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Fish exhibit a great variability in reproductive strategies. A particular mating system reported for some species of fish is the lek system. Studies on spawning behaviour of cod (Gadus, morhua) - a closely related species to haddock - have shown that the mating characteristics are consistent with lekking, meaning that cod are particularly vulnerable to disturbance by fishing on their spawning grounds. In this study the spawning behaviour of haddock, was examined and the prerequisites of lekking system were:'observed. Visual and acoustic displays ensured a clear area around a dominant male excluding less vigorous males and unripe females. Larger dominant males performed the solitary display for longer periods than smaller males, more during nights than days. Repeated female intrusions into the male's territory led to courtship and provided evidence that haddock mating is under female's control. Direct observations and DNA analysis demonstrated the existence of alternative male mating strategies. The ventral mount occurred generally outside the male territory. The acoustic repertoire of haddock, both during and outside spawning was described: sound unit structures and call duration were different and provided evidence of speci~s-specificsounds emitted by spawning males. The knocks emitted by each male during the solitary display had a constant waveform that differed from the knocks produced by the other individuals. Female and juvenile sounds were described for the first time. A simple mechanical model showed that a double pulsed sound waveform, could be obtained from two oscillators linked by a viscous-elastic spring, having opposite directions. A quadrupole motion of the haddock swimbladder was suggested. Detailed information about the haddock acoustic repertoire allowed us to detect for the first time haddock spawning grounds in the field (off Norway) by means of passive acoustic monitoring. Key words: haddock, lekking, fish sounds, wild spawning sounds.