Sunday, April 7, 2013

Dillon, part 1

     All right, I've been asked to get into Dillon Reservoir a number of times. I've been holding out because we had a meeting this past week regarding Dillon and I wanted to wait for new information. There's a lot to talk about with Dillon, so we'll take it one topic at a time.
     First of all, let's talk about the browns. Historically, Dillon enjoyed the status of having a trophy brown trout fishery. It was a destination, and people would travel to Dillon specifically to fish for the large (4-6 lbs or more or larger) browns. All reservoirs experience a reduction in productivity as they age, and Dillon is no different. It is definitely less productive now and the highly visible trophy brown trout fishery has declined over the past couple of decades. However, it is not gone entirely, and in fact has been showing some signs of resurgence lately. The brown trout population in Dillon is actually pretty dense - more dense than any of the other reservoirs in my area. We don't stock any at all; they're all the product of natural reproduction. the problem is, they just don't have anything to eat, and so they tend to be very skinny, and there has been somewhat of a "growth barrier" right around the 15-inch mark.

     The figure above shows the size distribution of brown trout in my 2008 gillnet survey. I'm using 2008 as an example because it's the most pronounced drop between 15-inch fish and 16-inch fish. But that drop exists in all the gillnet sampling I've done. I've only been netting Dillon every other year, with six overnight gillnets in established locations. Picking the suckers out of the nets is just too much of an ordeal to do it every year, and things don't change that much on an annual basis to necessitate netting it every year.
     Speaking of suckers, here's the species composition from 2012 in the gillnets, by percent:

     Every gillnet sample is very similar in this regard too. Here is a graph showing body condition by size in the brown trout population. There are multiple years' samples on this graph, pooled together.

     Because of that apparent growth barrier that I mentioned above, I ran trend lines for these fish as if they were two different populations. What we see at Dillon consistently over the years is that body condition in the brown trout declines as the fish approach 16 inches. Only a small percentage of the fish make it beyond the 16-inch mark, but the ones that do exhibit increasing body condition the larger they get. The reason for this is most likely that those few fish making it beyond 16" figure out how to prey on the 10" catchable rainbows that we stock annually. Brown trout will eat other fish half their length (or sometimes even larger), and there are some runts in those catchable batches that run as small as 8". 
     I'll talk about arctic char next week, but for now I'll finish off with a shot of the largest brown we picked up in the gillnet survey last summer. It went 24" and weighed a whopping 9.3 pounds, which calculates to a relative weight of 177. A very fat fish. So, rest assured that there are still fish like this out there. 


  1. Jon- Great information! I'm looking forward to hearing more about what is going on in there.

  2. I would be interested in some information on the typical habitat, diet, depth range etc of the Char in Dillon. I know very little about the species.