Because of the previously mentioned increase 

 in abundance during mid-summer of 1986, two 

 catch curves were used, resulting in early (0.335) 

 and late (0.440) seasonal estimates of monthly 

 survival rate. Although the above partitioning of 

 the data resulted in good fits, this apparent sam- 

 pling problem made the survival estimates unre- 

 liable. The 1987 monthly survival rate of 0.511 

 appeared to be based on more accurate density 

 estimates. Overall, survival may not have been 

 as high at WA as LR, although additional sam- 

 pling will be necessary to confirm this. The 

 monthly survival rates in the Niantic River were 

 less than the value of 0.69 reported by Pearcy 

 (1962) for the Mystic River estuary, which is the 

 only published estimate for young winter flounder. 

 Most monthly survival rate estimates for young 

 plaice in British coastal embayments also were 

 about 50% per month (Lockwood 1980; Poxton 

 et al. 1982; Poxton and Nasir 1985). 



Density-dependent processes in the fu^st year 

 of life following the larval stage are believed to 

 occur in a number of species and can greatly 

 affect subsequent recruitment to adult stocks 

 (Bannister et al. 1974; Gushing 1974; Sissenwine 

 1984). Density-dependent mortality was not ap- 

 parent for post-larval Niantic River winter floun- 

 der, at least over the range of abundances seen to 

 date. In fact, the highest survival rate estimated 

 was in 1987, when densities were greatest. 

 Density-dependent mortality was reported for 

 young plaice by Bannister et al. (1974), Lockwood 

 (1980), and van der Veer (1986). However, ex- 

 amination of their fmdings indicated that greatest 

 rates of mortality occurred only when extremely 

 large year-classes of plaice were produced (three 

 to more than five times larger than the average). 

 The numbers of juvenile winter flounder in the 

 Niantic River since 1983 were probably not ex- 

 tremely large or small enough to have produced 

 density-dependent mortality (should it exist), al- 

 though densities were apparently different enough 

 to have affected rates of growth. 



Impingement of winter flounder at 

 MNPS 



Annual estimates of the number of winter 

 flounder impinged on the traveling screens of 

 MNPS have been made since 1972-73 (NUSCO 

 1987). Since 1976, the winter flounder has been 

 the second-most abundant fish impinged at 

 MNPS, making up 5.9% of the total. About 

 two-thirds of the impingement occurred during 

 the winter with relatively little in summer. Fish 

 return sluiceways have been in place at Unit 1 

 since December 1983 (NUSCO 1986b, 1987) and 

 at Unit 3 since it commenced commercial oper- 

 ation (NUSCO 1988c). These sluiceways have 

 considerably reduced the impact of impingement 

 as the winter flounder is very hardy and had good 

 ( > 85%) survival following return to Long Island 

 Sound. The estimated impingement of winter 

 flounder at Unit 2 was 1,212 for October 1985 

 through September 1986, 547 during 1986-87, and 

 77 from October through mid-December 1987. 

 The 1985-86 estimate was less than 50% of the 

 next smallest aimual estimate. As in most previous 

 years, about two-thirds of the fish impinged in 

 1985-87 were less than 20 cm in length. The 

 numbers taken at Unit 2 during the past few years 

 have been small, due to varying plant operations; 

 declining winter flounder abundance; and possibly 

 the construction and operation of Unit 3, which 

 may have altered fish availability and movements 

 near the Unit 2 intake. The latter effect on fish 

 impingement is discussed in the Fish Ecology sec- 

 tion of this report. Although impingement is no 

 longer routinely monitored at MNPS, a require- 

 ment exists for reporting significantly large ( > 300 

 specimens per day) impingement events, should 

 they occur (NUSCO 1988a). 



Impact assessment 

 Approaches to impact assessment 



Considerable effort has been expended during 

 the past 15 years evaluating the impact of fish 

 mortality resulting from the operation of power 

 plants in the United States (Van Winkle 1977; 

 MacCaU et al. 1983). For adult fish subject to 



Winter Flounder Studies 



207 



