[hobiecat]

Redleader has mentioned lake trout that he has tagged and caught that have never achieved high growth rates..For example a twenty year old fish that is 20 inches long. I've also heard Jon Ewert wonder why some lakers make the switch to feeding on fish and grow large while others never seem to make that transition.

We've also seen pictures of larger lake trout from Granby that appear skinny..while others are fat and in what we would consider great body condition. Jon even wonders why that would be the case in the same reservoir under the same conditions. (See link below for pics and Jon's comments)

http://coloradoheadwatersfisheries.blogspot.com/2013/06/granby-spring-netting.html

Could it be that we have more than one Phenotype swimming in the reservoir?

There are three Phenotypes listed for lake trout...Lean. Siscowit, and Humps. Leans and Humps tend to prefer shallower areas of a lake than Siscowits..

http://pblabs.biology.dal.ca/research/index.php?t=Lake trout ecotypes

What's noted in this link is that the larger fish have fewer gill rakers than the smaller fish. The more gill rakers ...the more likely a fish will continue to feed on zooplankton/mysis..fewer gill rakers..more likely to feed on fish.


"a large fish-eating morph and a smaller invertebrate-eating morph. There is evidence for further variation within these morphs but the major split among life history types relates to diet and maximum size attained. The two morphs differ greatly in body morphology with fish-eaters having a more streamlined body shape, larger mouths, smaller fins and fewer gill rakers (used for filtering prey out of the water) than the smaller invertebrate eater."

http://eurekamag.com/research/025/3...lvelinus-namaycush-algonquin-park-ontario.php

"Counts of pyloric caeca and gill rakers were made on lake trout (Salvelinus namaycush) in three Algonquin Park lakes where the trout were dependent on plankton for food in varying degrees. In 798 trout from the three lakes, pyloric caeca counts ranged from 105 to 210 and varied with the size of trout. There was no association between numbers of caeca and type of food eaten. Gill raker counts were made on 930 lake trout. Numbers of rakers are not related to size or sex of lake trout in the sample. Gill raker counts ranged from 17 to 26 in the combined populations. An association exists between the numbers of gill rakers, their length, and the amount of plankton eaten by the lake trout. Accessory gill rakers on the medial surface of the gill arch are described. The number and development of these rakers were associated with the amount of plankton eaten by the lake trout."

So rather than just try to explain body condition relative to food supply..Does the phenotype of the lakers in question need to be determined and factored into the equation?

Also if there are multiple phenotypes in the reservoir..what is the proper balance between those that feed entirely on zooplankton and mysis and those that use fish as forage? Are different Phenotypes one of the reasons that some lakers never grow beyond a certain size..say less than 22..23 inches? Would a phenotype with more gill rakers...feeding entirely on zooplankton and mysis compete for food with juvenile kokes?

Also plankton/mysis eaters...the smaller fish reach maturity faster and enter into the reproductive cycle sooner than the fish eating phenotypes..Could having too many zooplankton/mysis eaters than contribute to overpopulation problems?


Just wondering.,

 

[hobiecat]

I guess when I mentioned zooplankton...I should have mentioned the mysis also. The fish with the most rakers..are also the lakers that continue to feed on mysis rather than switching to a fish forage diet. However, all juvenile fish start with zooplankton before moving on to the mysis. Also the Siscowits contain more lipids (fats) than either the humps or leans and have the ability to move up and down the water column easier due to larger air bladders...They're found deeper up to 300 feet or greater if the lake has the depth.

I'm just wondering if the difference between these morphs...also effects the fishery. I'm not too sure how much study was done on the different Phenotypes before stocking the lakes in Colorado...and if they actually ever chose between the different types ..if that testing wasn't done prior to the original planting of the fish.

There seem to be some studies that suggest planting the right Phenotype is critical when managing a lake for lake trout.

 

[hobiecat]

It even comes down to the spawn...It's mentioned that different phenotypes spawn in different locations...not a lot of crossover between the types.

 

[hobiecat]

I have mentioned to Ewert about the different species that have adapted in Granby and he didn't think there would be major differences but the darker suspended fish have obvious differences. When the lake trout were originally stocked in the 60s A lake trout was a lake trout and I doubt they have ever done much studies here on the Phenotypes.

I agree. Maybe a study for some CSU graduate student to undertake? I know that recent Phenotyping was how they determined that we weren't stocking "pure" greenbacks in the state.

 

[hobiecat]

I guess my reasoning is this...not too sure how it could help Granby..but take Turquois Lake as an example...They are continuing to stock lake trout in Turquoise Lake..I think at least as recently as 2010 or 2011. Which Phenotype should they be putting in that lake?

 

[hobiecat]

If you have a sucker problem in a lake...Don't expect to have a great impact on the suckers if stocking a phenotype of lakers with more gill rakers that feed on invertebrates their entire lives and compete with other game species for forage rather than switching to a fish forage diet.

 

[hobiecat]

So was the recommendation to stock deep water sculpin ever attempted at Granby? Page 8 under recommendation in the link.

Also which type of Artic Char are being stocked in Dillon...It seems to make a huge difference in Mysis control...and for the growth of the Char. Page 7 in the link..bottom left hand..

Sorry only some parts of this PDF are allowing me to copy and paste using my limited computer skills.

Study from 1987 ...very interesting..Proposed Biological Management of Mysis relicta in
Colorado Lakes and Reservoirs PATRICK J. MARTINEZ

http://wildlife.state.co.us/SiteCol...ications/ProposedBiologicalManagement1989.pdf

 

[hobiecat]

Jake went to Wisconsin for a meeting on that about 20 years ago when Martinez had that theory going but they never could figure out how to get them out of deep water from lake michigan without damaging them and gave up on the idea. The lake trout eat the Mysis anyway so having them go from mysis to sculpin to lake trout may not have changed much.

I think it would help a lot. It says those sculpin feed almost entirely on mysis...don't compete for zooplankton...live on the bottom and lake trout feed heavily on them...They also don't move from lake to lake and they grow to 9 inches. May allow the kokes to come back...since the kokes compete with the mysis.

That's if you can get them...