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Lake: Dillon Reservoir

New CPW biologist report for Dillon

Post By: Ewert      Posted: 3/20/2023 12:11:03 PM     Points: 100    
Greetings - just letting folks know that I posted a new report on Dillon at this link:

[log in for link]

I'll watch this thread for a week or so if anyone has any questions or wants to discuss. Thanks!
 Reply by: fishinaddiction247      Posted: 3/20/2023 12:18:52 PM     Points: 640
Thank you for what you do for the River Basin! Are you considering closing the Blue River upstream of Dillon like it used to be? Would like to see those brown trout have a chance to spawn up there again, very good strain of reproducing fish that make a run. Lots of people catching fish off redds in the fall...
 Reply by: eholm      Posted: 3/20/2023 12:28:42 PM     Points: 17861
Very cool! I love the idea of growing huge brown trout to eat up the suckers!

And wow that state record char caught (presumably) through the ice! I would love to see a story about that catch.
 Reply by: Dorfer      Posted: 3/20/2023 12:35:06 PM     Points: 2272
Very good report. Thank you for posting it. Is there a chart of the rainbow trout size distribution like Figure 6?
 Reply by: Ewert      Posted: 3/20/2023 9:34:41 PM     Points: 100
fishinaddiction247 -
I've definitely been putting some thought into that. I have an electrofishing station just upstream of the Highway 9 crossing, or right behind the Tiger Road bus stop. I hit that site every other year. Look at what the biomass estimates have been doing there.
I don't know that it's just angling pressure causing this consistent decline because there are some water quality issues there too, and the whole place has been heavily urbanized over the past ten years. But I also can't rule it out. I also used to thnk that I couldn't positively connect a decline in that spawn run to a decline in large fish in the lake - because the lake is so large, and browns spawn in many places besides just that section of the Blue. But now I'm not so sure about that either.
The kokanee run that used to pile into that stretch pretty heavily has also disappeared over the past ten years. We keep watching for it but we never see it any more.
So yeah, I think the time has come to have a serious conversation about re-instituting a spawn season closure on that section.
 Reply by: Ewert      Posted: 3/20/2023 9:43:33 PM     Points: 100
Dorfer -
No, I don't have graphs like that handy of the Rainbows. Main reason is that they're not very interesting or informative -- the vast majority of the Rainbows we get in our surveys were stocked that summer, so they're just standard 10" right off the truck stuff, and then there are a smaller number that have put on some growth - 12-14" depending on the batch and the timing when they got stocked. But really nothing much more than that. I mean, we pick up the occasional higher-quality Rainbow but it's rare.
The Brown Trout population is much more interesting to me because it's all wild reproduction, it's a resource that Dillon produces naturally and so I see that as far more valuable.
 Reply by: xavierk31      Posted: 3/21/2023 8:25:29 AM     Points: 6653
Really enjoyed reading through that, happy to see that the Char population might be supplemented with stocking again, despite the difficulties mentioned.
I noticed that there were no brook trout sampled. Were these never stocked in dillon and therefore only would be from the headwaters of the blue, or potentially tenmile creek? Or were they stocked and just never took?
 Reply by: Ewert      Posted: 3/21/2023 9:13:25 AM     Points: 100
xavier - interesting question about Brook Trout stocking in Dillon. Much to my surprise I do have records of them being stocked four times: once in 1980, twice in 85, and once in 92. The numbers and sizes are odd, and since there was no regularity to it I suspect it was a situation (which happens often) where the hatcheries say, "Hey biologists, we've got XXX extra Brook Trout that are not spoken for, who wants them and where?"
Brook Trout are aptly named, and are not really suited to do well in a large, deep, cold reservoir like Dillon. Any fish that would turn up there today would be incidental, fish that would have washed down from an upstream source like you suggest.
 Reply by: Troutbisquits      Posted: 3/22/2023 9:29:52 AM     Points: 2557
First thanks for the work you do on these reports. I have been reading them for years and love the info.

I have a question about artic char. In the 2020 Carter lake survey there was a rather large char captured. [log in for link]

I thought they have only been stocked at Dillon? Is this true? I am wondering if they have ever been stocked in Carter, and if they have not could this be a case of misidentification?

