CSU Fisheries Ecology Lab

Sustaining Kokanee and Lake Trout Fishing in Blue Mesa Reservoir




Blue Mesa Reservoir (BMR) near Gunnison is Colorado's largest coldwater reservoir.  BMR has been managed as a kokanee fishery and egg source since the 1960s and has provided some of the finest kokanee fishing anywhere.  The value of a thriving BMR kokanee fishery to Gunnison County was recently estimated at $5.2 million (Johnson  et al. 2008).  Thanks to kokanee, BMR has also produced four state record lake trout, in 1998, two in 2003 and the latest in 2007 (50 lbs).



Kokanee abundance has been declining at BMR for most of the last 20 years with kokanee abundance in 2010 at just 10% of its former level in the early 1990s (Figure 1).  This decline is occuring despite dramatic increases in the number of kokanee stocked into BMR. The Roaring Judy State Fish Hatchery has been raising kokanee at capacity (about 3 million per year) and no more kokanee can be raised for Blue Mesa Reservoir.  Before lake trout expansion at BMR, stocking less than half the current number of kokanee produced spectacular kokanee fishing.

Concurrent with the kokanee decline there has been a steady increase in the abundance of lake trout, as measured by catches quantified in creel surveys (Figure 2).

It is not currently known why the lake trout population expanded so rapidly over the past decade or so. This is a topic we are currently investigating.





Lake trout growth in weight (grams; Figure 3, solid lines), and their body condition (how heavy they are for a given length, in millimeters; Figure 4) has been declining over the last decade, concurrent with the decline in kokanee abundance. [to convert to English units: 25.4 mm/inch, 454 grams/pound]

Thus, it is obvious how dependent the BMR trophy lake trout fishery is on maintaining an abundant kokanee population. Without kokanee there will be no more monstrous macks in BMR.


Because lake trout growth in BMR is so fast, compared to most other populations in North America, we have worked very hard to verify our estimates of lake trout age and growth.  A total of 545 lake trout collected from BMR in 2009-2010 were aged from otoliths (below).  Samples were aged independently by 2-3 experts at CSU and a subset of samples was aged by a lake trout expert from Cornell University. All of our work confirms that lake trout in BMR grow very, very fast but do not live as long as in slower growing populations in the native range of the species.




Figure 5. Latest growth estimates of Blue Mesa Reservoir lake trout, estimated from otoliths.


Figure 6. Transverse cross section of the otolith ("ear stone") from the current state record lake trout, caught at Blue Mesa Reservoir in 2007. Amazingly, this giant lake trout was only 16 years old.  Anglers can be heartened that more giant lake trout will be produced quickly at Blue Mesa if kokanee numbers rebound.



Population modeling (Figure 7) using the best available data predicts that BMR lake trout could eliminate kokanee from the reservoir without immediate action. 

The figure below shows the average amount of kokanee anglers catch each year (sky blue, with the range shown as vertical line and crossbars) in kilograms (left; 2.2 lbs per kg) and numbers of fish (right).  In 2009 the kokanee harvest was the lowest on record and less than 18% of what it was in 1993.

Compare these numbers to the estimated amount of prey consumed by BMR lake trout in:

2010 (red bars), the amount they will consume into the future without any action to stop them (green), the amount they will consume into the future if CDOW keeps removing lake trout at the rate they did in 2009 (purple), and the amount consumed by lake trout if CDOW doubles their removal efforts and culls fish up to 965 mm (turquoise bars).

From these comparisons it should be clear that in 2009-2010 there were enough lake trout in BMR to consume ALL of the kokanee in the lake. Fortunately, they have not done so, yet, because as the abundance of kokanee has declined the macks are finding other prey species to eat (but are growing slower as a consequence).

Figure 7. 


Figure 5. Lake trout predation on kokanee at Blue Mesa Reservoir. Left panel: the average biomass (and range) of kokanee harvested during 1993-2009 (pale blue) and the predicted biomass of kokanee consumed by lake trout under four scenarios: 2010 population (red), into the future without lake trout suppression (green), continuing the 2010 suppression program into the future (purple) and doubling suppression effort and culling fish up to 38 inches (965 mm) into the future (turquoise). Right panel: number of kokanee estimated from SONAR (pale blue), and numbers of kokanee consumed by lake trout under the same 4 scenarios as on the left side.






Wondering what you can do to help bring back great kokanee and mack fishing at BMR? There are several things:

1. Support Colorado Parks and Wildlife biologists that have dedicated themselves to restoring kokanee.

2. Let your fellow anglers know that you care about kokanee at Blue Mesa, for the great fishing they provide, and as the key ingredient for producing trophy lake trout.

3. Help Colorado Parks and Wildlife thin out the mackinaw population by harvesting more macks.  There's no bag limit on macks under 38" and they are good eating. A favorite way to cook lake trout in the Great Lakes region is the "Door County Fish Boil". Why not give it a try?


8 sm. potatoes, scrubbed
2 onions, peeled & quartered
2 lb. white fish or trout cut into 1 1/2" pieces
2-3 tbsp. salt


Fill kettle with water. Heat to boiling. Using a basket add potatoes to kettle and boil for 12 minutes. Add onions and boil for 5 minutes longer. Add fish and salt and boil 8 minutes for white fish or 12 minutes for trout. Serve hot with melted butter. Water must boil at all times during cooking process. Serves 4.



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