Alternative Tactics in the Breeding Behaviour of Male Coho Salmon

Abstract

Abstract1. The movements and breeding behaviour of male coho salmon in Kanaka Creek, British Columbia, Canada, were studied by means of radio-telemetry and detailed observations of males in breeding groups. 2. Males were of three morphological types, large, three year old males (which we term alpha males), small, three year old males (which we term satellite males) and small, two year old 'jack' males (which we term sneak males). All three types of male were common in the population. 3. Males associated with females in breeding groups that included one alpha male and one or more of the two smaller male types. Alpha males dominated in breeding groups. Their movements tended to be restricted to a small section of the spawning stream where they had primary access to redd building females. 4. The majority of aggressive interactions were between resident alpha males and intruding large males, which the resident attempted to drive away. Alpha males would not tolerate another large male nearby and would attack until the intruder left the area. Aggressive interactions between alpha and satellite males or between alpha and sneak males were much less frequent. The alpha males moved from female to female within their section of stream, often returning several times to the same female over the space of a few days. 5. Satellite males adopted positions downstream from a breeding pair. Satellite males were highly mobile within the stream and were observed to join breeding groups at widely spaced locations without significant aggressive interaction with the alpha male. An established satellite male would oppose but not drive away other satellite males attempting to join the breeding group, allowing them to adopt subordinate positions further downstream. The first satellite male in a breeding group would also defend and court the female during periods when the alpha male was absent. 6. Sneak males were most often observed in the spawning redd with the alpha male and female. Sneak males were also found hiding outside the redd early in redd construction, when the cavity of the redd was still shallow, or if the sneak was larger than 30 cm. Sneak males were subject to little aggression from the alpha male or satellite males and displayed the lowest frequency of aggressive interactions of the three types of male. Sneak males would defend their position against other sneak males. Like alpha males, sneak males restricted their movements to a small section of the spawning stream. 7. Overall levels of aggression were lowest in breeding groups consisting of two or three males and much higher in groups consisting of just the alpha male or of four or more males. Intrusion by a large male that challenged the alpha increased the rate of aggressive interaction among all members of the group. 8. Our results suggest three unique breeding tactics among coho males, here termed alpha, satellite and sneak. On the basis of the information currently available, it cannot be determined whether the choice of tactic by a particular male is genetically determined, environmentally determined, or both.