 Reply by: eholm      Posted: 3/22/2023 9:43:33 AM     Points: 17861
Troutbisquits - I'll let Jon answer the question, but I would bet it was a splake.
 Reply by: Dorado1      Posted: 3/22/2023 12:48:21 PM     Points: 20

Thank you for your post on this platform! Is there any data available yet regarding the success/failure of the golden trout stocking of Old Dillon Res?
 Reply by: Ewert      Posted: 3/22/2023 1:17:06 PM     Points: 100
Troutbisquits - that's a funny question about Carter Lake, good catch.
One thing that happened while we were importing char eggs for Dillon is that the guys at Poudre hatchery held some back to grow over a winter and see if they could build a small broodstock to produce our own eggs. It was just kind of a fun/pet project that held a very low priority for hatchery space, since it would only benefit one water. At the time I tried to sell some of my counterparts on the idea of trying them in Reudi and Turquoise because they're similar to Dillon and both of those lakes have mysis -- but they also both have lake trout and we'd be putting char on top of them. They weren't interested. So when space inevitably became an issue at Poudre, the char that had been held over had to go. So this was in 2014 and the char at Poudre were a little over 11" long. I got most of them (1700) in Dillon, but they stocked 450 in Carter and 450 in Horsetooth. I think it might have had to do with timing -- the Carter and Horsetooth fish were stocked on 3/3/14 and the Dillon fish weren't stocked until 5/27. I think that probably what happened is that they had to make a small amount of space, because they were outgrowing their raceway, while waiting for Dillon to open up for the year, and the bios for Carter and Horsetooth were good with taking them. So it's not a mis-identification, it's definitely one of those 2014 fish that showed up in the nets at Carter. No splake have been stocked in Carter since 2007, so it would be less likely to run across one of those anyway. And they're not that hard to tell apart.
 Reply by: Ewert      Posted: 3/22/2023 1:40:20 PM     Points: 100
Dorado -- the Goldens were a bust at Old Dillon. That's a reg change I'm planning on proposing for this year, to delete that regulation there.
When they rebuilt that lake I had different hopes about how it would turn out in terms of its productivity. I thought it would behave more like an alpine lake and it doesn't. It has always had this unusual color to it - it's got super fine sediment that stays suspended in it. For the first couple of years I thought it would go away and that it was still settling in from the construction disturbance, but that hasn't been the case. I stocked Goldens there four years: 13, 15, 17, and 18, and the only batch that was ever successful as far as I could see was the very first one. I think that the other ones got eaten by the first batch because food is more scarce than I thought it would be. When the water doesn't clear up, that can really suppress the overall productivity of a lake because you don't have good phytoplankton production, which means poor zooplankton, etc, on up through the food web. I haven't really seen much in the way of bugs there other than midges.
The other thing is our limitation on Golden Trout -- we get them from out of state and so we don't have a lot of control over the size and timing. We also don't hold any over winter. We get eggs in the spring and we stock them out at the end of that same growing season at whatever size we were able to get them to. So they were going in at 1" in October, which didn't seem to work out too well in terms of survival.
So for the past three years I've switched to stocking holdover cutthroats -- meaning, we do hold over a small number of cutthroats in our hatcheries through one winter. So they're going in at 3-4" in late May or June instead. I need to get up there and net it again this year and see how that's going but it has to be working better than all those Goldens just disappearing on us. Definitely interested in receiving any reports from folks.
 Reply by: xavierk31      Posted: 3/22/2023 4:14:04 PM     Points: 6653
Thanks for the info on Old Dillon. I plan on spending some time out there this year when the weather warms up so Iíll definitely hit you up and let you know what I find.
 Reply by: eholm      Posted: 3/23/2023 12:27:52 PM     Points: 17861
Wow a little secret for Carter and Horsetooth! Very cool! The one sampled was big too! There could be a state record in there :-)
 Reply by: not too old to fish      Posted: 3/25/2023 2:20:11 PM     Points: 9233
Wouldn't the kokanee completely disappearing in the Blue most likely be a water or pollution problem. As you stated before Dillon is a large lake and there is a lot of room for the fish to move around in and I can't imagine the entire population being wiped out unless there is a total loss of reprodution for several years. I 'm no expert in this field but since Dillon is a domestic water supply I presume it gets tested on a reular basis including the main streams coming into it. What do you think?
 Reply by: smanoppo2      Posted: 3/27/2023 9:20:54 AM     Points: 579
Fished Old Dillon last spring and caught a lot of Cutts in the 7-9 inch range. Caught a lot of smaller ones too essentially at any depth and any part of the lake. Wasnít able to catch any Goldens though so your update on them is pretty unfortunate.

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