Hoenig et a\ Estimating survival rate over time for larval fishes 



491 



Our method was developed for the situation in which 

 cohorts can be sampled simultaneously. Thus, if in a 

 particular week gear efficiency was better than nor- 

 mal, any changes in catchabilities would tend to affect 

 both cohorts so that the ratio of catchabilities might 

 stay relatively constant. It may be tempting to use our 

 method to compare cohorts over the same part of the 

 ontogeny (i.e., cohorts of the same age occurring at dif- 

 ferent times of the season). However, in this situation 

 there is no reason to believe that variations in catch- 

 ability over time of the first cohort will be tracked by 

 variations in catchability of the later cohort. Hence, 

 there is no advantage in using our method over tradi- 

 tional methods of estimating absolute survival based 

 on declines in catch-per-unit-effort. 



Finally, we note that methods for estimating relative 

 survival have wide applicability beyond the study of lar- 

 val fishes. A classical problem is Lee's phenomenon in 

 which back-calculated sizes at the first annulus do not 

 agree with the observed sizes of the young fish. Jones 

 (1958) proposed as explanation that faster-growing fish 

 may have a different mortality rate than slower 

 growers. This can be studied by observing the propor- 

 tion at each age that have a small back-calculated size 

 at the first annulus. Hoenig and Lavdng (1983) describe 

 how the logistic model might explain the occurrence 

 of progressively skewed sex ratios with age. Differen- 

 tial survival has been studied by a variety of mark- 

 recapture methods in the context of fitness and natural 

 selection (see Manly 1985) and as a means of estimating 

 impacts of power plants (see Burnham et al. 1987). 

 Another application is to the evaluation of stocking suc- 

 cess as related to genetic strain of fish or hatchery 

 treatment. That is, the ratio of abundances of two 

 strains can be monitored over time to study whether 

 one strain survives better than the other. 



Acknowledgments 



We would like to thank Russell Millar, Geoffrey Evans, 

 Dennis Heisey, William W. Fox Jr., and an anonymous 

 reviewer for helpful comments. Victor Crecco and 

 Thomas Savoy generously provided data on shad lar- 

 val abundance for our example. 



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