Wednesday, December 4, 2013

We saw this coming . . .

     Ka-boom. Through the blowing snow in Grand County this morning if you listen closely enough, you can hear the low, dull sound of an implosion taking place. It's the sound of the Granby kokanee population crashing.
     We took our last kokanee eggs of the year on Monday, the 2nd. Our spawn season ended with a whimper. The total number of eggs we took from the Granby run was 357,425 out of 432 ripe females. My optimistic prediction for this year before the season was that we might get a half million. Granby needs to produce 1.2 million eggs just to sustain itself. Preferably, we'd like to use Granby eggs to stock other waters also, but that's obviously not happening any time soon. Below is the recent history of egg takes at Granby:

     Our first indication that this was going to happen was the trend in recent years of mysis densities in the reservoir. We sample mysis every year and estimate their density per square meter of water surface area. I like to look at the four-year rolling mysis density estimate because the average kokanee in Granby lives for four years. Here is that statistic, for the entire history of our mysis surveys at Granby:

     Mysis respond positively to high water years at Granby, and get knocked back during drought periods. The low point in 2004-2006 is the response to the Granby drawdown that took place during the drought of the early 2000's. When mysis densities are low, we get much better recruitment out of our kokanee stocking because there is less competition for the zooplankton that both mysis and kokanee eat. If you look at the "good" years of egg harvest, from 2004-2009, you can see that this recovery in the kokanee population was a response to that low-mysis cycle covering roughly the same time period. What we are seeing now is the fallout from the years of high mysis densities that peaked in 2010. 2013 was a lower-water year at Granby and the mysis density estimate was 280. We responded by loading Granby up with extra kokanee - we stocked 1,450,000 rather than the normal 1 million. So hopefully we got good recruitment out of those fish this past summer, but we won't see that in the spawning run until about 2015 or '16. What concerns me now is that the downward trend in this average appears to be flattening out, and for a real kokanee recovery to take place I think we need to see this statistic drop below 500 or less. If the snow keeps piling up, Granby will have another high-water year, mysis numbers will bounce back, and this trend will start heading upward again.
     Every year we run sonar surveys of our kokanee reservoirs to get an idea of the status of the kokanee populations and to predict what the spawning runs will look like. Our research section has a dedicated boat equipped with a scientific sonar rig for this purpose. It's basically a fish finder on steroids, with a lot more definition, giving you the ability to actually count fish and estimate their sizes. There is a lot of room for error in this estimate, but the trend definitely predicts what kind of kokanee run we're going to have. Here is the recent history of sonar surveys at Granby:
     The sonar surveys have been doing a good job of predicting the following year's egg take. So, the 2012 survey was showing roughly 1/3 the number of fish that were there in 2011. Lo and behold, our 2013 egg take was roughly 1/3 of our 2012 egg take. I don't have a 2013 number from this survey yet, but when I get it we will know if we've hit the bottom or if it will get even worse before it gets better. I'm betting on the latter.
     Now, the part that no one wants to talk about. We know that competition from mysis makes it very difficult for the kokanee population to persist. What about predation by lake trout? The recreational kokanee fishery is long gone from Granby. Serious kokanee anglers go elsewhere now.  However, we still need to maintain enough kokanee to provide a prey base for the lake trout, as well as survive to maturity to provide us with eggs. This is the irony of the misguided discussions you see on the discussion boards. The lake trout guys spin their arguments as some kind of kokanee-versus-lake trout struggle, when that is absolutely not the case at all. If you don't have a healthy kokanee population, what are trophy lake trout supposed to eat?
     As I've discussed before, I run a gillnet survey of the lake trout population every May. As expected, 2013 saw a significant decline in the body condition of the large lake trout that we captured. I handled some of the skinniest large lake trout that I ever have this year. Here is a data summary for the lake trout survey:
      I set each net for 6 hours, at the same locations each year. 2011 was the first year I surveyed the lake in this way. There were a couple big differences in 2013. The first was the number of large lake trout that I caught. This number was remarkably similar in 2011 and 2012 but took a big jump in 2013. I did have one net this year that caught nine fish over 24". I had not seen that before. However, even if you remove those nine fish, we still caught twice as many fish over 24" than in the previous two years. Does that mean we have a big increase in large lake trout? This netting survey does not have the degree of accuracy necessary for me to be able to say that. There is too much variation, thus room for error, in the catch rates. In addition, the lake was lowest in 2013, so this could possibly be a reflection of the fish being squeezed into a smaller volume of water. Are the numbers of large lake trout declining? No way. But I'm saying that based on professional judgement and opinion, not on statistical analysis.
     The most telling statistic here is the last two lines. Body condition for all lake trout captured remained the same as it has the past couple years. We know that lake trout smaller than 24" make a good living off the dense mysis population. However, when they grow beyond 24" they need to at least supplement their prey with a vertebrate food item, if not switch over completely. So the drop in average body condition of large fish is the big kicker. That is a large drop over a one-year period for a fish population that by nature changes slowly. And it is statistically significant. A body condition of 73.5 is a very poor fish.
     In an earlier post I had mentioned that it appeared to me that the variation in body condition was higher in 2013 among the larger fish. That is, we caught some really skinny ones, but there were also some fish still in decent body condition. The way to see if that is true is to look at the standard deviation in body condition for the samples from each year. So, for 2011 the standard deviation in this statistic for fish >24" was 8.4. In 2012 it was 8.3. In 2013 it was 8.6. So, my initial impression was wrong, and the variation in body condition was only very slightly greater in 2013 than in past years. We just had some ridiculously skinny fish. Here's an example:

