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.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/5185 |
| Date | 11 1900 |
| Creators | Atagi, Dana Yutaka |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Relation | UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/] |
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