Sound production by Atlantic cod during spawning

Using captive groups of Atlantic cod Gadus morhua from two Northwest Atlantic populations (western Scotian Shelf and southern Gulf of St. Lawrence), we quantified the temporal patterns and behavioral contexts of sound production during the spawning season. We found that sound production occurs most frequently during the peak of the spawning period, particularly after the onset of darkness. The rate of sound production by males in the western Scotian Shelf group was 8.4 times greater on average than that of the southern Gulf of St. Lawrence group, and this corresponded to differences in the mass of the sound-producing musculature. Based on behavioral observations during daylight hours, we found that sounds were associated with both courtship behavior and agonistic displays. Although sounds were produced on only 14.5% of the occasions in which a male and female swam together in a "ventral mount," limited data suggest that ventral mounts accompanied by sound were more likely to lead to spawning. While almost all sounds produced by Atlantic cod in our study matched the short "grunt" type previously documented for this species, we report evidence for another sound, described as a "hum," which occurs during the ventral mount immediately before gamete release. We hypothesize that sound production is related to competition among males for access to females and may help synchronize gamete release, underscoring the potential importance of sound production to Atlantic cod spawning behavior.