Presentation: State-of-Knowledge Initiative for the Special Committee on Sustainable Aquaculture of the British Columbia Legislature

Delivered November, 2006.

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Environmental Conditions in the Broughton Archipelago

[A full pdf copy for printing may be downloaded from the WAVES database.]

Currents in the Broughton Archipelago and adjoining straits and inlets are primarily driven by tides, wind and river discharge. These waters are strongly layered (stratified) with lighter freshwater at the surface and denser saltwater at depth.

Tidal currents are strong in the constricted passages and straits of the Broughton region, but the primary transport mechanisms are the estuarine flows (the seaward flow of low-salinity surface water over a deeper higher -salinity waters) resulting from river flows (rain and snow) and glacial runoff, and the near surface currents that arise from strong winds.

Recently, several long-term current meter measurements in key passages of the Archipelago and numerous temperature and salinity profiles throughout the region have allowed a much better understanding of the circulation and water property variations. However, wind observations remain sparse and the installation of weather stations is essential to further improve our understanding.

A preliminary numerical model describing the tidal and estuarine circulation the region has been developed3 and reproduces average spring (April – June) currents with reasonable accuracy.

Notable circulation features include surface estuarine flows of up to 15 cm/s [1] that generally move seaward from the heads of Knight and Kingcome Inlets and bottom return or landward flows of up to 12 cm/s that primarily enter through Fife Sound.

The seaward surface currents split at the junction of Knight Inlet and Tribune Channel, and where Kingcome Inlet separates into Sutlej Channel and Penphrase Passage. The proportion of flow into each channel at these separation points will vary with the winds and the volume of the main river discharges.

Observations suggest that the depth of the interface between the seaward and landward flows can vary from a few meters in Knight Inlet, and from 20 to 50m in Fife and Tribune Channels.

The three major rivers in the region are the Klinaklini, Kingcome and Wakeman Rivers. The Klinaklini River, the largest river in the region, is the only major river in the region that had been gauged for more than one year.
In 2006, the Kingcome and Wakeman Rivers were gauged but the data will not be available until early 2007.
The Klinaklini discharge typically peaks in July and is the main source of freshwater responsible for the low salinity surface waters of Knight Inlet and upper Tribune Channel.

Based on the relative size of their watersheds, the Kingcome and Wakeman discharges should be approximately 25% and 20% of the Klinaklini, but significant glacial input may alter these estimates.

Near surface salinities and temperatures in the region exhibit spatial and temporal variations that are correlated with the river discharges, atmospheric heating, and mixing from wind and tides.

In the spring, average surface salinities generally increase and average surface temperatures decrease when moving seaward from the heads of inlets. Surface salinities greater than 30 ‰ (parts per thousand) are generally not found until Queen Charlotte and Johnstone Straits.

However, in the Broughton Archipelago and outer reaches of Knight Inlet higher salinity water of 30 ‰ is found deeper (5 to 15m) in the water column. Average spring surface temperatures range from 12 °C near the heads of inlets to less than 9 °C in Queen Charlotte Strait and become cooler with depth.

Tidal and mean estuarine currents from the numerical model have been used to simulate the transport of passive particles emanating from several fish farm locations1, 2. They generally show movement into Queen Charlotte Strait within several days (4 to 10 days).

However, there are several caveats that limit the reality of these simulations, particularly if the particles are intended to represent sea lice:

    i. the absence of wind-forced currents (expected to have large temporal variations and magnitudes as large
    as for the tides and estuarine flows);
    ii. a steady rather than variable estuarine flow;
    iii. inaccuracies in the model numerics and resolution;
    iv. the absence of behaviour (e.g., horizontal and vertical swimming) for the particles.

Krkosek et al.^4,5 assumed a simple advection-diffusion circulation model for interpreting his sea lice observation.

The diffusion was meant to represent tidal motions while the advection was either specified uniformly (assumed), or indirectly estimated from lice observations. Most of his advection speeds were less than those measured with current meters.

Though this model is clearly an over-simplification of the regional dynamics, the sensitivity of the paper’s conclusions to these simplifications was not assessed.

However the consistency of the lice observations (i.e., the data not the model) in proximity to a salmon farm in the Krkosek research is inconsistent with the DFO model expectations and remains to be explained.

[1] a 10 cm/sec flow is equivalent to 0.36 Km/hr and 8.64 km/day km/day km/day km/day km/day

References

1. Brooks, K. M. The affects of water temperature, salinity and currents on the survival and distribution of the
    infective copepodid stage of sea lice (Lepeophtheirus salmonis) originating on Atlantic salmon farms in the
    Broughton Archipelago of British Columbia, Canada. Rev. Fish. Sci., 13(3), 177-204 (2005).

2. Brooks, K. M and D. J. Stucchi. The Effects of Water Temperature, Salinity and Currents on the Survival and
    Distribution of the Infective Copepodid Stage of the Salmon Louse (Lepeophtheirus salmonis) Originating on
    Atlantic Salmon Farms in the Broughton Archipelago of British Columbia, Canada (Brooks, 2005) - A response
    to the rebuttal of Krkosek et al. (2005). Rev. Fish. Sci., 14: 13-23 (2006).

3. Foreman, M.G.G., D.J. Stucchi, Y. Zhang, and A.M. Baptista. Estuarine and Tidal Currents in the Broughton
    Archipelago. Atmos.-Oceans. 44(1), 47-63 (2006)

4. Krkosek, M., M. A. Lewis and J. P. Volpe. Transmission dynamics of parasitic sea lice from farm to wild
    salmon. Proc. R. Soc. B. 272: 689-696 (2005).

5. Krkosek, M., M. A. Lewis, A. Morton, L. N. Frazer and J. P. Volpe. Epizootics of wild fish induced by farm
    fish. . Proc. Nat. Acad. Sci. 103(42), 15506–15510 (2006).