40 DAHLBERG 



SURVIVAL CURVES AIMD ENTRAIIMMEIMT LOSSES 



Constructing egg-to-adult survival curves (e.g., Fig. 1 and Table 1) 

 is recommended as the first step in assessing larval entrainment 

 impact. The curves in the figure illustrate a snapshot approach in 

 which ratios of adults and earlier stages represent survival rates. 

 Families of such curves permit graphic comparison of such variables 

 as year-to-year differences, augmentation, and biased sampling, e.g., 

 avoidance of sampling gear. The latter is a common source of error in 

 lacustrine studies (Hackney, 1977) and may be responsible for the 

 high mortality rate determined by Noble (1972) for walleye 

 postlarvae in one bay of Oneida Lake (see Fig. 1). 



The predictive value of survival curves is greatest in stable 

 populations (Horst, 1977a). When we are restricted to specific 

 population data, we can construct representative survival curves by 

 averaging data acquired over several years, as in Fig. 1, or by selecting 

 data for a typical year. Survival curves for naturally spawned (1969, 

 1971, and 1973) and augmented (1968, 1970, and 1972) popula- 

 tions of walleyes (Forney, 1976a) are presented in Fig. 1. 



Egg production (13 x 10^) of walleyes in Oneida Lake was 

 estimated by Forney (1976a) from fecundity-at-age data. Walleyes 

 hatched near May 10 (day in Fig. 1) and were pelagic for 

 approximately 40 days. The larvae occupied 971 x 10^ m^ of water, 

 which is the upper 5.5 m (Clady, 1975) and 64% of the total lake 

 volume. Postlarvae were transported by surface currents and yolk-sac 

 larvae by subsurface currents (Houde and Forney, 1970). 



Since larvae were not fully recruited into the plankton until the 

 swim-up stage (day 10), numbers of larvae at hatching and survival 

 from hatching to swim-up (13.6%) were estimated from the ratio of 

 these stages in the augmented population. Assuming the difference in 

 numbers of 10-day-old larvae in augmented and unstocked popula- 

 tions (3.192 X 10"^) resulted from the stocking of 2.347 x 10^ 1- to 

 3-day-old larvae (Forney, 1976a), the survival rate was 13.6%. There 

 is a possibility that stocked larvae experience higher mortality than 

 wild larvae (Forney, 1976a). 



Populations of demersal young-of-the-year and age I juveniles 

 were estimated from trawl studies (Forney, 1976a). Survival to 

 adults is shown in Fig. 1 by plotting parent stock size, including all 

 mature age classes (age IV and older), as determined by mark and 

 recapture studies (Forney, 1976b). 



Assuming a feasible open-cycle cooling-water-intake flow of 

 1 million gpm (5.45 x 10^ m^/day), cumulative numbers of larvae 

 entrained were calculated and illustrated by a survival curve (Fig. 1). 



