Friday, February 5, 2016

Green Mountain timeout, part 1.

     I'll start with the good news about Green Mountain before getting into the bad news.  Last June, I spent four days out there running gillnet surveys.  We set 40 gillnets in random locations all over the lake, for six hours apiece. Over the course of that survey we handled 86 lake trout. These lakers looked the best that I have seen them at Green Mountain over the past decade. Below is the size distribution of the lake trout that we caught.
     Out of all the times I have run gillnet sampling on Green Mountain, 2015 gave us the highest catch rate of fish greater than 24", and the average size of all the lake trout caught was the largest. In the four years prior to 2015, when running these surveys, I caught an average of one lake trout over 24" for every 23 hours of gillnet soak time (we have 8 gillnets fishing simultaneously, so each day we're out there we get 48 hours of gillnet soak time). In 2015, I caught one for every 10 hours. 
     Also, when we look at relative weights of lake trout, fish over 24" at Green Mountain are in consistently better condition than fish under 24". Below is the relative weight plot from 2015 to illustrate that. Body condition for fish less than 24" averaged 72.1, while fish larger than 24" averaged 95.7. In fact, we have seen this relationship - increasing body condition as the fish get larger - for all of the past 5 years there. However, the difference between the two groups in 2015 - 23.6 points - is the largest gap that I have observed there. Another way to put it would be to say that the trend line on the graph below has the steepest slope out of any of the years that I have surveyed GM.
     All this information means a lot more when you compare it with other lakes. At Granby, fish over 24" have consistently POORER body condition than fish under 24". Below is the relative weight plot for all the Granby lake trout that I have weighed (899 fish) from 2011-2015. 
     Across all those years, relative weights for the Granby fish less than 24" was 82.7, and for fish larger than 24" was 77.1. I have searched our database and run this calculation for many of the lake trout waters across the state including Jefferson, Taylor, Ruedi, Twin, Turquoise, Blue Mesa, and Williams Fork (of course). The fact is, Granby is the ONLY lake trout fishery in the state that sees a consistently lower (and statistically significant if that matters to you) body condition in large fish than in small fish. 
       Here's how it works out in pounds of fish. Based on these length-weight relationships, the average 30" fish at Green Mountain weighs 10 lbs., while a Granby fish of the same size weighs 8.6 lbs. A 36" fish at Green Mountain averages 18.6 lbs., while a Granby fish of that size averages 15.1. And if you take it up to 40", a Green Mountain fish will average 26.7 lbs., while the Granby fish weighs in at 21. The result of poor body condition in these large fish is extremely slow-or zero- growth in length over time, and less egg production. The population seems to become "decadent" in a way, like an overgrown forest that has suffered from a lack of disturbance and renewal. We've all seen what that results in. 
     You may remember that we changed the harvest regulation at Green Mountain Reservoir beginning in January of 2011. We increased the bag limit of lake trout to 8 fish of any size, separate from the bag limit of other trout. There is no size restriction of any kind on the 8 fish bag.  In the years since that regulation change, I regularly ask the game wardens as well as other folks around Green Mountain if people are taking advantage of the 8-fish bag on lake trout and harvesting limits of them. The answer has always been that they absolutely are. As far as we can tell, anglers have been taking full advantage of the increased bag limit there since we enacted it.
     I'll take this opportunity to point out a statement that I wrote in the 2010 issue paper which proposed to increase the lake trout limit:

"This population should easily sustain increased harvest rates, and may exhibit increased quality as a result of the thinning effect that increased harvest may produce."

     I'm not making the claim that my recent data at Green Mountain is unequivocal proof of the above statement, and that my data isn't subject to any of the possible pitfalls of such a data set. However, I will say that this data gives me good confidence that the probability of the above statement being correct is far higher than the probability of it being incorrect.
     This is the stuff that a fisheries biologist bases his/her decisions on, in combination with other sources of information such as public input.  One of the main purposes of this blog is to try to fully expose the reasoning that goes into my - and any biologist's - fishery management decisions.  Even though it goes on in my head and across countless spreadsheets and databases, it's a public process. It's what you pay me to do when you buy a fishing license. And  for that, I have to say a heartfelt "thanks" because I love my job very, very much. Entering my tenth year here, I still have to pinch myself nearly on a daily basis and make sure I'm not dreaming. I consider myself to be extremely lucky to have this job.
     Anyway, I digress.  What I was getting at is that I think there is a little bit of an information gap in the world when it comes to making the details of this stuff accessible to the public. It mainly appears in the pages of specialized scientific fisheries journals that people typically don't have easy access to. I believe one reason that there's not that much of it out there is because as a group, we biologists tend to assume that there is not general interest among the public in this level of detail. But I think that there is a sector of the angling public that is interested in as much detail as we're willing to take the time to give. It may not be the majority of license buyers, but a decent slice of them, I think.  That's the group of anglers that this blog aims to serve. There will always be a majority of people who have no interest in the fact that there is a lot of effort, consideration, and decision-making behind each and every fish that they find on the end of their line.  I'm not going to reach those people no matter what, and that's fine.
     There's a selfish motivation to do this also - it makes me a better biologist to work this stuff out in written form. Makes me consider things more thoroughly and carefully.
     I'm going to hold off on the bad news about Green Mountain until my next post.