The Idaho Department of Fish and Game’s wild steelhead monitoring infrastructure has been improved with the addition of a fish-detection system in Marsh Creek, a tributary of the Middle Fork Salmon River.
It allows Fish and Game biologists to monitor wild steelhead as they move around Idaho. the fish have been fitted with passive integrated transponders which can be read by antenna spans buried in the stream bottom.
Adult steelhead are especially difficult to count in their home streams, Fish and Game Biologist Eric Stark said, because they spawn in the spring when streamflows are high and water is often murky. Spawning surveys, like those conducted for adult Chinook salmon, are not possible because steelhead are not visible in the spring and weirs are often blown out with high flows.
The Marsh Creek project consists of an upstream and a downstream span of antennae, about 600 feet apart. A 6-inch trench was dug across the stream channel and the antennae embedded into the substrate before the moved material was used as backfill. The antennae are anchored into the bottom of the river with duck-bill earth anchors, connected to cables and run overland to a central controller, which is linked to a satellite modem to transmit data.
Most wild adult steelhead detected at tributary arrays are tagged as adults when they pass Lower Granite Dam, which the fish pass through on their upstream migration through the Snake River, Stark said. When an individual fish is detected at a tributary array, not only can biologists estimate how many returned to a given tributary, but they can further describe what the population looks like from all the information they have on each fish.
Idaho wild steelhead populations remain listed as threatened under the Endangered Species Act and returns have continued to decline. Monitoring of Middle Fork Salmon River populations serves as a standard to which other populations can be compared, since these populations are mostly in wilderness, he said.