In 2014 it was a hunch.
In 2015 it seemed like a long shot.
In August it was wishful thinking.
In September it seemed possible.
But today it is without a doubt confirmed. Brook trout use Loyalsock Creek as overwintering habitat.
In my post last week I hinted that this was probably the case. There were a few tags that suddenly appeared in the mainstem shortly after what I had guessed was peak spawning season. But, we couldn’t be sure. There’s always a question of whether a dropped tag floated downstream. And, even with some tags entering the mainstem and going upstream, it’s always possible that they were gobbled up by a bird.
But, there’s no question about it now. This week we’ve been doing the final round of 2016 tissue collections and finished a little early on Thursday. On a wing and prayer, with daylight dwindling, I sent the crew to an area on the mainstem Loyalsock that had a few tags and was shallow enough to wade through. I was hoping to maybe move the tags around, which would confirm that it was in a fish, and with any luck actually capture a brook trout. But, even if a trout was there, the probability of catching it was small. Backpack shockers are intended for small streams where you can push fish into habitats they can’t escape. In larger waters, the fish feel a little tingle and start running. And, walking in Loyalsock is like sliding around on greased bowling balls. But, I had to try.
So, Dan and I hoisted on the backpacks and started shocking. Chubs, madtoms, smallmouth bass. All signs of a cool water fishery and not what you want to see in trout waters. We shocked past where the receiver said the tag was, but it didn’t move. It started to seem likely that it was a drop. We fine-tuned the signal to a large rock and Dan started shocking all around it. I stood on the side anxiously awaiting the outcome and prepared for disappointment. Standing there, I see a white mass come out from under the rock and held my breath in excitement waiting for Dan to identify his catch. Sure enough, not only was it a brook trout, but it was tagged fish 38.16. Success! And, after searching for more tags, we ended up catching a much smaller, untagged brook trout. They are there, and they are fairly abundant.
That fish was originally tagged in East Branch in September. In the early weeks of fall tracking we saw it swimming in a shallow pool several times, likely preparing to spawn. And then it disappeared. Based on my experiences this summer, that usually meant that something had eaten the fish and taken the tag far away. But, in the days that followed I noticed tags go “missing” at a much faster pace than I ever saw over the summer. Looking to rule out possible downstream movements, I tracked Loyalsock one day. Sure enough, there they were.
Now that we are nearly at the end, I can confidently say over 1/3 of fish tagged in September moved into the mainstem Loyalsock (and that proportion might be much higher once I determine how many tags were dropped in the stream. And, I’m sure more are on their way. There are a few fish that have been moving downstream in the past week and are probably on their way to Loyalsock now.
Tributary to mainstem trout movements are not as uncommon as you might think. And, though only a handful of studies have documented this movement pattern, it makes a lot of ecological sense. Coming off of a stressful summer and increased activity with spawning, adult trout are literally starving. And, they know that they need to prepare for a long, cold winter ahead. In the winter fewer insects are emerging and the once bountiful streams lack significant sources of food for trout. But, mainstem rivers have many small fish that are not only plentiful, but have a higher caloric value than small insects. In fact, during sampling the last two days we found several trout with small trout tails hanging out of their mouths. Diets once made almost entirely of insects have quickly shifted to fish-dominated.
The mainstem also offers some thermal buffer relative to the smaller tributaries. As stream temperatures continue to drop, trout will enter into periods of dormancy known as torpor. Trout don’t actively choose to go into torpor; it’s a physiological response to cold water because, unlike humans and other endotherms, fish cannot maintain their body temperature. Their body temperature is the same as the water they are swimming in, and when they are cold their muscles don’t function as quickly as when they are warmer.
Torpor is likely, at least in part, an evolutionary response to a lack of food. While in torpor, fish metabolism is very low and they require very little energy to remain alive. However, while torpor may allow fish to survive cold winters, it also decreases their ability to continuing growing and reduces the amount of energy they can put towards producing offspring. But, by staying a little warmer during winter, fish occupying mainstem rivers will spend less time in torpor, more time feeding, and they can put more energy into growth and reproduction.
So, in short, trout migrating to the mainstem have more food sources, grow larger, and maybe even produce more and healthier offspring. More interesting, though, is that only a subset of the population seems to engage in the behavior. Some seem hardwired to do these movements, and others content staying in the small streams. What causes this? Great question, but I have no idea. At least not yet. I still have over a year to figure it out.