Seasonal patterns and individual differences in the calls of male haddock Melanogrammus aeglefinus

Haddock Melanogrammus aeglefinus males observed in captivity proved to be vocally active throughout the whole year, although the highest levels of vocal activity were reached in the spawning season between February and April. During this season, the most common type of sound consisted of sequences of regularly repeated, almost identical, double-pulsed knocks (KN). These sequences were mostly associated with a particular behaviour pattern, the patrolling display, which has previously been shown to play a key role in territoriality and spawning. Differences in the double-pulsed KNs produced by three individual males were demonstrated by two different statistical methods and it is suggested that these differences may allow individual males to be discriminated by other fish, perhaps conveying fitness-related information on the sender. Some aspects of the calls changed seasonally, perhaps reflecting observed variations in the mass of the drumming muscles. When using passive listening to locate M. aeglefinus spawning grounds, the physical characteristics of the sounds may indicate how close the individual males are to spawning.