Variation with Fish Length, Sex, Stage of Sexual Maturity, and Season in the Appearance and Volume of the Drumming Muscles of the Swim-Bladder in the Haddock, Melanogrammus aeglefinus (L.)

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Abstract
Observations made as early as 1947 and data collected in 1953 to 1954 and 1956 to 1957 indicate that there is considerable variation in the size and appearance of the drumming muscles of the swim-bladder of haddock with fish length, sex, sexual maturity, and season on the Grand Bank and St. Pierre Bank. For haddock of the same length the data indicate that the drumming muscles of mature male haddock are larger than those of immature males and also of immature and mature females of the same length. In addition the data indicate that a periodicity is exhibited in the size of the muscles of the mature male, the muscles being nearly twice as large during the pre-spawning and spawning period in May and June as they are in October and November. This phenomenon does not occur in female haddock, the drumming muscles of which are considerably smaller than those of male fish of the same size and show no variation in size with the season.It is suggested that the difference in size of the drumming muscles of male and female haddock can serve as a means of determining the sex of haddock landed in the gutted condition by commercial trawlers, at least for the mature fish which make up the bulk of the commercial trawlers' catches. There is some difference between male and female haddock in the colour of drumming muscles, but this seems to be of secondary importance for distinguishing between the sexes.Male haddock probably use the drumming muscles chiefly to produce low-frequency swim-bladder sounds which serve as rallying calls for female and other male haddock to join in spawning activities.

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