Graduate Thesis Or Dissertation
 

Movements of spawning sockeye salmon in Hidden Creek, Brooks Lake, Alaska

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  • The movements of a population of adult sockeye salmon (Oncorhynchus nerka) were followed in a small tributary of Brooks Lake, Alaska until spawning was nearly complete in 1963. The objectives of the study were to describe the movements of sockeye salmon on the spawning ground of an entire small stream, to determine factors related to such movements, and to consider the significance of the movements to the species. The spawning run was inventoried at a weir located near the mouth of the tributary. Portions of the run were sampled as they immigrated, and the sampled fish were tagged to enable subsequent identification of individual fish on the spawning ground. Following the initial immigration, daily observations were made to ascertain the location of all fish and the activities of the tagged fish. Tagging had no material effect on either dispersal in the stream or pairing of spawners. The spawning run totalled 2,495; the ratio of males to females was 2.27:1.00, The run consisted preponderantly of males that had spent two winters in the ocean (two ocean-years) and females that had spent three winters in the ocean (three ocean-years). They entered the tributary over a period of 14 days in two waves separated by an interval of three days. The overall median life span in the stream was 12 days in the first wave and eight days in the second wave. The difference was primarily due to predation on the salmon by the brown bear (Ursus arctos). The average female established a redd in 1.4 days; thereafter the females rarely moved far from their redds. The average male required 2.3 days to establish himself in a spawning locale; thereafter most males remained within a relatively small area. The males showed a decided tendency to remain in areas with a high density of females on redds. Fewer females than males overshot their spawning sites. No relation was found between the movements of spawners and either variations of water level or surface water temperature of the stream. Bear predation disrupted spawning activities only locally and temporarily. Males of three ocean-years paired with more females per male than did males of one or two ocean-years. But because of their greater numbers two-ocean-year males participated in more spawnings than did three-ocean-year males. The mean duration of pairing of a particular male with a particular female on a redd was 1.25 days. I concluded that homing within the tributary did not occur; therefore its spawning stock must be a homogeneous unit. The role of the male is primarily one of fertilization of the eggs, not one of defense of territory. Because of the abnormally high ratio of males to females, competition between males for spawning partners resulted in most males of one or two ocean-years being relegated to attend females paired with larger males. Conflict among the males was apparently largely confined within two separate classes of males: males 50 cm or shorter jockeyed among themselves for seemingly preferred positions of attendance next to a pair on a redd, while males 51 cm or longer competed for spawning partners. The polygamous habit in sockeye salmon usually results in a seeming excess of males on the spawning ground beyond the number required to fertilize the eggs. I propose that this "surplus" of males provides a safety factor in times of low numbers in a stock and the competition needed to disperse the males among the available females; both provisions insure fertilization of the available eggs, However, crucial experiments need to be done on an entire population of sockeye salmon with artificially reduced ratios of males to females to determine whether the movements of the spawners would result in an efficiency of egg fertilization sufficient to make harvesting of the "surplus" males feasible.
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