TABLE 18. Monthly survival rate estimates for young winter flounder taken at two stations in the lower 

 Niantic River from 1983 through 1987 as determined by a catch curve. 



Year 



Monthly survival rate (S) at station: 

 LR WA 



1983 

 1984 

 1985 

 1986 

 1987 



0.552 

 0.564 

 0.569 

 0.553 

 0.597 



0.661 



0.335 (early); 0.440 (late) 



0.511 



Average annual survival rate 



0.567 



0.507 



Determined from the average of the corresponding estimates of instantaneous mortality rate Z. 



Weekly mean lengths at LR during 1983-85 

 were similar by year until about late June; means 

 were 6 to 8 mm greater thereafter during 1983 

 (Fig. 34). Growth in 1986 and 1987 was alike, 

 but means were less than in previous years. 

 Smaller differences were seen in growth among 

 years at WA. Average size of young appeared to 

 be greater in July during 1987, but if 95% confi- 

 dence intervals were shown, no significant differ- 

 ences would have been found in August and Sep- 

 tember. The reasons for these differences among 

 years are not known. Water temperatures ap- 

 peared to be comparable; seasonal 19-week means 

 ranged from 18.9''C in 1987 to I9.5°C in 1985. 

 Life history data such as food preferences and 

 rates of feeding were not available, but both were 

 considered important factors in the growth of 

 young plaice (Steele and Edwards 1970; Poxton 

 et al. 1983). Apparent annual changes in growth 

 may also have been caused by differentia] move- 

 ment of larger young away from the station, 

 which also would have increased the apparent 

 mortality rate. However, few young were taken 

 during the summer at trawl monitoring program 

 stations. Thus, neither large-scale movements 

 offstation nor differential movements by size 

 seemed to have occurred, at least into areas sam- 

 pled by trawl. 



A tentative explanation for the differences ob- 

 served among years is density-dependent growth, 

 especially at LR. Densities there in 1986 and 



1987 were greater than during 1983-85. Benthic 

 production and food availability at the stations 

 may have been a limiting factor for growth. 

 Density-dependent growth in juvenile fish has 

 been regularly observed (Gushing and Harris 1973; 

 Ware 1980; Poxton et al. 1983; van der Veer 

 1986). This may be demonstrated conclusively 

 in forthcoming years, if greater densities are ob- 

 served with concurrent slower rates of growth. 



Mortality 



To determine instantaneous mortality (Z) and 

 weekly and monthly survival rates (S), catch 

 curves were constructed using annual abundance 

 data from LR for 1983-87 and WA for 1985-87. 

 This method assumed that young comprised a 

 single-age cohort which was followed from week 

 to week during the sampling season. Catch curves 

 and estimates of Z and S for 1983-85 were given 

 in NUSCO (1987). The catch curves for LR data 

 from 1986 and 1987 had relatively good fits with 

 r^ of 0.70 and 0.89, respectively (Fig. 35). Re- 

 markably similar values of Z were obtained over 

 all years, resulting in monthly survival estimates 

 ofO.552 to 0.597 (Table 18). Estimates of survival 

 were more variable at WA (Fig. 36). The fit to 

 the data in 1985 (NUSCO 1987) was not as good 

 (r =0. 56) and monthly survival rate was estimated 

 as 0.661. 



Winter Flounder Studies 



203 



