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Spatial distribution and spawning migration of Kokanee (Oncorhynchus nerka) in Nicola Lake, British Columbia Lorz, Harold William

Abstract

Vertical distribution and onshore movement of maturing kokanee were studied by means of extensive gill net sets in Nicola Lake between 1958 and 1961. In late spring of 1959 maturing kokanee were found largely in the upper 30 feet of the lake but gradually dispersed to occupy all depths to 100 feet by mid-summer. A diel vertical migration occurred in which maturing kokanee moved surfaceward during the day and downward at night in 1959. In 1961 the reverse condition was observed wherein kokanee avoided areas of bright illumination during the day but moved surfaceward at night. No effect on vertical distribution of kokanee by extensive and rapid fluctuations of the thermocline, initiated by wind induced seiches, was noted in 1959 or 1961. The 1961 vertical distribution appeared closely associated to light intensity. Seasonal and diel changes in diet were observed in 1959. Chironomid pupae were the dominant food organisms eaten in late spring and summer. Planktonic crustaceans were consumed in greatest numbers in late summer, autumn and spring. Kokanee captured in mid-summer in the surface water (0-25 feet) generally had been feeding on planktonic crustaceans whereas those taken from below 25 feet contained largely chironomid pupae and larvae. Onshore movement of mature kokanee toward a spawning stream was initiated by falling light intensity and intensified by strong onshore winds. Possible mechanisms of location of the spawning stream were investigated. Migration to an inlet spawning stream and movement within the stream were recorded at two traps, one situated at the stream mouth, the other 1000 feet upstream. Movement into the spawning stream occurred only at night and was unaffected by changes in stream temperature and flow. A significant correlation was found between daily number of kokanee entering the stream and strength of onshore winds. Differences in sex ratio of the spawning runs and length of mature kokanee were recorded and possible causal agents discussed.

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