Friday, July 1, 2016

Summer mix

     I am reminded in June annually that by far my favorite thing in the world of wildlife that I have seen firsthand is colored-up cutthroat spawning in a mountain stream that is clear enough to be able to watch all of their social interactions. Never mind the fishing part of it. Completely unnecessary, and in my opinion somewhat on the invasive side when that is the scenario.  This is the early-summer sweet spot of the year when a lot of my time is spent in the presence of such activity, and it's now more than ever that I remember how ridiculously lucky I am to have the job that I have. Next week, I get to go take a three-day pack trip to one of the most remote and beautiful alpine lakes in the state, collect eggs from the spawning cutthroat there, and deliver them to our hatchery in Glenwood where they are raised into the broodstock that then produces the eggs for all the alpine lake stocking that we do on the western slope (minus the San Juan River drainage). I mean, could there possibly be a more enjoyable work assignment than that? When I'm standing next to a stream full of spawning cutthroat in some jaw-dropping setting, I feel like I won the lottery. So thank you, from the bottom of my heart, for buying a Colorado fishing license.

     From time to time, when an interesting scientific paper comes out, I'll discuss aspects of it here. The Idaho folks have put out one of the best lake trout studies in recent years, describing work they've done in Priest Lake. It was recently published in the North American Journal of Fisheries Management. One of the great things about this study was that they collected one of the biggest age-growth data sets (628 fish) for a lake trout population that I have seen, shown below.
    Priest Lake is similar to Granby in its food web, with lake trout, kokanee, and mysis shrimp all introduced at different times. Also like Granby, there is a negative relationship between the size of lake trout and their body condition, suggesting that food availability becomes a limiting factor as the fish get larger, and that the lake is overloaded with large predators that do not have an adequate forage base available.
     The most striking thing to me when looking at this data set is the degree of variability. The solid line describes the average, or typical, length for a given age. I think that often, once you slap that line on a graph, your eye is drawn to it and it's easy to forget to consider the other information contained in the figure. Using the equation that describes the line, which is in the lower right hand corner, it tells us that a 10-year old lake trout is 498 mm, or 19.6 inches. A 20-year-old is 636 mm, or 25 inches, and a 30-year-old would be 709 mm, or 27.9 inches. In fact, based on the equation, it would in theory take 47 years to produce a 30-inch fish, and no fish in the lake would ever reach 40 inches. But that's obviously not the case. The reality is that there are fast growers and slow growers. The 31-year-old age group ranged in size from about 21 inches to about 40 inches. The  20-year-old age group ranged from about 18 inches to about 35 inches. So you may catch a 31-year-old fish that is 21" and you may catch a 20-year-old fish that is 35". I think that we all tend to overlook this degree of variability when thinking about lake trout growth rates.  It appears to me that what's going on here is similar to what we see at Granby -- beyond 24", you're really talking about two populations of fish. You have the ones who make the prey switch to vertebrates, and the ones who, for whatever reason, never make that switch and just stop growing, seemingly perfectly content to just keep eating mysis for their entire lives. It's maybe even a little misleading to apply one growth curve to this population. At 600 mm, or 24", it should really split out into two, one describing the growth of the fish who made the prey switch and one describing the fish that did not. You can see right where those two curves would be, and there's even kind of a blank space in between the two groups on the right half of the graph.
     The problem is, in order to produce such a data set, you would have to run stable-isotope analysis on each of the 628 fish that were aged here, which tells you what the fish preferred to eat throughout its life. Then you would be able to assign the fish into two groups, the vertebrate predators and the mysis eaters. That wasn't the point of this study, and it would become pretty expensive. It was a huge amount of work just to age this many fish - in order to age lake trout you have to prepare the otolith by setting it in epoxy, and then sectioning it with a saw, cutting very thin slices through it in order to be able to read the annual rings. There is a reason why it is very rare to see an age-length data set that is this large. Anyway, the Idaho folks did a great job on that study and there are a lot of interesting things in their paper.

    I've spent a lot of days on lakes over the past 8 weeks. Wolford, then Granby, which I talked about last post. Then a couple days on Shadow Mountain (my least-favorite lake of all time), followed by 5 days on Williams Fork. We ran the most intensive gillnet survey that's ever been done on Williams Fork, with 40, six-hour gillnet sets, making for 240 total hours of gillnet soak time. Then a couple days on Dillon.
     Williams Fork was a lot of fun and I learned a lot. The biggest laker we picked up there was 39" and weighed 35 3/4 lbs. It was very similar to the last really big laker we handled at WF, in 2013. That one weighed 36 pounds and is the heaviest fish I've ever netted at Williams Fork. So this one was the second-heaviest by a quarter of a pound. Here's a pic of this year's big fish:

