Haddock have drumming muscles attached to the swim bladder. Rapid contraction sets the gas-filled bladder vibrating and radiates a low-frequency pulse into the water. Males produce these sounds seasonally, in the spawning period, and the muscles are larger in mature males than in females.
Recordings describe a progression rather than a single call. Slow, separated knocks accompany early courtship; as a male displays to a female the repetition rate increases until the individual pulses fuse into a continuous humming sound near the moment of spawning. The behaviour is closely tied to the male's display swimming.
This has practical consequences. Because the sound is loud, species-specific and confined to spawning, passive acoustic recorders can locate and time haddock spawning aggregations without catching anything. That matters for a species where cod and haddock share grounds. What remains uncertain is how much a female's choice depends on the acoustic signal itself as opposed to the visual display and the male's size, since the two are hard to separate in the field.