A tiger muskie conversation.

From James (Gasner?? - is that you?), a couple days ago:

In the past you mentioned the possibility of stocking Tiger Musky in several lakes to control the sucker population. What lakes are you considering for this? Specifically I am interested in Shadow Mountain. What if any progress/further thought has gone into this.

     I thought I would discuss this a bit for this week's post. For those of you that don't know how we're structured, the biologists in the aquatics section are divided into management biologists (which is what I am), who cover a specific geographic area, and the research group, headquartered in Fort Collins. These guys don't have a specific geographic area of responsibility. Their function is to answer bigger-picture questions across the state to help us manage or waters as effectively as possible. So each of them has various research projects going on all over the state. They are assigned to specific subject areas. So, for instance we have a researcher dedicated to native cutthroat issues, one for eastern plains fishes, one who specializes in physical habitat work in rivers, and so on. I'll be sharing some information that our lake and reservoir researcher, Jesse Lepak, conducted last year. 

     One of the questions that we asked Jesse to work on is to investigate the utility of tiger muskie as a sucker control agent, and how to most effectively do that. The proliferation of white and longnose sucker is a constant frustration to those of us that manage coldwater reservoirs. For many of these reservoirs, white and/or longnose suckers make up 60-80% -- or sometimes more -- of the total fish population, by both number of individuals and total biomass. You can't help but wonder what the sportfish productivity of a water could be if that much biomass wasn't tied up in sucker bodies, which really don't benefit the system at all. Out of the large reservoirs that I cover (Dillon, Green Mountain, Wolford, Williams Fork, Granby, Shadow), Shadow Mountain is the worst in this regard. The whole thing is unintentionally designed to be prime sucker habitat. It is a shallow, flat pan, almost entirely 15-17 feet deep. It fluctuates very little - which is usually a good thing, and is actually relatively productive. However, unfortunately most of that productivity is tied up in suckers, as far as the fish population is concerned. Below is the length-frequency histogram for both brown trout and white suckers from 2011 (I don't have all the 2012 data entered yet but it's very similar).

     As you can see, white suckers dominate the fish population both in numbers (329 captured, or 67% of the total catch) and in size. The size distribution of the two species is really interesting in Shadow, because there's not much overlap - the browns don't get much beyond 14", and the white suckers really take off beyond that size. It's obvious that the browns can't get "on top" of the white sucker population.

     So anyway, Shadow is a great candidate to try to do something to change the white sucker population structure and see if we can get a positive response out of the sport fish community. When you spend time on that lake, it just seems made for tiger muskie, with a lot of the type of habitat that these fish prefer. 

     Enter Dr. Lepak. Over the past year Jesse collected tiger muskie (TGM) from a handful of places around the state to see what they're actually eating. Below are some slides for a presentation that he gave to us at a staff meeting a couple weeks ago.

     The slide above shows the locations that TGM's were collected, and how many were captured at each water. As you can see, it's hard to get a large sample of them, but he did end up with a decent number. Jesse took tissue sample biopsies from each fish captured to perform stable isotope analysis. This is a fantastic tool which looks at the molecular composition of a fish's body tissues and tells you what food items have made up the fish's diet throughout its whole life. At the same time, Jesse also collected rainbows and white suckers from each lake for use in the analysis.

     Above is the result of the stable isotope analysis. The TGM's that Jesse and his crew collected had consumed 20% white suckers and 80% salmonids. About half of the salmonids were stocked rainbows, the other half unknown, but they were likely wild browns or other wild trout in these lakes. We've known from other studies that esocids have been observed to preferentially feed on salmonids even if another type of fish is present in greater numbers, but this is the most direct observation we've had of what our TGM's are eating. 

     So now that we have that information, we can run what is called a bioenergetics model. This one is a quick thumbnail version, but gives us a realistic scenario of what a TGM consumes throughout its life, up until it's 14 years old and about 22 lbs. in weight. Jesse cut the TGM's some slack on this one and went with a 70/30 trout-to-sucker ratio in the diet as opposed to the 80/20 that he actually observed. He used Parvin Lake temperature data, just because it's a heavily studied lake and was the one that had the best temperature information available. Temperature is important in these models because it tells you at what speed fish consume and use their food items. The cost column is an accounting of how much it costs our agency to raise and stock the trout that the TGM eats throughout its lifetime. 

     The bottom line is that by the time the TGM in this particular scenario is 14 years old, it has consumed $462 worth of hatchery-stocked trout. Over that same period of time it has consumed 165 suckers. Or, you could say that we're paying that TGM $2.80 for every white sucker that it eats. If I had the time, I could remove a whole lot more than that for a lot less money.

     There is a bright side to all this. In order to force TGM's to eat more suckers and not eat stocked trout, all you have to do is stop stocking trout. It's obvious to me from this information that if you really want TGM's to hammer the whiter sucker population, that's a necessary step. So, to give the original question a very long-winded answer, I would still like to go down this road at Shadow. I would think of TGM as a biological "treatment" that we would be applying to the lake, that would play out over 10 years or so, to be followed by a reduction in TGM density and the restoration of some level of trout fishery there.  Here are the main hurdles as I see it:

• We have to get permission from the US Fish and Wildlife Service to do this. This is because Shadow is on the west slope, in the Colorado River basin, and drains into the endangered fish recovery area (eventually). I believe that I can make a very good argument with the Service that they should allow this in Shadow, because of the extremely remote chance that a TGM from Shadow would make it down into the endangered fish reach -- and even if that did happen, one of the great beauties of TGM is that they are sterile hybrids, incapable of reproduction. I've made some inroads on getting this permission, just through informal conversation at this point. But it seems promising.
• Numbers. To date, we have always had to obtain our TGM's from out of state. It seems that we have never had a reliable source, and when we do get them it's never as many as we would like. To do it right at Shadow, we would need roughly 13,000 fish (10/acre). In 2012, we stocked 11,874 TGM's in the WHOLE STATE. I don't believe I would be able to persuade all my counterparts in the entire state that I deserve all of the TGM's that we could get in a given year. But -- we're working on this one. My counterpart to the east, Ben Swigle, has imported pure muskellunge into the state and is attempting to raise them to spawning size in a private water on the east slope. If all goes well, we may have the means to make our own TGM's before too long. If and when this happens, hopefully this quantity issue will be resolved. Our fingers are crossed on this one.
• Public buy-in. We need to get the acceptance of people who like to fish Shadow. It may not be the most valuable fishery in the world (or in Grand County, for that matter), but it does get its share of traffic. Some of those people will not be thrilled that we're going to cease the stocking of trout there for a decade or so. I suspect that the recreational fishing traffic that would be lost would be replaced with a different set of anglers coming specifically to pursue the TGM, but it would probably not be as many people, and it's tough to say whether or not the economic activity would be equivalent.

     Then there's the issue of Grand Lake. We haven't yet discussed at all the fact that there is free movement of fish between Shadow and Grand. It would be foolish to assume that TGM wouldn't at least occasionally move through the channel and check things out over on that side. We're having some good success with a new strain of rainbow fingerlings that we've been stocking in Grand, and I wouldn't want this TGM project to interfere with the progress that we're making there. My gut feeling is that because of the difference in habitat, the occurrence of TGM turning up in Grand Lake would be fairly minor and rare, but what about if that's the only place where trout are available, and they figure that out? What if Grand Lake became a magnet for the TGM's just because of the presence of trout there, and they didn't do their job in Shadow at all? Maybe that's a far-fetched scenario, but it does warrant some consideration.

So that's pretty much where my thinking is at the moment on the subject. If you're a Shadow Mountain regular, I'd love to hear your thoughts.