The Fraser River plume: some preliminary observations on the distribution of juvenile salmon, herring, and their prey |
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| Authors: | MICHAEL A ST JOHN J STEVENSON MACDONALD PAUL J HARRISON RICHARD J BEAMISH EDWARD CHOROMANSKI |
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| Institution: | Department of Oceanography, University of British Columbia, Vancouver, Canada;Department of Fisheries and Oceans, West Vancouver Laboratories, West Vancouver, Canada;Department of Oceanography, University of British Columbia, Vancouver, Canada;Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, Canada;Department of Fisheries and Oceans, West Vancouver Laboratories, West Vancouver, Canada |
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| Abstract: | Zooplankton and fish densities in the southern Strait of Georgia were observed to coincide with variations in surface salinities resulting from the outflow of the Fraser River. Vertical net hauls in the euphotic zone revealed that copepods, amphipods, and euphausiids were significantly more abundant per m3 in the brackish estuarine plume (surface salinities - 10–15 ppt) when compared to the area covered by the freshwater of the Fraser River plume (0–10 ppt) and the region of the Strait of Georgia (25–30 ppt) unaffected by the outflow of the Fraser River.
The estuarine and riverine plumes had significantly higher fish densities (adult and juvenile herring, and juvenile salmonids excluding chinook]) than the Strait of Georgia region, with no significant differences in densities of juvenile chinook salmon observed between regions. The highest catches of juvenile salmonids were at the boundary between the estuarine plume and the Strait of Georgia. Zooplankton found in the stomach contents of both adult and juvenile herring suggested that the herring were filter-feeding on the zooplankton in the estuarine plume. Juvenile salmonids fed primarily on small unidentifiable juvenile fish. The existence of increased densities of prey items in the estuarine plume is proposed to be the primary mechanism resulting in increased residence time in this region by outmigrating juvenile salmonids. Utilization of aggregated zooplankton could lead to increased salmonid growth rates and therefore to enhanced survival of individuals utilizing the Fraser River plume environment. |
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| Keywords: | Fraser River estuary salmon herring juvenile fish zooplankton prey of juvenile fish |
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