Effects of coho salmon (Oncorhynchus kisutch) recolonization on resident cutthroat trout (O. clarki clarki): competition or cooperation?
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In 1901, the Landsburg Diversion Dam was established in the middle of the Cedar River in Washington, effectively cutting off access to habitat for migrating salmon populations. For a hundred years, no salmon dwelled in stream reaches above the dam, but the resident cutthroat trout populations remained. This was the case until 2003, when a fish ladder was built into the Landsburg Dam, reopening passage to spawning habitat for salmon and reintroducing the species to existing trout populations. In this project, coho salmon (Oncorhynchus kisutch) and cutthroat trout (O. clarki clarki) were used to study the competitive effects of recolonization in two neighboring streams, Rock Creek (salmon present) and Williams Creek (salmon absent), in the Cedar River watershed. Though it has been found that opening up passageways to previous spawning sites leads to successful recolonization of the target species, few studies have determined if reintroducing a species to an ecosystem that has survived without it for so long has negative effect on resident species. Previous studies determined that salmon exert dominance over cutthroat trout in certain conditions. Conversely, some recent studies have found no negative impact on cutthroat trout from coho salmon recolonization. Species population densities were computed and tested for correlation to observe temporal and spatial population trends and determine if coho negatively influence the population dynamics of cutthroat trout. Our results indicate that both species populations in Rock Creek are growing since the fish ladder was built. However, coho populations are growing more rapidly than the cutthroat trout in the shared stream. However, the trout population in Williams Creek is growing at a faster rate than that of the trout in Rock Creek. Nonparametric testing indicated that there is a significant and positive relationship between coho and cutthroat pool densities, thus refuting our hypothesis. Our results suggest that there is not a significant competitive effect as a result of the coho recolonization. Stream conditions could be improving for both species and aiding in population growth. Future testing should compare the mixed stream (salmon present) with more isolated streams (salmon absent), like Williams, to confirm if the discrepancy between cutthroat trout population growths, in habitats without salmon versus habitats with salmon, is a result of the coho presence or if it is due to improved habitat conditions in general.