UBC Theses and Dissertations
Estuarine use by juvenile coho salmon (Oncorhynchus kisutch) : is it a viable life history stratecy Atagi, Dana Yutaka
Across its geographic distribution in North America and Eurasia, juvenile coho salmon (Oncorhynchus kisutch) utilize an array of different freshwater habitats prior to moving seaward. These freshwater habitats include streams, lakes, beaver ponds, and off-channel sites. In recent years, pre-smolt juvenile coho salmon use of other habitats such as estuaries has been documented. This finding raises the question of whether or not estuarine use by pre-smolt coho salmon is a viable alternative life history? Juvenile pre-smolt coho were present in the upper intertidal portion of the Salmon River estuary from May through November with peak abundance occurring in August. Coho salmon use of an experimental tidal channel was similar to that of a nearby natural channel. Coho densities were similar in the two habitats. Mean residence time in the upper estuary was estimated to be 13.4 and 7.1 days in 1989 and 1990, respectively. It is postulated that the presence of a low tide refuge in the experimental channel lengthened residence time and increased site fidelity. Both young-of-the-year and yearling pre-smolt coho were captured in the estuary. By late summer, high estuarine growth rates of young-of-the-year fish enabled them to grow to the size of yearling coho. The age structure of the estuarine coho population shifted from predominantly young-of-the-year coho in spring and early summer to yearling coho in late summer and fall. Agonistic behaviour has been proposed as a potential cause for the emigration of stream resident coho to the estuary. Observations of juvenile coho in a stream and in an estuarine tidal channel indicated that stream coho defended territories and were highly aggressive while estuarine coho aggregated into small groups and were infrequently aggressive. In mirror image stimulation (MIS) experiments, stream resident forms were found to be more aggressive than either estuarine resident or stream emigrant forms. These differences in social organization and agonistic behaviour were appropriate for an existence in a resource-limited, stream environment and a resource-rich, high predation risk, estuarine environment. In field enclosures with paired groups of fish, estuarine coho were dominated by smaller stream resident and stream emigrant conspecifics. In the laboratory, a comparison showed that estuarine coho dominated both stream residents and stream emigrants. This was probably due to size advantages associated with high estuarine growth rates and occurred in spite of estuarine coho exhibiting lower levels of aggression in MIS experiments. These apparently conflicting results suggest that facultative switching of behavioural modes (i.e. non-aggressive to highly aggressive) in estuarine reared coho can occur. Juvenile pre-smolt coho inhabit specific localities in the Salmon River estuary for several weeks. Physiologically, they are able to withstand the level of salinity encountered in estuaries and often exhibit high rates of growth. Behaviourally, they exhibit low levels of aggression but they can dominate smaller, more aggressive stream forms. These results increase a growing body of evidence suggesting that estuarine use by juvenile pre-smolt coho salmon may be a viable alternate life history.
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