| Summary: | The opossum shrimp (Mysis relicta) was introduced into Okanagan Lake,
British Columbia (BC), in 1966 in order to serve as an intermediate food item for
kokanee salmon (Oncorhychus nerka). However, beginning in the early 1970s,
kokanee began a sharp decline in abundance. In the search for reasons for the
kokanee decline, two factors were identified: mysid competition with kokanee
over zooplankton resources and reduced nutrient loads to the lake. Between
1970 and 2000, the M. relicta population increased 20-fold and nutrients in the
lake fell to one quarter of 1970 levels.
Using the Ecopath with Ecosim (EwE) software, an Ecopath model of
Okanagan Lake in 1970 was built. This model attempted to account for all
biomass within Okanagan Lake and contained 16 groups and 2 fisheries. This
base 1970 model was then run through the Ecosim module and biomass was
predicted for all groups from 1970 to 2020. For the 1970 to 2000 period, mysid
and kokanee biomass were tracked by the program with high accuracy. Two key
findings for this period were that Mysis relicta appears to have been responsible
for the original kokanee decline, but reduced nutrient loads to Okanagan Lake
are currently keeping the kokanee at depressed levels.
From 2000-2020, Ecosim was used in a forecasting mode and solutions
were examined which may help rehabilitating the kokanee population. The
current mysid fishery does not have the capacity (30t year-1) to catch the number
of mysids required (300t year-1) to aid kokanee populations to any great degree.
Nutrient additions appear to be able to boost kokanee abundance in the lake
without increasing mysid populations greatly. However, a combined approach
involving nutrient additions with an intensified mysid fishery could allow kokanee
abundance to approach 1970 levels. === Science, Faculty of === Zoology, Department of === Graduate
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