Sunday, May 1, 2016

Spring river work observations

     We spent six days during the last half of April running raft electrofishing surveys on three of our standard stations on the Colorado River.  I thought I would share some of my observations from those projects. 
    On April 19 & 21, we surveyed the Paul Gilbert - Lone Buck units of our State Wildlife Areas below our office. We put the raft on the river right below the Byers Canyon bridge and started our station at the base of the first riffle downstream of the bridge. The downstream end of the station is the bend where the Lone Buck unit of the SWA borders on private land.  So this survey covers about 7,000 linear feet of river.
     Flows were absolutely perfect for this operation on those days. The Windy Gap gauge was reading right about 260 CFS. The weather was great too - we couldn't believe that we didn't encounter a single angler on the water either of those days. Given what we found down there, if I hadn't been working those days, there would have been no better place to be fishing than that reach of river. We did sneak in a few casts after work, and the fish were all over the prince nymphs. 
     We had not done a spring survey of this section of river since 2013. It's hard to get the timing right on this reach in the spring. Some years, runoff comes up very quickly and the river becomes too high before we have a chance to get out there; other years, the Windy Gap pumps get turned on relatively early and suddenly we don't have enough water in that section to float a raft. So we were happy to catch just the right conditions this year.
     The main purpose of this survey was to see what kind of progress our HXC rainbows have made since the 2013 survey. This section of river has been known to host a lot of rainbow spawning activity historically. If rainbows ever do become dominant in the river again, this is a location where we would consider using the rainbows as a wild broodstock, collecting eggs from spawners and using them to stock other waters. 
     I also wanted to know how overall densities of both browns and rainbows >14" are looking. If you've followed the information I've been gathering on the Parshall-Sunset reach, you may be aware that we've had some years in recent history where the population estimates have flirted with slipping below Gold Medal standards. I wanted to see whether or not that was the case on the Gilbert-Lone Buck reach in the spring.
     Fortunately, all the information we got out there was good news. I have not run full population estimates yet, and these numbers that I'm reporting are not final -- they're just my quick initial counts and calculations. For brown trout over 14", we marked 145 fish on the 19th. When we came back on the 21st, we captured 124 and 40 of those were marked. That yields a population estimate of 444 browns >14" currently on the Gilbert-Lone Buck reach, or 26 fish per surface acre. The Gold Medal standard is 12 fish per acre, so we're looking good there.
     Our 2013 survey took place on May 6 & 8, so a couple weeks later, but close enough for direct comparison. Our population estimate for brown trout >14" from that survey was 178 total fish, or 11 fish per surface acre. So this year's survey showed us a big improvement in that category.
     On the rainbow side of things, more good news, but maybe not quite as good: For fish >14", we marked 27 fish. On the recap run we captured 33 fish, 8 of which were marked. That gives us a population estimate of 105 fish >14", or 6 per surface acre. That means that of the trout >14" on the Gilbert-Lone Buck reach right now, 19% of them are rainbows. In our 2013 survey, we estimated 83 rainbows on this reach >14", or 5 fish per surface acre. Because of the lower number of large browns at that time, rainbows contributed 32% of the total trout over 14". 
     So: a 149% increase in large browns from spring 2013 to spring 2016, and a more modest 27% increase in large rainbows (a difference which could easily be accounted for in the margin of error). Many of the rainbows were in pre-spawn condition. Below is a picture of one of the ripe males. You can see his belly colored up nicely in this shot. That dusky color on the belly is a classic indicator that he's in peak party mode.

     We did capture a single mountain whitefish in the run at the top of the station, immediately below the Byers Canyon bridge. This marks a new farthest-upstream point that we've found whitefish, and this fish also happened to be the largest one we've captured to date in Middle Park -- an upstream colonizer.  Next week, we'll be running a survey upstream of Hot Sulphur Springs, and it will be interesting to see if we find any there.  I would be shocked to find them above Byers Canyon this year -- however, I'm sure they'll find their way there in the near future.
     On Monday and Thursday, the 25th and 28th, we ran a survey on the State Bridge reach.  The section that we monitor there is exactly two miles long, beginning at the bridge itself. The bridge is actually the downstream boundary of my area of responsibility, but every year I get together with my Glenwood Springs counterpart, Kendall Bakich, and we run these Colorado River surveys together with our crews. Due to the size of the river there, we use two electrofishing rafts working in tandem, one on river left and one on river right. This was the third time that we had surveyed this reach, the past occasions being in 2013 and 2015.  We were anxious to get out there this year, because the two previous surveys had given us very different numbers, which are summarized in the table below.

     The above figures are for brown trout only. These two data sets were a head-scratcher, because that's more variation than we typically see over only two years, and we didn't have any specific explanation for that kind of a swing in the population. It's also hard to draw conclusions about trends when you've only got two sampling occasions. Hence, our strong interest in getting back out on this stretch this year.
     Like the other station I discussed, I've only done quick counts of fish over 14" to get an early indication of what the data will tell us. But we captured 211 browns >14" on the mark run and 194 on recap day, 46 of which were recaptures.  That gives us a population estimate 879 browns over 14", or 25 fish/acre.  This suggests that once we run estimates of all the population parameters, they will probably split the difference between the 2013 and 2015 surveys. That is a relief, because it eliminates the concern that we were seeing some kind of unexplained, rapid population decline taking place. It also suggests that this survey will again yield results that are well above the Gold Medal standard.  This is important, because we just added Gold Medal designation to the section of the Colorado River from Gore Canyon down to McCoy, based in part on the data we've collected at this station.
     As is usually the case on that part of the Colorado, we saw many browns with freshly-swallowed mottled sculpin in their throats. The feeding frenzy on stonefly nymphs is also cranking up. Blue-winged olives were coming off on both days we were out there, with a more impressive hatch showing up on the 28th. We did see fish rising to these, sporadically.  The other notable food item that I saw in a lot of throats was really small caddis larvae, with the little cone-shaped casings.
     The rainbows on this section also looked good. We marked 64 fish over 14" on Monday, and captured 52 on Thursday, 15 of which were recaptures. That gives us a population estimate on this reach of 214 rainbows >14", or 6 per surface acre.  These estimates show rainbows making up 20% of the trout over 14". 
      In the end, I am astounded at the similarity of the estimates between the two sections of river that I've discussed. They yielded almost identical values. These are very different sections of river, with different prey assemblages. The river at State Bridge is much larger, running at least five times as much water as the section on the State Wildlife Area at the time of the survey. 
     Also, the big management difference between the two sections is the fact that I've been actively stocking rainbow trout fry on the upstream reach, and no stocking has taken place on the State Bridge reach. However, one thing that we have seen with the stocking of HXC fry is that there is a lot of downstream dispersal from the point where fish are stocked. The mature rainbows at State Bridge look exactly the same as the mature HXC's up on the state wildlife areas, and I strongly suspect that even though the State Bridge reach has not been stocked, some stocked fish have managed to find their way that far downstream and are contributing some WD-resistant genetics to the mix.
     One of the few benefits to this winter reprise that we've been having the last couple weeks is that river flows have remained somewhat moderate and water clarity is still good. Early runoff is actually progressing in a really nice, steady fashion, rather than everything suddenly blowing out in the blink of an eye.  Therefore, we're going to try to squeeze in a couple more raft electrofishing surveys during these first couple weeks of May, before the focus of the field season shifts entirely to standing water until sometime in July.