     That fish has a body condition factor of 132, if you're curious. I'll discuss that further below, and on the relative weight plot for this survey, you can see how outstanding this fish is compared to the general population.
     The second biggest lake trout we picked up, a 25-pounder, had two recently eaten, partially digested lake trout that were both about 16" sticking out of its stomach. The coolest thing about that was that one of those lakers had a full stomach that was still intact and packed full of recently-eaten crayfish. So a nice picture of three levels of the food web here:

    Williams Fork is experiencing a nice peak in crayfish production right now for whatever reason and you can see it benefitting the whole food web. Many smaller lake trout had stomachs very full of crayfish. Actually, I shouldn't say "for whatever reason" because I think it's a response to drawdown that occurred as a result of the 2012 drought. When the reservoir refills after a drought period, we tend to see a "new reservoir effect" that creates a surge in productivity for the next few years, and you can watch that surge work its way through the food web.
     WF is always a decent crayfish lake but it does seem to behave in somewhat of a cyclical manner. Hopefully this also means it's a good zooplankton year. I certainly saw a lot of daphnia kicking around in the water just looking down into it. We could really use a rebirth of the Williams Fork kokanee run about now. It was nice to see that fish that's pictured above, because it is an example of the food web "bypassing" the traditional kokanee prey base. If that is going on a lot, and we've got good zooplankton production right now, maybe that's good news for a recovery of kokanee numbers.
     Here's a 12-pounder that we picked up out on the flats that had recently eaten a kokanee that ran about 13":

     You may remember that we liberalized lake trout bag limits at Williams Fork in 2011, to allow for an 8-fish bag and no more than one fish over 30". We also liberalized the bag limit at Green Mountain, which I've discussed in previous posts. But we did not include the one-over-30 provision at GM. I am looking at this as a kind of a paired experiment to see if the one-over-30 regulation produces any differences that we can see in the lake trout population over time. My hypothesis is that lake trout anglers so rarely harvest fish over 30", let alone more than one of them, that it makes no difference whether or not a lake includes that restriction. So now, I have a data set from each lake, Green Mountain in 2015 and Williams Fork this year, both of which involved the exact same amount of effort - 40, six-hour net sets in randomly selected locations. The reason why these data sets are from consecutive yeras rather than the same year, is that I run the surveys at the same time on the calendar. It takes 5 days to run a 40-net survey, and I don't have 10 days to devote to just these two lakes every year. But I can commit to 5, hence the every-other-year schedule that I'm on now. 
     We can now start to look at those data sets to see if we can detect any differences. Here are the length-frequency histograms for the two lakes:

     The most common size of fish at Williams Fork was 11", while the most common size at GM was 15".  The total number of lake trout we captured at WF was 122, while Green Mountain yielded 86 fish. With 80% confidence intervals, those catch rates look like this:

     Not much overlap there. We can apply a simple t-test to see if there is a significant difference between these two data sets. This yields a p-value of 0.14, suggesting that there's an 86% chance that the two lakes have different densities of lake trout. Plenty of evidence for me to say that Williams Fork currently has a higher density of lake trout than Green Mountain. Is this a result of the difference in the regulations?
     Because the difference in the regulation involves fish over 30", what can we say about that? In 2015, at Green Mountain we captured 12 fish over 30". At Williams Fork this year we caught 5. Put another way, at Green Mountain it took an average of  20 hours of gillnet soak time to pick up a fish over 30", while at Williams Fork it took an average of 48. This suggests that the density of fish larger than 30" is higher at Green Mountain, which is the opposite result that you would expect given the regulation. 
     Let's look at relative weights between the two lakes. Here are the relative weight plots by size. The axes on these graphs are the same for easy comparison.

     Green Mountain is showing a greater increase in body condition as fish get larger. That is, the slope of the trend line is steeper, as illustrated by the equation. However, the reason for this is not that the large fish are in better shape at GM. It's because the small fish are in better shape at Williams Fork. That is why the intercept value in the Williams Fork equation is higher. This can be seen in the table below, which displays body condition for fish on either side of the 24" mark:

Green Mountain 2015
Williams Fork 2016

     You can see that the difference here is that small fish at Williams Fork were in significantly better condition. Again, this is a direct reflection of the good crop of crayfish currently in the reservoir. The condition of the large fish is essentially the same between the two lakes.      
     All right, time to wrap it up. Bottom line is that so far, there is no evidence that the one-over-30 regulation at Williams Fork is resulting in any differences in the lake trout population that I am able to detect in comparison with Green Mountain. We'll see what the future holds. There are currently a ton of small lakers in Williams Fork, and I think that people aren't harvesting them because a 12" lake trout isn't nearly as desirable to harvest as one that's in the 16-22-inch range. But it sure wouldn't hurt anything if people were willing to harvest a bunch of those 10-12-inch fish. 
     Please be sure to use the comment section to mention topics you'd like to see me address. I suspect people are tired of hearing about lake trout and it's time to turn  to other subjects. Let me know what those other subjects should be, and thanks for your interest.


