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Aspects of the reproductive ecology of bull trout (Salvelinus confluentus) in the Chowade River, British Columbia Baxter, J. S.


In the Chowade River in northeastern British Columbia, bull trout (Salvelinus confluentus) males were observed to exhibit two reproductive strategies. The most common strategy was that of an individual being a dominant male, and controlling access (through direct aggression) of other males to a reproductive female. A less common strategy was that of an individual being a sneaker male, and not engaging in aggressive encounters with other males to gain access to a female. Dominant males and sneaker males differed significantly in size (length and mass), growth, reproductive pattern, morphology (mainly kype growth) and sexual colouration. Sneaker males were more similar in morphology and colouration to reproductive females, suggesting that female mimicry by sneaker males may be occurring. Dominant males were observed to be more aggressive to other large brightly coloured males than they were to smaller and duller coloured males. Experimental trials on dominant male aggression towards coloured models demonstrated that dominant males were more aggressive towards brightly coloured (male like) models than they were towards less coloured (female like) models. These results suggested that the reduced secondary sexual characteristics of sneaker males may enable them to get within close proximity to a spawning pair to allow sneak fertilizations. These studies demonstrate that variation in male reproductive strategies occur, and provide the possible mechanism by which smaller males may spawn successfully without actively courting or guarding a female. In terms of sexual selection the study suggests that as the spawning period progresses, there may be a shift in which type of strategy is at an advantage. Microhabitat at bull trout redd sites was also measured, and found to be similar to data reported from other northern latitudes. Bull trout were highly specific in areas of the upper river where they spawned, and where a high degree of redd superimposition was observed. Bull trout females selected areas of groundwater discharge for spawning that may provide high quality incubation sites for eggs. An experiment designed to measure egg to alevin survival at selected and non-selected sites with similar habitat attributes other than groundwater infiltration, demonstrated that egg to alevin survival was significantly higher at selected sites than randomly available sites. The areas selected for spawning were areas of groundwater discharge and had more stable and warmer incubation temperatures than non-selected sites. This study has conclusively demonstrated that bull trout do spawn in areas of groundwater upwelling, and that individual female's reproductive success may be increased by site selection.

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