 

 tial culling practices of commercial fishermen 

 (Crecco et al. 1984^). 



^Crecco, V. A., and T. F. Savoy. 1987b. Fishery Management 

 Plan for American shad in the Connecticut River. Unpubl. 

 manuscr., 117 p. Connecticut Department of Environmental Pro- 

 tection, Hartford, CT 06106. 



'O'Leary, J., and H. E. Booke. 1986. Connecticut River anad- 

 romous fish investigations. Mass. Coop. Fish. Res. Unit Proj. Per- 

 formance Rep., F-45-R-2, 37 p. 



"O'Leary, J., and H. E. Booke. 1987. Connecticut River anad- 

 romous fish investigations. Mass. Coop. Fish. Res. Unit Proj. Per- 

 formance Rep., 32 p. 



^Crecco, V. A., T. Savoy, and L. Gunn. 1984. Population 

 dynamics studies of American shad in the Connecticut River. CT 

 Dep. Environ. Prot. Final Rep. AFC 13, 76 p. 



•^Crecco, V. A., L. Gunn, and T. Savoy. 1981. Connecticut 

 River shad study, a progress report. Unpubl. manuscr., 87 p. 

 Connecticut Department of Environmental Protection, Hartford, 

 CT 06106. 



Table 1.— Estimates of total female American shad parent stock, female spawn- 

 ing stock above and below the Holyoke Dam, mean June flows from 1967 to 1987, 

 and total number of adult recruits from the 1967-82 year classes. SE = stand- 

 ard errors about the estimates x 10^. 



468 



