Sunday, March 17, 2013

Fraser River: Safeway

     Continuing with our discussion of the Fraser River, this week I'll share some information about what I call the Safeway station. I call it that because it's a 600-foot reach of river directly behind Safeway, in the town of Fraser.
     This is on the downstream end of what was called FREP, the Fraser River Enhancement Project. The town of Fraser partnered with us, TU, and other entities to construct a stream habitat improvement project. Work on this reach was finished in 2005. This is one of my favorite stream habitat projects I've seen. The main reason is that to the untrained eye, or if you're not looking for it, it's not obvious that the stream was worked on. That's the best way for habitat projects to turn out, in my opinion.
     We've got a good survey history at this site; I've electrofished it every year that I've been here, and my predecessor Bill Atkinson established the station in 2003. This has always been a fascinating reach. It represents the transitional zone where the river is changing from a (historically) brook trout-dominated montane stream above (see last week's post), to a classic brown trout stream below in the Fraser Flats. Since the completion of the habitat project, this section has held a nice mix of browns, rainbows, and brook trout. The graph below displays the biomass estimates over the years for all three species:
      You can see that in 2003, prior to the habitat project, things were in really bad shape. The total trout biomass estimate, for all species combined, was 12 pounds per acre. In 2012, that estimate was 163.
     The big spike in rainbow trout densities seen in 2008 is the result of me stocking Colorado River rainbow brood culls in the river. We did that for a few years right after the habitat project was done. The idea behind that was to get some rainbows in as "place holders" in the river in order to create a rainbow fishery and compete with the browns as they recolonized the reach. It wasn't intended to be a long-term thing. 2008 was the last time I stocked any of those. I had originally hoped that we might possibly see some reproduction out of those fish, but it didn't happen.
     I've been stocking HXC fry in this reach since 2010, at the same time as I've stocked Confluence Park. That is the cause of the rebound in rainbow trout biomass that has occurred in 2011 and 2012. These plants have performed nicely here as well, but we haven't seen the complete takeover here that we have seen at Confluence Park. 
     Below are the rainbow trout size distributions for the past three years:

     In 2010, we were pretty much starting out from scratch with the rainbows. The brood culls hadn't stuck around, and that year's HXC plant actually went in at 3.5", on September 13 (after this survey), so they don't show up here. In 2011, the 2010 plant was present in good numbers and appeared to have formed a nice little year class, and doubled their average length at 7". The 2011 fry plant shows up at 2", and that bar is somewhat higher than the graph. In 2012, we can see that the 2010 fish have put on another four inches and are now hitting 11" (2" larger than the same year class up at Confluence Park), and the 2011 plant grew nicely, to 8". It was a little bit disappointing that there weren't more fish in that 8" size group in 2012, suggesting that the 2011 plant had possibly experienced relatively high mortality. That's probably to be expected here, because we do see browns up to 18" or better at this site, so there is certainly more predation going on here than at Confluence Park.
     Now, here are the brown trout size distributions over the same time period:



          The graphs going all the way back to 2003 look very similar to the 2010 and 2011 graphs. In fact, the one concern that I have had about the habitat project is that prior to 2012, it appeared that there was very little or no successful brown trout spawning taking place - as suggested by the absence of age-0 or age-1 year classes. I thought that this was possibly a result of the habitat project representing a trade-off which sacrificed spawning and fry-rearing habitat in favor of maximizing adult holding water. However, the reach had maintained a decent adult brown trout fishery, and the theory I had to explain this was that adult brown trout migrated into this reach at a steady rate -- probably from Fraser Flats -- and stayed, because the habitat quality is high. 2012 threw me for a loop. As you might have guessed, many of those bars are cut off in order to maintain consistent axes on the three graphs. But the 3" bar actually extends up to 107, and the 7" bar extends up to 33. In other words, in 2012 this reach was just loaded with age-0 and age-1 browns, which we had never seen here before, and which seemingly appeared out of nowhere. The reason I say that is because the group at 7", which would have been born in 2011, should have showed up in large numbers in the 2011 sample at 3" or so, but they weren't there at all in 2011. 
     The big age-0 year class, at 3", is easy enough to explain. Low water years result in very high survival in baby brown trout. They don't have to fight a big runoff, and warm water results in higher growth rates and a longer growing season for them. But I haven't thought of any good reason for the appearance of the big age-1 group. That will probably remain a mystery.
     Anyway, that's the Safeway station in a nutshell. It's a fascinating place to observe the ever-shifting dynamics of these three species and their interactions.

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