SAVOY AND CRECCO: MORTALITY OF AMERICAN SHAD 



noted that the highest survival rates of American 

 shad eggs occurred over gravel and rubble sub- 

 strates. Since these areas generally have higher 

 water velocities, eggs lodged within the interstices 

 of the gravel and rubble are less likely to be eaten 

 by predators or covered by silt. Dense aggregations 

 of eggs may also be highly susceptible to fungal 

 agents (Leach 1925), which could then act as effi- 

 cient density-dependent predators. However, since 

 predators of American shad eggs and larvae in the 

 Connecticut River have not yet been identified, 

 density-dependent losses are difficult to quantify. 

 Also, predation mortalities in general may be either 

 density-dependent or density-independent, depend- 

 ing on the functional response between the predator 

 and the prey (Hassell 1978), and whether predators 

 actively search for discrete aggregations of eggs and 

 larvae or prey on randomly encountered eggs and 

 larvae (Gulland 1987). 



In the absence of direct egg mortality estimates 

 of American shad, the accuracy of our egg mortal- 

 ity estimates {ZEt) from Equation (7) are difficult 

 to evaluate, particularly since the standard errors 

 about the estimates are highly variable (Table 6). To 

 provide an independent estimate of daily egg mor- 

 tality, we used the multiple regression method of 

 McGurk (1987) to estimate the average daily egg 

 mortality rate. This method requires an estimate of 

 the patchiness and the mean dry weight of a shad 

 egg. We derived Lloyd's patchiness index (x = 

 7.26, SE = 0.376) for American shad eggs in the 

 Connecticut River from the 1974-75 egg surveys in 

 the Holyoke Dam impoundment (NUSCo 1977). Sub- 

 stituting the mean weight of a shad egg (0.00025 

 g, Leggett 1969) and the mean patchiness index into 

 McGurk's equation 4, yields an instantaneous daily 

 egg mortality rate of 0.34 which closely approx- 

 imates our average estimate (0.36) from 1979 to 

 1987 (Table 6). 



Lastly, since it is presently unknown if younger 

 (smaller) virgin female American shad produce 

 smaller, less viable eggs than older shad as was 

 reported for cod, Gadus morhua, (Knutsen and 

 Tilseth 1985), the contribution of the age structure 

 to the parent progeny relationship (Rosenberg and 

 Doyle 1986) should be considered. This issue and an 

 examination of whether egg deposition rates are 

 density-dependent should be addressed by monitor- 

 ing egg retention rates among postspawning Ameri- 

 can shad. 



ACKNOWLEDGMENTS 



We would like to thank Rita Lorenzetti-Langan, 



Mary Payette, Thomas Stanford, and all the other 

 people who have contributed field and laboratory 

 time or helpful advice. We also thank the two 

 anonymous reviewers for their comments on the 

 manuscript. 



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