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Abundance and production of zooplankton and Kokanee salmon (oncorhynchus nerka) in Kootenay Lake, British Columbia during artificial fertilization Thompson, Lisa Colleen


Experimental fertilization was conducted on Kootenay Lake, B.C. from 1992 to 1997 to compensate for nutrients lost behind hydroelectric dams upstream of the lake. Declining nutrient loads were correlated with lower in-lake nutrient concentrations, chlorophyll a concentrations, and macrozooplankton densities, and a dramatic decline in kokanee salmon {Oncorhynchus nerka) stocks. A simulation model of the lake suggested that increased zooplankton production resulting from fertilization might be shunted into increased abundance of Mysis relicta, an exotic crustacean that competes with kokanee, and that nutrient additions might actually hasten the kokanee decline. In an attempt to test this prediction, nutrients were applied at the north end of the lake, and the response of the food web was monitored along the expected longitudinal productivity gradient. The food web structure along the lake suggests that a trophic gradient of grazeable phytoplankton abundance was established, but that M. relicta may have grazed down any increase in zooplankton production in the fertilized end of the lake. Kokanee distribution and size-at-age along the lake did not correlate with the nutrient gradient. Surprisingly, M. relicta abundance decreased during the experiment, while kokanee abundance increased four-fold, and Gerrard rainbow trout {Oncorhynchus mykiss), which prey mainly on kokanee, also increased in abundance. M. relicta is vulnerable to mortality due to export out of the lake during high flow years, whereas zooplankton replace flow-related mortality through rapid reproduction and kokanee can actively avoid export. High surface water turnover rates, due to large winter snow accumulation during the experiment, likely contributed to increased M. relicta mortality. This physical factor may have shifted the competitive equilibrium between kokanee and M. relicta, by suppressing an increase in M. relicta abundance, and allowed kokanee to take advantage of increased zooplankton availability. Caution should be exercised in extrapolating the results of fertilization in Kootenay Lake to other large lakes where fish populations have been affected by hydroelectric dams or competition from exotic species introductions. Nutrient additions may not reach the desired target species unless the responses of exotic competitors are suppressed by physical factors operating independently of the dynamics of the food web.

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