     This fish is an embarrassment. This is a starving fish. I've seen plenty of anglers' pics of fish like this over the past year. These fish are telling us loud and clear that the predator population does not have an adequate prey base to support it. 
     I know that no one wants to kill large lake trout. If this is what the anglers want out of Lake Granby then we can continue down this path. But I want to make sure that everyone fully understands that choosing not to put pressure on the lake trout population at this point in time is a POLITICAL decision and there is not a single piece of biological evidence to support it.  One thing we can't do, is increase the numbers of rainbows being stocked to prop up the lake trout, thereby substituting a cheap prey base with one that is as much as ten times more expensive. In fact, the fishing for other species is likely to decline as the starving lakers turn to anything and everything to survive on.
     Another common misconception that pops up on the discussion boards frequently is that growth rates of lake trout in Granby are slow, period. End of story. The truth is that growth rates CHANGE based on predator-prey dynamics, and we have directly observed that change. Many people are familiar with the old tags that are still in some fish in Granby. Those tags were used in a growth study in the '90's, and this is where peoples' information about lake trout growth comes from. Here is the most important information from that study, which was conducted by Pat Martinez:

     This data is old, but it reflects a time at Granby that is very similar to our situation now. In 1998 the kokanee population had crashed completely and no eggs at all were taken at Granby. The kokanee had to be restarted with Blue Mesa fish.
     But here is the take-home message from that study: Lake trout growth rates CHANGE according to conditions in the reservoir. If there is no prey for the lake trout to eat, of course they are going to be slow. But they don't have to be slow as a rule.  Imagine a scenario with one, 24" lake trout in the whole lake and a huge kokanee population. Do you think that one fish would have an extremely slow growth rate? Of course not. Its growth rate would be through the roof. This is why I get frustrated when people say, "if you kill a 30-year-old lake trout, it's going to take 30 years to replace that fish." This is dead wrong because there is a 29-year-old lake trout waiting in the wings to take that 30-year-old's place. And one less large fish to compete with will increase that 29-year-old's growth rate. So it doesn't take 30 years to replace that fish. It takes one year. Could Granby's growth rates match what we see in Blue Mesa? No way - Granby simply isn't productive enough for that, and the mysis issue will always limit the potential. But could they be faster than they are now? Absolutely.
     Here's another thing to consider. In 1994 Wayne Hubert and a couple other guys published an article called "Interpreting Relative Weights of Lake Trout Stocks." In that article, they took data from 58 different lake trout populations throughout North America, across the range of the fish. The figure below is the most important part of the article:

     It's kind of hard to make sense of but bear with me. This is a cumulative frequency distribution. The X-axis, labeled Mean Wr, is the exact same equation that I use to rate body condition in the Granby fish. What this shows is that 100% of the populations studied had an average body condition below 135. That makes sense, because a relative weight of 135 is a very fat fish. About 65% of the populations had an average relative weight less than 100, and about 35% had an average greater than that. What I'm getting at, is look where Granby falls on this distribution - at the very bottom. Our fish are skinnier than 97% of the populations in North America. In fact, we may have the skinniest lake trout in the world.


