Many species of fish are vocal and several studies have shown that these sounds emitted by fish have a communicative function. One group of marine fish, the Gadidae or codfish, contains a large number of vocal species. Codfish calls are relatively simple and species-characteristic. However, one gadoid, the haddock, produces a range of different calls in different social contexts. Both male and female haddock produce short sequences of repeated 'knocks' during aggressive encounters. But during reproductive behaviour male fish produce calls that vary in their characteristics as courtship proceeds. We have recently re-examined the repertoire of sounds produced by a small group of spawning haddock, and have attempted to relate the sounds made by males to the different patterns of behaviour shown by the fish. We have also characterised the sequences of behavioural acts that lead up to spawning. Haddock sounds were classified into single and multiple 'knocks'. Each 'knock' was composed of two low frequency pulses. Multiple 'knocks' were further split into 5 different categories based on sound duration and 'knock' repetition rate. The behavioural acts Follow, Flaunt, Solitary Display and Lateral Display were significantly associated with sound production. Sounds became longer and showed higher 'knock' repetition rates as the male fish became more aroused and came closer to spawning. It is suggested that the sounds serve to bring male and female fish together, in the same part of the ocean, and that sounds also play a role in synchronising the reproductive behaviour of the male and female fish.
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