PETERSON ET AL.; EARLY UFE STAGES OF ATLANTIC TOMCOD 



Water Level 

 in Tank 



Water Level 

 in Compartment 



I I 



5 cm 



Figure 2. — Diagram of incubation chambers used to rear tom- 



cod eggs. 



.9!^ 



4- 



2- 



. - 30 7oo 

 o- 10 %o 

 • - Fresh Water 



30 %. 



■^«efi 



FW. 



10 



20 



?.*Ssf°o "o*^?*"- 



.^.^?i ■■•^;^«»*'"°o 



_L 



10 20 30 40 50 



Time ( Days after fertilizotion ) 



60 



(range, 91-111). The columns were immersed in a 

 freshwater bath cooled (2°-4° C) by recirculation of 

 water to refrigerated header tanks. Water flowing 

 to the columns passed through titanium coils in 

 the bath. Water temperature decreased from 4.5° 

 C at the beginning of January to 2.0° C in mid- 

 February (40-d postfertilization), then increased 

 to 2.5° C by the end of February (Figure 3). Eggs 

 were incubated in salinities of (2 columns), 

 10.1 ±0.3 (1 column), 20.2 ±0.6 (1 column), and 

 30%o (2 columns). About 250-300 eggs were incu- 

 bated in each column. Temperature and salinities, 

 by conversion of specific gravity of water with 

 Knudsen's (1962) hydrographical tables, were 

 measured daily. 



Columns were checked for egg and larval mor- 

 talities every 2-3 d. Every third day, three eggs 

 were removed from each salinity and preserved in 

 10% Formalin for subsequent study of degree of 

 development. About 100 newly hatched larvae 

 were measured ( ±0.1 mm) from each salinity and 

 the percentage of deformed larvae noted. 



Water content of 10 eggs (combined) from each 

 salinity was measured every fifth day by measur- 

 ing loss of weight after drying for 16 h at 40° C 

 under vacuum. Specific gravities (sp. gr.) of eggs 

 were measured by glycerol flotation at 10° C as 

 described by Peterson and Metcalfe (1977). 

 Specific gravity of egg solids was calculated from 



Figure 3. — Developmental stages and incubation temperatures 

 for laboratory experiments on tomcod development. Upper 

 panel: Appearance of developmental stages of tomcod eggs incu- 

 bated in freshwater. Lower panel: Incubation temperatures for 

 tomcod eggs at various incubation salinities. Arrows indicate 

 median hatching dates at each salinity. 



total sp. gr. and water content where solid sp. gr. = 

 egg dry vvrt/(egg vol. - vol. HgO), egg vol. = wet 

 wt/sp. gr., and vol. H^O = water content/sp. gr. of 

 HgO. Egg diameters were measured microscopi- 

 cally to the nearest 10 ^tm. Buoyancies of newly 

 hatched larvae, due to air content of the swim 

 bladder, were measured by a Cartesian diver 

 technique (Saunders 1965). 



Photic responses of larvae were observed by 

 placing groups of five larvae in a Petri dish, half of 

 which was painted black, on a finger bowl full of 

 ice. Uniform overhead illumination was used. 



Statistical Procedures 



Differences in water content and dry weights 

 among eggs incubated in the various salinities 

 were tested by one way ANOVA with individual 

 differences detected by means of Duncan's Multi- 

 ple Range Test. Changes in water content and dry 

 weights during larval development were analyzed 

 by linear regression methods. 



149 



