Reproductive Behaviour of the Cod (Gad Us Callarias L.)

Abstract

AbstractThree weeks before spawning, aggressive and reproductive behaviour was seen in a group of thirteen captive cod. By aggression directed against males and females the most vigorous males were able to establish territories in the pre-spawning period. Less vigorous males and unripe females formed an inactive group outside the territory. All spawning observed occurred in the territory of the dominant male. The amount of reproductive activity was closely linked with dominance in the males. Behaviour culminating in spawning was initiated by a female entering the territory of the male. Males were able to distinguish the sex of mature intruding fish during the spawning period. The distinction appeared to be based on the slow swimming, unafraid approach of the female compared with the quick movements and readiness to flee shown by the males. The male approached ripe females and presented a lateral view of the flaunting display. In this display the median fins are fully erected and the male moves in front of the female with many unnecessary turns sometimes accompanied by a low grunting sound. Ripe females followed flaunting males for ten to fifteen minutes. Then if the female became stationary at the bottom of the tank it was prodded ventrally by the male, which caused it to swim upwards. At the surface the male mounted the female dorsally and then immediately slipped down one side into a ventral mount with the male inverted below the female. In this position the cod spawned while swimming in circles at the surface. No interest was shown by the rest of the group in the spawning. After spawning the spent female returned to the bottom of the tank and rejoined the group of inactive cod. The pattern of spawning behaviour was often initiated but failed to reach completion because of the inappropriate response of the female at some stage. During successive entries the courtship behaviour was carried to later stages until the female swam to the surface and spawned. Aggressive behaviour during the spawning period seems to be important in allowing only the most vigorous males to spawn and preventing interference by other males in the courtship pattern. The production of sounds by the males was used to enforce aggressive behaviour and stimulated the female. It is suggested that, as much of the behaviour leading to spawning involves vertical movements, natural cod shoals may be stratified with a layer of vigorous males above a group of females and less vigorous males.