FISHERY BULLETIN: VOL. 78, NO. 1 



margin of the eye. Hatching began in late Feb- 

 ruary to mid-March. A series of relatively early 

 stage eggs was also obtained in late February to 

 mid-March 1978 (Figure 6), indicating a possible 

 second spawning of tomcod in late January to 

 early February. 



Laboratory Observations 



Survival to Hatch and Length at Hatching 



Tomcod egg survival to hatching was highest in 

 freshwater (Table 2). Fifty-eight percent of the 

 freshwater eggs hatched, compared with 50, 37, 

 and 13% at 10, 20, and 30%o salinities, respec- 

 tively. About half of the mortality at and 10%o 

 occurred at about day 30 (stage 15). Above 10%o 

 high mortality also occurred at earlier stages of 

 development. 



Table 2.— Percentage survival to hatching, total larval length 

 at hatching, and median time to hatch for tomcod eggs incubated 

 at four salinities. Standard deviations are given for larval 

 lengths. 



Larvae hatched in freshwater were significantly 

 longer than those from higher salinities (7.54 mm 

 for freshwater vs. 7.25 and 6.31 mm at 10 and 

 20%o, respectively). Larvae at 30%o had severe spi- 

 nal curvature and could not be measured accu- 

 rately. Hatching was earlier at the higher 

 salinities. 



The developmental success of tomcod eggs var- 

 ied with salinity, so various parameters associated 

 with water balance were measured on eggs reared 

 at 0, 10, 20, and 30%o. These parameters are all 

 interdependent so that changes in one may result 

 in concomitant changes in others. 



Specific Gravity 



The sp. gr. of freshwater (FW) eggs was constant 

 throughout development (Figure 7) at 1.030, im- 

 plying that weight and volume were not changing 

 or that they were changing in such a way that the 

 sp. gr. was constant. In contrast, eggs incubated at 

 20 and 30%o decreased in sp. gr. throughout de- 



152 



050 



045 



1.040 



1035 



1030 



,025 



, 30 7oo 



20 %o 



10 7oo 



F W 



_L 



20 ~ 

 Time (Doys after 



40 

 fertilization ) 



60 



Figure 7 .—Specific gravity of tomcod eggs incubated at various 

 times from fertilization at various incubation salinities. Each 

 point is based on the mean of measurements made on 10 eggs. 

 Lines fitted by eye. FW = freshwater. 



velopment. Specific gravity at 10%o was constant 

 for the first 25 d of incubation, then decreased 

 linearly. The sp. gr. of water at 10, 20, and 30%o 

 (10° C) are 1.009, 1.017, and 1.024, so that eggs 

 were denser than the incubation medium at all 

 salinities and by approximately the same amount. 

 For example, the sp. gr. of FW eggs is 1.030 com- 

 pared with 1.000 for freshwater, a difference of 

 0.03 sp. gr. units. The sp. gr. of 20%o eggs (extrapo- 

 lated to time at 20%o from Figure 7) is 1.045 

 compared with 1.017 for the incubation medium, 

 again a difference of 0.03 sp. gr. units. Changes in 

 sp. gr. may be associated with changes in water 

 content, loss of solids through metabolism, change 

 of salt content of eggs, or a combination of these 

 factors. 



Water Content 



Mean water content of FW eggs was 2.83 mg/egg 

 (Table 3) with no trends throughout development. 

 The water content and percentage water content 

 of FW eggs for the first 27 d of development were 

 significantly higher than the values obtained at 

 the other incubation salinities (P<0.05, ANOVA, 

 Duncan's Multiple Range Test). The percentage 

 water content increased from 86.4 to 89%o over the 

 final 25 d of egg development, attributable to de- 

 creases in egg dry weight (Tables 4, 5). 



There were no significant differences among the 

 percentages of water content of eggs reared at the 

 three higher incubation salinities for the first 27 d 



