Predation Deterrents

The dams have installed a number of predation deterrents to increase the percentage of salmon smolt that successfully migrate out each season. The spillways have been renovated to reduce pressure changes and physical impacts undergone when the salmon pass over. Walls have been constructed to ensure that the smolt going over the spillway are directed into a deep part of the river to make them harder for gulls to pick off. The two most visible (and probably most expensive) deterrents are the avian line arrays and hazers.

The avian line arrays are arrays of very thin cables that stretch from one side of the river to the other over the areas of highest predation. This sounds very simple, but it becomes complicated when stretching the cables over a mile wide river in an area where a pretty nice day can involve sustained winds upwards of 25 MPH.

The above picture is from the navlock peninsula at John Day. The large tower on the left is one of the avian line anchors. They have to be this tall because of how much slack the cables need to have in order to span the river and put up with the high winds. They actually just redid the avian lines at John Day. They had used very thin braided steel cable before, but they found that it broke far too often. They switched over to some sort of nylon I believe.

Since you can't see them, I suppose I should explain what they look like a bit better. When I first heard about them I expected an array similar to a large net. There are nowhere near that many lines. And they only run straight across the river, no crisscrossing. They are also spaced pretty far apart. Maybe 15 feet between lines at the center of the river.

Despite seeming pretty far apart to keep birds out, these lines appear to be pretty effective. The birds can definitely slip between them, and sometimes they do, but far more rarely than they forage outside of the lines. I'm not sure if it's because they can't see the lines very well or if it's the noise of the wind going over all the lines (which is pretty loud), but something keeps most of the birds out of the areas with avian lines. Which is where the hazers come in.

The hazers seem to have a pretty sweet job. They're USDA Wildlife employees, and what they do all day is cruise around the river in the little boat pictured above and shoot fireworks at the gulls. There are a few varieties of fireworks. Their favorite is very similar to a bottle rocket, it spirals a long distance and whistles loudly (I think they call these screamer rounds). And then there are a couple that shoot up a ways and detonate with a bright flash of light and a loud bang. One of them shoots off four bright white sparks very similar to a traditional firework. All of them are fired from some sort of pistol. I try to spy on them from time to time, but they typically only shoot during high activity, when my eyes need to be elsewhere.

Like I said, the avian arrays keep most of the birds away from them. This means that all the birds congregate right at the end of the arrays to forage. The hazers work pretty well at dispersing them. There are always a couple gulls that stick around to forage, but that is pretty insignificant. Without the hazers present I once counted over 80 gulls in a zone. So, reducing that to a couple of actively foraging gulls makes a big difference.