There is a limited amount of information on Atlantic cod spawning behavior which may suggest complex mating systems. Researchers are aware that mating behavior in Atlantic cod can include reproductive strategies such as sound production by males and mate selection by females. Although these behaviors have been observed, the causes and consequences of such behavior, and their specific interplay within the mating systems continue to be studied. Atlantic cod are considered "batch-spawners", as females only release 5 to 25% of their total egg complement at any time. ("Assessment and Update Report on Gadus Morhua (Atlantic Cod)", 2003; Rowe and Hutchings, 2004)
One study on the acoustic sound production of Atlantic cod provides some insight into possible mating behaviors. Drumming muscles are present in both males and females, yet males tend to have more pronounced muscles. The mass of the drumming muscles increases in males prior to spawning and larger males have larger muscles. This suggests that the amplitude of sound production might be a determinant in the success of spawning and selection by females.
Observations of Atlantic cod behavior support the hypothesis that females are responsible for mate selection. The biology of the drumming muscles in males, as well as the circling behavior of numerous males around prospective females supports the female selection hypothesis. It is worth noting that dominance hierarchies can also be established. Males with greater body sizes and those who were successful in spawning sometimes appear to dominate the population and act aggressively towards “lesser” males. (Rowe and Hutchings, 2004)
Recent research suggests that anthropogenic noise pollution in the water (via oil/gas exploration and drilling) could pose a threat to the success of sound production and the role it plays in the reproduction process. (Rowe and Hutchings, 2004)
Many stocks of cod exhibit migratory behavior during their reproduction season due to seasonal variations in water temperature. Typically, a cod population moves into warmer waters during winter and early spring to begin spawning. Although spawning can occur year round, peak spawning levels occur in the winter and spring. As the population moves inshore it may disperse temporarily to feed if large amounts of prey are present.
Cod spawn annually, and spawning takes place within a three month period. Cod employ a ventral mount position in which a male uses his pelvic fins to clasp onto a female and then position himself properly beneath her. Cod spawn in dense concentrations of more than 1 fish per cubic meter and multiple pairs of fish can be observed spawning in the same water column. Spawning occurs near the ocean bottom in temperatures between 5 to 7 degrees Celsius. The eggs that are produced are pelagic, and drift (often towards the surface) for approximately 2 to 3 weeks before hatching and reaching the larval stage.
There is some debate as to the age of sexual maturity for cod. Age and size at maturity often vary amongst different populations with northeastern populations maturing around 5 to 7 years and southern populations maturing between 2 to 3 years. A recent finding suggests that cod are moving towards a reduction in age and size for sexually mature fish. In 1959 the median age of maturity was 6.3 years for females and 5.4 years for males. In 1979 the age of maturity was listed as 2.8 years for both sexes. Now, the median age of sexual maturity is between 1.7 to 2.3 years and corresponds to a length of 32 to 41 cm. ("Assessment and Update Report on Gadus Morhua (Atlantic Cod)", 2003; Fahay et al., 1999; Rowe and Hutchings, 2004)
There is no indication that any parental involvement exists on the behalf of either females or males after the eggs are released. The high mortality rate of the offspring (eggs) is attributed, in part, to the lack of parental care. The reproductive strategy of high fecundity levels may be a response to the absence of protection the eggs receive once released into the water. Although the survival rate is low, the sheer number of eggs produced is huge. ("Assessment and Update Report on Gadus Morhua (Atlantic Cod)", 2003)