Tuesday, October 10, 2017

Crayfish Assault on the Dam at Oxford Mills

Orconectes rusticus x propinquus working its way toward the spillway
On October 3, 2017, I'd just returned from two months away from home, and Fred was eager to show me the assault on the dam at Oxford Mills, so I checked that there was still charge in the GoPro that my Dad had won by entering his "Bear Tree" video in a trail camera contest, and pulled on my gumboots.

Fred put fresh batteries in the big light, and we drove the 10 minutes north to the dam on Kemptville Creek in Oxford Mills - the site of our winter "Mudpuppy Night in Oxford Mills" project.

Water levels had finally dropped to wadeable, after being "spring flood" for the whole of this wet summer. For a few days he'd been collecting crayfish claws, legs, and carapaces, left by predators, and expected a repeat of last fall's mass migration of the hybrids - Orconectes rusticus x propinquus, rushing the dam against the spillway current as if their very lives depended on it.

My attempts to pair the GoPro with my smartphone when we received it from my Dad last winter were not successful, so I was filming "blind" - just pressing the button and dipping it into the water, at the end of an extended "selfie stick", and following the beam of Fred's flashlight - hoping that the little black glass-fronted cube at the end of it would capture half decent images of whatever it was pointed at, both under and above water. We were not disappointed!

climbing the wall
I didn't realize how close the GoPro would focus, so could have gotten much better images than these, which I've cropped from screen captures of the three short videos downloaded when we got home.

First we climbed down from the retaining wall on the east side of the dam, and approached the first spillway. There was one of the rascals, scaling the corner of the dry stone and concrete wall, heading for the chink you can see directly above it.

Wading across the flat limestone bedrock at the foot of the three spillways, we caught glimpses through the rushing water of several crayfish struggling to hold their hard-won positions in the torrent.

Three crayfish jockeyed for positions at the far corner of the west spillway. We watched as one attempted to round the corner, tail-first.

attempting the corner

You can see the shapes of other crayfish on the bottom, all facing into the current. The clearest of these is in the lower right.

Fred wrote this as part of a discussion about the origins of life:

Just look at the dam in Oxford Mills over the past couple of weeks, when the ancestors of the rusty x propinquus hybrid Crayfish have had 30 years in which each generation was the offspring of those that went the farthest upstream from the mouth of the Jock River, where they also entered and went up the Rideau River and thence Kemptville Creek: "At both the east and west spillways in Oxford Mills there were piles of Crayfish trying to get upstream into the current, and being swept downstream when they got too close. Many were walking around on the ground & rocks out of the water, and on the E side several were climbing up an interior angle of the dam, and one was going into the hole of a missing rock 30cm above the ground level. This was an assault like one sees in drawings of medieval sieges of castles. My confidence that the dam will keep them out of the upper creek has completely disappeared. I'm sure that with the trickling flow that there is in a low-water summer, they could have climbed the surface of the logs and been over the dam in significant numbers in one night." - and this assault goes on at the cost of hundreds eaten by Raccoons and Herons.

Here is our video of the event on 10 October: 

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