FISHERY BULLETIN: VOL. 86, NO. 3 



ity rates (ZAt) and scaled juvenile abundance (J,) 

 was positive and statistically significant (P < 0.04) 

 (Table 3). Since the average density-dependent 

 mortality (Z^post = 0.30) present during the post- 

 juvenile stage comprised only 6% of the mean total 

 postjuvenile mortality rate {ZA = 4.85), density- 

 dependent mortality is not large enough to alter the 

 significant linear correlation (r = 0.78, P < 0.01) 



betv^^een the juvenile indices (J^ ) and adult recruit- 

 ment (Rf) for the 1967 through 1982 year classes 

 (Fig. 3). 



The prejuvenile mortality rates (ZEJ,) from 1967 

 through 1987 v^ere positively correlated (r = 0.70, 

 P < 0.0004) to total egg production (EggS() (Fig. 4). 

 Egg production alone explained 49% of the varia- 

 tion in prejuvenile mortality (Table 3) and the slope 



Table 3.— Estimates of density-dependent (Zp), density-independent mortality (Z,) and the fraction of density- 

 dependence (ZqIZi^Zq) during the postjuvenile (1967-82) and prejuvenile (1967-87) mortality phases for 

 American shad in the Connecticut River. SE = standard error, and numbers in parentheses = 95% C.I. for 

 Zq,ZEJ, = prejuvenile mortality, Z4, = postjuvenile mortality. 



Model 



Parameters 



SE 



Z, Zol{Z,,Z^) 



Prejuvenile 

 ZEJ, = a + b(Eggs,) 



ZEJt = a + b(Eggs,) 

 + c (J Flow,) 



Postjuvenile 

 ZA, = a + b(J,) 



a = 5.466 



b = 1.88 X 10"^ 



r^ = 0.49 



a = 4.287 



b = 1.97 X 10"^ 



c = 0.0028 



r^ = 0.80 



a = 4.53 



b = 3.49 X 10" 



r^ = 0.275 



0.313 



4.43 X 10"^ 



0.303 



2.87 X 10"^ 



0.0005 



0.15 



1.51 X 10"^ 



1.13 5.47 0.17 



(0.862-1.392) (0.136-0.203) 



1.18 5.34 0.18 



(1.007-1.351) (0.158-0.202) 



0.30 4.53 0.062 



(0.035-0.560) (0.008-0.110) 



1300 



« 1025 



c 

 <0 

 (0 



3 



o 



I 750 



u 



3 



< 475 



200 



— n 

 10 



—J- 

 20 



— I— 

 30 



— r- 

 40 



— I— 

 50 



— I— 

 60 



70 



Juvenle Index 



Figure 3.— Relationship between total adult recruitment (R,) and the juvenile abundance indices (/ATZ), ) of American 



shad from 1967 through 1982. 



472 