  1. Great stuff Jon. I hope a lot of people read your discussion on the predator-prey dynamics of kokanee and lake trout. I've certainly talked to Bill Pate a lot about the negative flack CPW has gotten for trying to maintain the opportunity to catch trophy lake trout in Blue Mesa through netting. Sometimes maintaining trophy fish means you simply need less mouths to feed. The same condition has happened in many tailwaters across the west. We as fly anglers have subscribed almost universally to catch and release in recent years and densities of fish in many popular tailwaters are sky high but body length and condition are less than they used to be. We as anglers have to understand that we can't always "have our cake and eat it too." We can either have lots of fish or trophy fish but it's a rare situation where we can have both, at least for long. Sometimes killing a few fish is actually in the fishery's best interest.

  2. Excellent post, as always! Do you see any revisions to the regulations at Granby? Possibly decreasing the limit on kokanee or increasing the limit on a ‘slot sized’ lake trout to try to encourage angler harvest of the mid-sized fish. Another thought would be to try to supplement their diet with a cheaper self-sustaining alternative such perch, ciscoes, or even suckers. I wouldn’t think that perch would do a very good job feeding the lakers as the perch would most likely stay shallow when the lakers are deep and vice versa.
    I’m not trying to play arm chair biologist, just uneducated and curious. Thanks

    1. Thanks Steve. A regulation change at Granby is definitely a discussion that we'll be having with stakeholders this year.
      The food item that I've thought about repeatedly is lake whitefish. That's what the lakers are living off of in Flathead Lake, now that the kokanee are gone from there. There are plenty of suckers in Granby, and it appears that the lakers will eat them when they have to, but they don't choose to if there are other food items available. In places like Flathead and the Great Lakes, lake whitefish eat mysis and feed lake trout.

  3. Good idea on the lake whitefish... I have also been studying the lake trout data at Flathead and it seems to be a very solid dynamic between the two species.

    I have a question for you, Jon... Could a temporary increase in the laker limit (say for 2-3 years) with a "one-over" caveat (let's say 24 or 26 inches) help in the long term to create a stronger trophy lake trout fishery?

  4. Good job, Jon with the picture of the skinniest fish we netted, you could have included a picture of the fat one with a sucker in it's throat or the fat one with the small lake trout we pulled out of the throat, but I guess you are trying to make a point.

    You know Granby gets more fishing pressure than any other Lake Trout water and that skinny fish may have a hook in it's stomach and or been caught and released several times but don't let those facts get in the way of your agenda. You say a 30 year old lake trout can be replaced in a year... Well, that borders on the most Ridiculous statements ever. Actually we could kill off half the large Lake trout and we would have half the opportunity to catch a large lake trout.

    You know Granby won't be going back to the pre mid 90 years before they cleaned up the water, so that theory doesn't hold water. How about explaining why Granby got shorted on catchable Rainbows the last several years to around 1/3 of the norm at a critical time coinciding with the bottom of a down Kokanee cycle-- while all your other waters got increases.

    It's no wonder the Rainbow shore fishery has crashed and is in very poor condition. No Rainbows stocked prior to memorial day. Hundreds of shore fisherman and No Rainbows. Granby will continue to be what it has always been: A great lake trout fishery and a good shoreline Rainbow (when they get stocked) and Brown fishery and an inconsistent Kokanee lake due to environmental conditions (high and low water levels).

    To maintain Granby's status as a trophy laker destination and a solid trout (shore) destination, a few things need to happen. Lake trout Regs need to be changed to a 1 over 20" along with 4 or more under AND we need to get our fair share of Rainbow catchables that every other lake gets.

    You know Granby gets more anglers than any of your other waters and giving us the short end of the stick on Rainbows due to political issues is a huge problem that takes away angler opportunities.