differences may occur from year to year depending upon prevailing condi- 

 tions and a regression calculated in one year may or may not hold for 

 other years. However, this cannot be ascertained for winter flounder 

 spawning in the Niantic River without additional estimates of fecundity. 



Mortality and survival estimates 



Survival and its complement, total mortality, of the winter flounder 

 has been calculated by dividing the estimated number of a specific year 

 class in one year by the estimated number in the previous year. The 

 mean of several of these calculations was reported as 0.3844 (NUSCo 

 1981a). This method, updated and shown on the upper portion of Table 

 24, is called cohort- specific by Ricker (1975). He noted that most 

 methods of calculating mortality rely heavily on the existence of equilib- 

 rium conditions and are overly sensitive to fluctuations in year class 

 strength. The cohort-specific method is independent of equilibrium 

 conditions, whereas the time-specific techniques (comparison of the 

 numbers present in successive age groups within a year) assumes equilib- 

 rium conditions. Examination of the Niantic River showed that in many 

 cases no or unrealistic survival estimates were calculated by the cohort- 

 specific simple ratio method because a larger number of winter flounder 

 of a specific age were estimated in one year than was found in the 

 previous year. These errors were probably caused by inaccurate population 

 estimates, errors in aging, and bias in the selection of the aging 

 samples. This particular method of calculation itself is also subject 

 to bias and according to Gulland (1969) and Ricker (1975), the most 

 proper way to determine survival (S) using this procedure is to calculate 

 the instantaneous mortality coefficient, Z, which is equivalent to -In 



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