Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Although inadvertently introduced to North American's Laurentian Great Lakes, pink salmon (Oncorhynchus gorbuscha) now constitute the world's only self-perpetuating freshwater population of this species. Their proliferation and growing impact on the extant fish fauna are elevating the species to one of considerable importance here. Despite this, the reproductive ecology of Great Lakes populations remains largely unknown. This study of the pink salmon breeding in the Carp River, a tributary to eastern Lake Superior, is a response to the need for this information. Considered are their upstream migration patterns, fish size and condition variation, migrant reproductive maturity and gonadal investment, male spawning performance, the determinants of male and female length of breeding life, rates of carcass recovery, and the wandering of spawners to other streams. Contrasts between Carp River and anadromous populations and between even- and odd-year runs into the same study stream are also emphasized.;Plasticity in the life histories of salmonid species is associated with the occurrence of alternative male breeding patterns. Because pink salmon display essentially invariable life histories, they seem unlikely to demonstrate similar breeding pattern diversity. Nonetheless, in behaviour studies some Carp River males were seen to participate in spawnings by attempting to sneak fertilizations rather than by competing for access to females in characteristic salmonid fashion. These same individuals also resembled females in shape. Such behaviour was seen only at high spawner densities when single large males were apparently unable to monopolize nesting females. Consistent with these observations, morphological studies confirmed the occurrence of males which in shape resembled females more than did other males. Secondary sexual character development (hump size) was reduced in these individuals, likely as a result of their reduced condition. These female-like males did not differ from typical males in age, level of maturity, investment of biomass in testis, or length of breeding life. Thus, the occurrence of an alternative male breeding pattern in this population is interpreted as being a facultative response by males to reduced competitive ability.



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