Growth 



Examination of the length-frequency distribu- 

 tion of larvae collected in 1986-87 showed a sep- 

 aration between the first three developmental 

 stages by predominant 0.5-mm size-classes (Fig. 

 25). Stage 1 larvae were primarily in the 2.5 to 

 3.5-mm size-classes (95%), Stage 2 were 3.0 to 

 4.5 mm (86%), Stage 3 were 4.5 to 7.5 mm 

 (86%), and Stage 4 were 6.5 to 8.5 mm (92%). 

 These predominant size-classes for each develop- 

 mental stage were consistent with previous fmd- 

 ings (NUSCO 1987), indicating that development 

 and length were closely related. This allowed for 

 the approximation of developmental stage from 

 length-frequency data. 



A comparison was made of the length- 

 frequency distribution between Niantic River and 

 Bay in 1986-87 (Fig. 26). The pattem found was 

 similar to that of the spatial distribution of devel- 

 opmental stages for the same year (Fig. 19). 

 Smaller size-classes predominated in the river, 

 with over 60% of them 3.5 mm or smaller. In 

 contrast, over 50% of the larvae in the bay were 

 5.0 nun and larger. These patterns were similar 

 to those found in previous years (NUSCO 1987) 

 and was further evidence that a majority of the 

 larvae hatched in the Niantic River and then grad- 

 ually flushed into the bay. Based on the large 

 decline in the river from the 3.0- to the 4.0-mm 

 size-class, it is likely that this was the size range 

 where most of the mortality occurred. Larvae in 

 these size-classes would have been yolk-sac larvae 

 (Stage 1) and first-feeding Stage 2 larvae. In a 

 bioenergetic study on winter flounder larvae, 

 Laurence (1977) found that they had a low energy 

 conversion efficiency at first feeding compared to 

 later development, and that this stage of develop- 

 ment was probably a "critical period" for mortality. 

 The "critical period" concept was first hypothe- 

 sized by Hjort (1926) and discussed by May 

 (1974) for marine fishes. In many cases, the 



strength of a year-class was thought to have been 

 determined by the availability of sufficient food 

 after yolk-sac absorption was completed. How- 

 ever, the occurrence of a "critical period" depended 

 upon a number of environmental and species- 

 specific factors (May 1974). The small increase 

 in frequency of larvae from the 5.0- to 7.0-mm 

 size-classes in the river may have been caused by 

 the net import of these larger larvae into the river 

 due to the behavioral retention mechanisms dis- 

 cussed in NUSCO (1987). Previous sampling 

 during ebb and flood tides at the mouth of the 

 river showed a net loss of larvae smaller than 5 

 mm from the river, but a net import of larger 

 larvae. This was attributed to vertical migration 

 by larger larvae in relation to tidal stage as a 

 retention mechanism to remain in the river. These 

 larvae apparently swam up from the bottom dur- 

 ing flood tides and remained near bottom during 

 ebb tides. The decline in frequency after the 

 7.0-mm size-class probably was due to 

 undersampling as larvae metamorphosed and be- 

 came less susceptible to capture with a plankton 

 net. 



The effects of temperature on larval growth 

 was examined in the laboratory in 1986 under 

 four temperature regimes. Larvae were reared 

 from hatches on March 6 (treatments I and II) 

 and April 1 (treatments III and IV). Linear re- 

 gression was used to estimate growth rates and 

 reasonable fits were obtained with r values of 

 0.85 and higher (Fig. 27). The rate of yolk ab- 

 sorption was similar in all treatments, but growth 

 rates differed (Table 14). Temperature compari- 

 sons between treatments were based on the first 

 40 days from hatching. Growth rates were sig- 

 nificantly lower in lowest and highest temperature 

 regimes compared to the intermediate treatments. 

 This limited laboratory study suggested that larval 

 winter flounder have an optimum temperature 

 range for growth, and as the temperature decreased 

 or increased from the optimum, grovi^h rates de- 

 creased. 



Winter Flounder Studies 



187 



