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:
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:
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:
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.