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Water Levels Effects on Granby

by: Bernie Keefe 11/9/2013

It will be fun this year seeing Dike 3 recede back into the depths.  It rises like a beached submarine every four to six years only to dive back to the depths a couple of years later.  I started guiding over ten years ago and I have seen it all.  High water, low water, and everything in-between.

What I haven't seen is horrible fishing.  Each year is not the same, but the fish remain and the habitat withstands the test of time.  Everyone notices the lake trout losing weight and gaining it back but did you ever stop to think about how water levels affect a lake like Granby and specifically a fish like the Lake Trout?  With so many changes, why doesn't lake trout fishing success change? The story is complex and includes two main acts.  Water levels affect feeding patterns of all species but also affects the recruitment at spawning time.  Let's start with how water levels affect feeding behavior.

It all starts with microscopic organisms known as the Phytoplankton (immobile microscopic organisms). These microscopic organisms are not mobile and stay in the top zones of the water column.  Zooplankton (mobile microscopic organisms) eat Phytoplankton but also avoid light.  They migrate up the water column to feed in the evenings and then drop down again to avoid the light in the mornings.  Since sunlight rarely penetrates more than 30 feet into the water the Zooplankton can be found in the top 30 feet of a lake. 

The next level of knowledge necessary to understand lake biology is the Mysis shrimp.  These small organisms were introduced into Lake Granby to feed the kokanee. But Mysis live at the bottom of the lake and Kokanee do not so the food source never materialized.  Instead, Mysis shrimp eat zoo plankton and here is the first detail where water levels come into play---if the water levels are low,   then there  is more of the lake in the 30-foot depth.  With more of the lake at this depth the Mysis are at the same depth as the Zooplankton (who migrate every day) and the Mysis shrimp eat more of the Zooplankton. 

Now we are finally to the Kokanee Salmon.  The Kokanee are filter-feeders and use their gills to filter vast amounts of microorganisms from the water.  They feed mainly on zooplankton and if you follow kokanee on the sonar they follow the daily migrations of the Zooplankton.  Kokanee also prefer a very narrow temperature range.  Their preferred temperature is 54F. At times this temperature range puts the fish outside of the depths of the Zooplankton and they do not feed.  When water levels are down the Mysis eat more of the Zooplankton and the Kokanee struggle to
get their fill.  When water levels are up, Mysis have a tougher time feeding on Zooplankton higher in the water column.

Finally, the Lake Trout prefers colder water (48F) and except at ice-out typically lives in waters 40 feet deep or more.  At extremely low waters levels these fish would be pushed to marginal waters in the deepest parts of the lake.  In some lakes the low waters would warm too much and the lake trout would struggle to stay alive.  Since Granby has many areas with great depths the typical low-water year does not exterminate the lake trout, it just pushes them to new locations.  Small lake trout
(less than 21" in length) often feed on Mysis shrimp and larger lake trout feed on Kokanee
Salmon.

With this knowledge it is easy to see how each part of the food chain operates and  that in low water years more of the feeding zones overlap creating competition. 

To win the competition for food your species must have had a good year for reproduction (called recruitment).  This is the second piece of the puzzle and the key player is the Kokanee Salmon.  The Kokanee only live for four years.  At the end of the fourth year they move upstream to reproduce and, like all salmon species, die after reproduction.  In high water years Salmon do not reproduce well.  So, even with consistent water levels there will always be a shortage of Kokanee four years
after a high water year.

Mysis Shrimp thrive in low water years because their habitat overlaps more with the Zooplankton.  When Mysis levels are high Kokanee suffer from lack of food.  Are you following me here?  High water levels and low water levels are rough on Kokanee recruitment.

High water levels the lake does not stratify. The kokes and plankton lose there "protection zone" the shrimp will live deeper and nothing will eat them. They travel u[p at night and eat the plankton. Then in the morning the kokes have nothing to eat. Low water levels the lake does stratify and the kokes and other fish can eat the Mysis shrimp. The plankton has warmer water and can reproduce.  The kokes do not spawn well in Granby, stocking is the only real way to keep these fish in the lake.
The kokes have crashed, but the CPW is stocking 1.5 million kokes in Granby  this spring.

Falling water levels reduce lake trout recruitment.  Lake trout lay their eggs in 20 to 40 feet of water during October and falling water levels can put those eggs in jeopardy, especially as water engineers drop lake levels for winter.  When lake trout numbers are down Kokanee thrive because fewer of them are eaten by the lake trout.  Falling water levels also stress larger fish creating the potential for losses at that end of the spectrum too. 

        Low Water Recruitment  Low Water Feeding  High Water Recruitment    High Water Feeding
Mysis Shrimp          Good                 Excellent                      Poor                                    Poor
Kokanee Salmon      OK                     Tough                        Poor                                   Great
Lake Trout               Poor        Ok, if not too warm                Good                                 Good

Too many low water years in a row, combined with lake trout mortality from hooking, fighting, and trips to the hot oil will eventually decline the overall lake trout  population.  These low water years will eventually slow down lake trout recruitment  as well leading to a decline in the fishing success.  While this is possible, we have yet to see more than two low water years in a row during the last few decades.

Now that you know the details, how does all of this affect lake trout fishing success?  It doesn't!  Since Granby has enough depth to support lake trout even in low water years, the lake trout will only be skinnier or fatter

Blog content © Bernie Keefe
Member comments
shmiley1, CO   11/10/2013 4:27:49 PM
Nice write up Bernie! I often try to explain this to folks especially in relation to the kokes but it's pretty hard to show the whole picture without turning it into a drawn out speech.
 
Bernie Keefe
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