Fish Traps and Computer Chips Track Chinook Salmon

By Arnie Brimmer, Idaho Department of Fish and Game Is it an alien space craft? Is it a new fangled hydropower generator? What is it for? These are some of the questions I hear about the rotary screw trap I operate on the Lemhi River to collect juvenile Chinook salmon on their way to the ocean. The trap looks like something straight out of a Jules Verne novel - imagine a stainless steel horizontal sugar cone without the scoop of ice cream. Holes in the "cone" allow most of the river water to pass through it. River water enters the front, wide part of the cone and pushes against an auger or "screw," which spins the trap. Water and any fish are pushed to the back of the trap, where a well holds the fish. While the trap is being operated, it is checked every day. It is a lot of work, but we do it to learn more about the life cycle and survival rates of juvenile Chinook salmon in the Lemhi River. The information we collect helps us manage this imperiled stock of fish better. Most of the juvenile salmon I catch are tagged with a PIT (Passive Integrated Transponder) tag and released alive back into the river to continue their migration to the ocean. PIT tags consist of a hollow glass capsule about the size of a grain of rice and contain a small computer chip with hundreds of feet of coiled copper wire. Each computer chip has a unique 10 digit code of letters and numbers imbedded in it - kind of like a fish's Social Security number. While this is an interesting piece of technology, by itself it really doesn't help us much. Enter the computer. With the addition of a computer, we are able to record the species, length and weight for each fish. The computer tells us whether a particular fish has been caught more than once - important information when making a population estimate. In addition, dams on the lower Snake and Columbia rivers are equipped with devices that can read the tags as tagged juvenile salmon pass. This tells us how long it takes a particular juvenile Chinook salmon to travel from the Lemhi trap to a particular dam. It also lets us know how many survived. If the survival rate is low, that tells us there could be a problem in the migration corridor and where to start looking for solutions. In addition, growth rates and adult Chinook salmon return rates can be calculated with information gathered from PIT tags. Arnie Brimmer is the regional fishery biologist in Salmon.