46 DAHLBERG 



maximum egg production of a species, and applying low larva-to- 

 adult survival rates. These problems are corrected by using mean 

 lifetime fecundities and applying appropriate combinations of 

 survival rates and larval reductions. 



Applying the survival rate for the hatching-to-adult period is 

 reasonable v^hen larval entrainment losses are based on the reduction 

 of total larval production rather than the number of larvae actually 

 entrained. This reduction in larval production can be estimated by 

 multiplying larval production by the fraction of larvae entrained 

 throughout the pelagic period. The results are in reasonable 

 agreement with projections based on the assumption that the 

 percentage reduction of adults is equivalent to the percentage of 

 larval exploitation. Interpretations of these results should consider 

 that the projected adult loss is distributed among all mature age 

 classes and is in proportion to the adult age-class composition in a 

 stable population. 



With the exception of the general mode of application of the 

 Horst model (Eqs. 1 and 3), the calculations of N^ are probably 

 biased toward a possible maximum or worst-case estimate by the 

 assumptions that (1) an intake randomly draws on the total larva 

 population, (2) there is no avoidance reaction to induced intake 

 currents, and (3) there are no compensatory responses. 



Although larvae are transported by wind-generated lake currents 

 (Houde and Forney, 1970) and by power-plant-induced currents, it is 

 unlikely that they will be randomly recruited into an intake 

 throughout the pelagic period. Spatial distribution and movements of 

 larvae should be considered when possible. In Oneida Lake, postlarvae 

 concentrate in shallow bays along the southern shore (Noble, 1972). 



Avoidance of an intake probably increases with size of larvae, as 

 does avoidance of sampling gear (Noble, 1972). Although young 

 walleyes are pelagic up to 40 days and to 35 mm in length (Fig. 1), 

 Noble did not capture postlarvae over 18 mm long. 



Natural compensation for entrainment exploitation may occur in 

 the exploited year class as increased survival or growth or in 

 subsequent generations if fecuiidity also increases with grovii^h. 

 McFadden (1976) indicated that only partial compensation can be 

 expected. High compensation is apparent in Lake Erie walleyes, 

 which maintained their stock size despite total mortality rates of 

 50% in yearlings and 80% in older fish (Regier, Applegate, and 

 Ryder, 1969). Additional growth compensation would not be 

 expected, however, if this population had reached its physiological 

 maximum for growth (Regier, Applegate, and Ryder, 1969) or if the 

 supply of forage fish was low (Moyle, 1949). 



