The geometric mean annual survival rate (S) for the five estimates was 0.572, corresponding to an 

 instantaneous mortality rate (Z) of 0.558 (Table 16). The value is larger than the survival estimate of 

 0.526 determined by Howe and Coates (1975) for winter flounder south of Cape Cod and even greater 

 than that for fish from Great South Bay, N.Y. (0.27-0.28; Poole 1969) and Rhode Island (0.35-0.49; Berry 

 et al. 1965). The estimate may be biased because of violations in required assumptions. Ricker (1975) 

 noted that Robson and Chapman's formula assumes that survival rate is constant at all ages, that all 

 year-classes are recruited at the same abundance, and that all ages are equally vulnerable to the sampling 

 gear. When these conditions are not met, estimates of S are biased and confidence intervals are too 

 narrow. Ricker also noted that differences In year-class strength are usually found for most stocks and 

 that the best estimate of S will be made using a catch curve with equal weighting. 



Accordingly, catch curves were constructed for each year of data. Examination of the plotted log 

 frequencies of age showed that the curves were nonlinear. Furthermore, some bumps in the curves 

 increased with year, indicating probable non-uniform recruitment with stronger year-classes influencing 

 catches more so than average or weak ones. Tendencies for age 3 and age 7 and older frequencies to be 

 lower than the predicted regression line also suggested less than complete recruitment at age 3 and increasing 

 mortality in older age groups. Even so, for all years examined, the mean age of specimens age 3 and older 

 ranged only from 4.1 to 4.5. Annual estimates of Z ranged from 0.515 to 0.708, with a geometric mean 

 of 0.611. 



Table 16. Survival (S) and instantaneous mortality rate (Z) of age 3 and older 



Niantic River winter flounder determined using the method of Robson 

 and Chapman (1961). 



Year 



95% CI 



0.600 

 0.573 

 0.555 

 0.568 

 0.498 

 0.558 



Annual mortality rate = 1-S 



50 



