The physical oceanography of British Columbia's inside passage with respect to the return migration of Oncorhynchus nerka
Creators: name: TerHart, Bert Adrian
Publisher: name: University of British Columbia
Year: 2010

description: Data from five conductivity-temperature-depth (CTD) surveys collected during 1985 and 1986 in support of project MOIST -Meteorological and Oceanographic Influences on Sockeye Tracks- are used to describe the salient oceanographic features of the waters lying between Vancouver Island and the British Columbia mainland coast. Using these data, four oceanographic regimes are clearly defined on the basis of salinity structure. Temperature-Salinity diagrams are used to discuss water types and mixing ratios in these regimes. Vigorous tidal mixing over shallow sills and/or in narrow channels produces tidally mixed fronts that separate oceanographic regimes. The tidal evolution of two of these fronts located near Weynton Passage and Cape Mudge are discussed by means of 24-hour CTD stations. Seasonal variability of the residual estuarine circulation is examined and estimates of the seaward flow in the upper layer of a very simple two-layer geostrophic model were found to be in reasonable agreement with the few direct measurements made in this region. Seasonal variability of the general hydrography is described. Ultrasonic telemetry provided horizontal and vertical distribution time series data for return migrating sockeye salmon (Oncorhynchus nerka). Concurrent high spatial resolution CTD data was used to specify the ambient temperature and salinity fields in the immediate vicinity of the tagged sockeye. Spectral analysis of the depth and ambient oceanographic data time series revealed periodic vertical movements at approximately 15 and 33 minutes per cycle for fish tracked in the slightly stratified regimes of Queen Charlotte Strait, western Johnstone Strait and the Strait of Georgia. High frequency large amplitude periodic vertical movements were characteristic of fish that did not make significant progress towards the Fraser River: low frequency small amplitude vertical movements were characteristic of well oriented fish. Aspect ratio, defined as the horizontal distance travelled divided by the vertical distance travelled, gave an indication of the relative degree of homeward orientation. Vertical distribution and orientation were also related to the frequency and duration of successive vertical excursions. Fish depth and vertical swimming velocity were found to be positively correlated in regions of weak stratification and/or for well oriented fish. Ambient density gradients were not found to inhibit vertical movements as the rate of doing work against hydrodynamic drag was several orders of magnitude greater than that of doing work against a varying buoyancy force. In the presence of strong temperature and salinity gradients, tracked sockeye were most often observed at depths not associated with the maximum gradients. In stratification regimes where temperature and salinity gradients were nearly uniform with depth, tracked sockeye were observed at depths uniformally distributed throughout the thermo- and haloclines. Minimum vertical swimming velocities were generally associated with minimum vertical gradients. These observations suggest that the tracked sockeye frequently swam through but did not reside in the region of the maximum gradients. Dimensional analysis suggested that physical variables alone are insufficient to specify the vertical distribution of the tracked sockeye.