Sound Production By the Cod (Gadus Callarias L.)

Abstract

AbstractMale and female cod greater than 37 cm. in total length were heard to produce a low grunting sound in captivity. The sound has a peak frequency of about 50 cycles per second, and a duration of about one fifth of a second. The swimbladder shows features which would increase its value as a sound producing organ if vibrated by the drumming muscles attached to it, but this drumming could not be produced by electrical stimulation of recently killed cod. Cod in captivity most frequently produced sounds in February and March associated with spawning and from September to November when the fish were very aggressive. Sounds associated with spawning increased in frequency after sunset but at other times of the year the sounds ceased at dusk. Grunting sounds made during the threat display of aggressive male and female cod were intimidating if produced within about six inches of the threatened fish. In the spawning season grunts were only made by the males and were used with aggressive behaviour to remove immature females and less vigorous males from the vicinity of dominant males. Grunts accompanied the courtship display of the male and stimulated the female to respond more adequately to the display and to swim upwards to spawn. Males mistakenly mounted by other males grunted and broke up the pairing. Sounds were produced by cod frightened by strange objects, when startled or when fleeing from aggressive cod or a natural predator (Conger conger). Severely frightened cod assumed a cryptic coloration, pressed down on the bottom and were silent even when strongly stimulated. Electrical stimulation of living cod caused some grunts, usually with signs of fear. Grunts accompanying choking were assumed to be involuntary. Aggression with the production of sounds was not used in competitive feeding but stimulation by food increased the frequency of aggressive behaviour after a short delay. Cod rarely grunted when stroked and did not produce sounds at temperatures below 4° C. Cod in a new environment were usually silent.