Seasonal trends in adenylate nucleotide content in eggs of recruit and repeat spawning Atlantic cod (Gadus morhua L.) and implications for egg quality and buoyancy

Seasonal and ontogenetic variation in egg buoyancy (egg specific gravity; ρ) (n = 63) and nucleotide content (n = 46) were examined for wild-caught Atlantic (Barents Sea) cod (Gadus morhua) held in captivity over two successive spawning seasons, i.e. each female (n = 5) was studied both as recruit and repeat spawner. All eggs were naturally spawned and fertilized, and incubated under optimal condition in flow-through aquaria. Egg diameter and egg dry weight declined steadily during the spawning period, while stage-specific ρ was approximately constant between egg batches (typically around 15 in total). Within each egg batch, i.e. during egg incubation, ρ significantly decreased from the time of gastrulation to before hatching, accompanied by increased contents of ATP and ADP. Altogether, we found that adenylate energy charge (EC) (EC = ([ATP] + 0.5 [ADP]) / ([ATP] + [ADP] + [AMP]) positively affected egg buoyancy (P = 0.013) in concert with egg developmental stage (P < 0.001) and egg diameter (P = 0.014) (LMM). The presently studied eggs were considered to be in good quality and showed generally very high fertilization rates. Although the number of analyzed females in this complex repeated measurement experiment was limited due to logistic restrains, it can be expected that cod eggs in the field would show comparably similar trends in ρ and levels of nucleotides.