  1. Jon,

    I have not heard any news on the Salmon flies that were re-introduced into the Arkansas River. Any word on their success? I was also curious if there is any information on biomass/fish populations on the Eagle River? It seems to be in very good shape considering how much pressure it receives.

    1. Mike, as far as the Arkansas goes, read through the comments section on my May post. I addressed that question there. As far as the Eagle goes, it's outside my area, and managed by my counterpart in Glenwood Springs, Kendall Bakich. Her email address is Thanks!

  2. It's nice to see you are learning more about Lake Trout with hands on experience that you don't get from studies and books.
    I hope you are documenting well the food webs of your multiple encounters with large Lake trout preying on small ones. As you can see your request for more harvest on smaller 12" lakers at WF is naturally taking place by the few large ones that are a huge benefit to the overall health of the lake and they don't only eat Rainbows and Kokanee as Martinez's old bias studies stated were the vast majority of their diets. The one large fish limit is much more important than just biological reasons, it gives the lake a trophy status as a destination fishery. The Lake trout angling pressure has increased dramatically since the reg change, just like the Pike limit it brings more anglers looking for a quality experience to the lake. It also keeps from having an 8 fish open any size limit that favors the big fish killers in lieu of the ones that care about the fishery. It gives the dpw a better status that they appreciate what they have in lieu of places like GM where I have encountered dpw employees encouraging the harvest of all Lakers with no regard towards the large ones.
    Is there No hope of Whitefish being introduced in Granby?
    Tiger Muskie in Shadow mtn and Willow cr.?
    What positive improvements to our lakes for the anglers are possible?

  3. Jon,

    Williams Fork has insane populations of crayfish, Green Mountain does not. WF has fast growers. Green Mountain for the greatest part has slow growers. This makes it hard for me to understand your comparison and experiment.

    Green Mountain has snowmobile access and gets by far more angler pressure and harvest than Williams Fork. Many Many more lakers are harvested every year from GM than there are at WF. More big Lakers are killed from GM than you think. I promise you this. This also makes it very hard for me to wrap my head around your Hypothisis and exactly what you are trying to prove with your experiment. We know that GM had lots of little lakers that needed to be thinned out. We know that by raising the limit it has made a difference in body condition and growth rate of the smaller lakers.
    You cannot dispute that fact that large lakers eat little lakers, or that large lakers eat suckers. You also cannot dispute that Big lakers, especially at GM, take 20 to 40 years to replace. We also know that numbers of small fish for big lakers to eat at GM by far outnubers the amount of predators to eat them. So your High School graduate theory does not hold up at GM.
    We know that GM needs less small lakers and less suckers. We also know that a magnificant fish at GM is hard to replace. Its has been put to fact by your gill net studies that are very few fish over 36" in GM.
    Please can you see the fact that big lakers at GM are a huge benefit to the fishery. It is damaging the fishery by not having a protective bag limit for big lakers at GM.

  4. Mọi người có biết rằng Mụn rộp sinh dục và bệnh sùi mào gà là hai bệnh có tỉ lệ tái nhiễm lại là rất cao. Có rất nhiều nguyên nhân dẫn tới tình trạng này như chữa trị không triệt để, không đúng cách hoặc chữa trị gián đoạ, cũng có thể là tái nhiễm lại từ đối tác… Do đó, mọi người cần phải hết sức lưu ý. Khi có dấu hiệu bệnh cần phải đi thăm khám và điều trị triệt để ngay. Chữa bệnh sùi mào gà và mụn rộp sinh dục không hề đơn giản nên mọi người nên tuân thủ theo đúng nguyên tắc điều trị của bác sĩ đưa ra.

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  6. I enjoy fishing at Williams Fork and have had reasonable success. This spring (May 2017) in particular has been outstanding in terms of quality of the fish. I have caught seven in the last month and all but one have been 19-22 inches in length. Each fish has had a belly full of 1-2 inch crayfish in their stomach. Only one fish had a partially digested fish in in stomach, the biggest laker I caught at 22 inches. I am curious as to when the lake is stocked when kokanee salmon?

    Also, a statistical question. Why is the average used with regard to size discussion rather than the median? It seems like the median value is a more representative indicator of what an angler might expect to catch when the data is reporting on lake trout where large lakers skew the average value, yet are very unlikely to be encountered. Just my observation...

    With regard to the amount of crayfish at WF, I will admit that there does seem to be a lot of crayfish patrolling the shoreline at WF. In fact, when fishing with sucker meat, the crayfish do a number on the bait within about 30 minutes and I think that the lakers are likely to not take the sucker meat when it is covered with a lobster sized WF crawdad! However, unlike past year when the laker seemed to be full of 4 inch kokes, they now seem to be gorging on the crayfish. Doesn't seem to hurt their body condition any as all fish caught in 2017 seem very healthy. I have not fished GM for lake trout.

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