SAVOY AND CRECCO: MORTALITY OF AMERICAN SHAD 



Table 8.— Relationship between the number (x 10®) of American shad at each life history 

 stage and adult recruitment from each year class, r = correlation coefficient and P = 

 probability levels from 1979 to 1987. 



'Predicted or observed recruitment in thousands. 



^Correlation coefficient for egg abundance and recruitment was based on the 1967-87 data (Tables 

 1.2). 



1979 to 1987 to total egg production (Eggs^) in a 

 linear model: 



ZEt = a + 6(Eggs,). 



(13) 



As before, significant density-dependence would be 

 shown if the slope (6) in Equation (13) was positive 

 and differed significantly from zero. 



The cumulative amount of density-dependent mor- 

 tality during the egg and early larval stages com- 

 bined was estimated by summing the instantaneous 

 total mortality rates of early larvae (ZELf) and 

 eggs (ZEt) and then regressing the total {ZEf + 

 ZELf) against egg production as in Equation (13). 

 This procedure was repeated for each subsequent 

 stage by adding their respective instantaneous total 

 mortality rates. We then estimated the relative 

 magnitude of density-dependent mortality (Z^) at 

 each stage by multiplying the slope (6) of each 

 regression equation by the geometric mean egg pro- 



duction (GEgg) from 1967 through 1987. The 

 density-independent mortality rate {Zj) at each 

 stage was expressed by the y-a.xis intercept (a) of 

 each regression equation. The percentage contribu- 

 tion of density-dependent mortality {%Zf)) for each 

 period was the ratio of Z^ to total mortality {Z^ + 

 Zj) times 100. 



Results 



Our results showed that significant density- 

 dependent mortality first occurs during the early 

 larval stage and persists for all stages thereafter 

 (Table 9). The percentage contribution of density- 

 dependent mortality i%Z^) rose from 24% of the 

 total during the egg stage to 41% during the early 

 larval stage, and then declined during the mid- and 

 late larval stages as the magnitude of density- 

 independent mortality increased. The mean density- 

 dependent mortality rate for the egg and early larval 



Table 9.— Relationship between the cumulative mortality rates and parent stock sizes (Eggs,). 

 The cumulative mortality rate was egg mortality (ZE) plus early larval mortality (Z2), midlarval 

 mortality (Z3), late larval mortality (Z4), and juvenile mortality (Z5). Zg = density-dependent 



mortality rate, Z, = density-independent mortality rate 



o/oZo = 



percentage density-dependent 



mortality, %Z, = percentage density-independent mortality, t = student /-statistic. 



'Significant student f-statistic at the P < 0.01 level and SE(b) = standard error of b. 



477 



