The importance of underwater sounds to gadoid fishes

The codfish family includes more than 500 species that vary greatly in their abundance in areas like the North Sea and are widely fished. Gadoids (codfish) gather at particular locations to spawn, where they exhibit complex reproductive behavior with visual and acoustic displays. Calls have been described from seven species, including the Atlantic cod and haddock. They vocalize by means of a specialized apparatus, consisting of rapidly contracting striated muscles (the drumming muscles) attached to the gas-filled swim bladder. Several gadoids, such as the ling and the Greenland cod, possess drumming muscles and are likely to make sounds. Non-vocal gadoids, such as the poor cod, lack these muscles. It is suggested that the sonic apparatus was present in the early species of the gadoids, with some species having lost their sonic ability. Interestingly, silent gadoids are mainly small schooling fishes. Gadoid species are most sensitive to sounds from 30 to 500 Hz. Gadoid hearing can be masked by ambient sound but also by anthropogenic sounds, which may therefore adversely affect their reproduction, with potential effects upon discrete local stocks. Listening for gadoid sounds provides a reliable, non-invasive way of locating spawning sites, which can enhance the protection of reproducing fish from human impacts.