ROGERS: EFFECTS OF TEMPERATURE AND SALINITY ON WINTER FLOUNDER 



at temperatures of 10°C and above some mor- 

 talities usually occurred during gastrulation. At 

 salinities of 35 to 40%, abnormal development of 

 the embryos was observed. The embryos were 

 shorter and thicker than normal and died just 

 prior to hatching. Collapsing eggs were noted at 

 37.5'Ii and above. Embryos incubated at 40% died 

 during gastrulation and throughout development 

 at all temperatures while all embryos held at 45% 

 died during gastrulation. At both 40 and 45% em- 

 bryos exhibited shrinkage and often collapsed. 



DISCUSSION 



The results indicate that although temperature 

 and salinity are both significant, the major effect 

 of increased temperature is to decrease the incu- 

 bation period, whereas salinity is the factor 

 which has more effect on the successful hatching 

 and survival of winter flounder embryos and lar- 

 vae (Figure 4, Table 4). It is apparent however, 

 that an interaction between the two does occur 

 since, at the optimum experimental temperature 

 (3°C), the salinity range over which high percen- 

 tages of viable hatches occurred was extended by 

 10% (Figure 1). At higher than optimal experi- 

 mental temperatures, the survival curves appear 

 to be dictated primarily by salinity; however, 

 survival occurs over a broad enough range that 

 the embryos and larvae can be described as 

 euryhaline with regard to the natural environ- 

 ment in which they are normally spawned. At all 

 temperatures tested, there was a decrease in in- 

 cubation time at higher salinities, a phenomenon 

 which was also reported in studies done on 

 Clupea harengus (Holliday and Blaxter 1960) and 

 Pacific cod (Forrester and Alderdice 1966). Those 

 authors speculated that the relationships of 

 temperature and salinity with hatching are de- 

 pendent on conditions that minimize the energy 

 required of the embryos in maintaining osmotic 

 equilibrium with their environment. Salinity also 

 appears to influence the time of embryo mortal- 

 ity. Observations on eggs indicated that mortality 

 usually occurred either at gastrulation, in 

 salinities of 40 and 45% at all temperatures, or 

 just prior to hatching in the lower salinities. Bat- 

 tle (1930) noted increased mortality of the four 

 bearded rockling, Enchelyopus cimbrius, at 

 hatching in low salinities and she attributed this 

 to poorly developed tail musculature. McMynn 

 and Hoar (1953), working with embryos of the 

 Pacific herring, Clupea harengus pallasi, ob- 



served that with the closing of the blastopore at 

 the end of grastrulation, embryos had a greater 

 ability to tolerate low salinities. However, many 

 embryos died just prior to hatching or when 

 partly emerged. Holliday (1965, 1969) observed a 

 similar occurrence in cod, Gadus callarius, and 

 plaice, Pleuronectes platessa. He felt that the low 

 specific gravity of such salinities made it difficult 

 for larvae to free themselves from the chorion so 

 that they died partly emerged. He also main- 

 tained that chorions did not rupture as easily at 

 low salinities. This phenomenon is also clearly 

 demonstrated for winter flounder in Table 3. The 

 highest percentages of abnormalities which were 

 aborted or partially hatched occurred at salinities 

 below 15%. 



Results of these laboratory experiments indi- 

 cate that successful incubation of embryos oc- 

 curred over a temperature range which exceeded 

 normal spawning season temperatures by as 

 much as 10°C, but coincide quite closely with 

 natural observations for salinity, although there 

 is a shift in survival toward slightly higher 

 salinities than would have been expected. It is 

 possible that the adults, while being held in the 

 laboratory, were conditioned to slightly higher 

 salinities than would have been encountered in a 

 spawning migration into estuaries. This might 

 explain the differences between natural popula- 

 tions and results of laboratory experiments. 



Most winter flounder populations move to in- 

 shore and estuarine waters to spawn (Perlmutter 

 1947; Bigelow and Schroeder 1953; Saila 1961), 

 but there are also spawning populations that re- 

 main in offshore shoals (Bigelow and Schroeder 

 1953; Marak et al. 1962). Field observations in 

 two estuaries of Narragansett Bay and in the Bay 

 itself indicate that spawning occurs at salinities 

 ranging from 11 to 32%. Plankton tows taken in 

 upper Chesapeake Bay produced one egg in 20% 

 with maximum numbers of larvae occurring be- 

 tween 6 and 14% (Dovel 1971). Salinities in sus- 

 pected offshore shoal spawning areas range from 

 32 to 35.5%, at the bottom (Bumpus 1973), so an 

 overall spawning range from 5 or 6 to 35.5% is 

 indicated for natural populations. The normal 

 temperature range for spawning is 0° to 3.3°C 

 with maximum temperatures for any appreciable 

 egg production and spawning being 4.2° to 5.6°C 

 (Bigelow and Schroeder 1953). Since the eggs are 

 demersal and adhesive, they are not subject to 

 transport into areas of unsuitable temperatures; 

 being estuarine, they are subjected instead to 



57 



