Hatcheries may be Confusing Fish's Sense of Direction

The metal parts used to construct fish hatcheries, such as iron pipes and steel rebar, affects the navigation ability of young steelhead trout needed for migration, according to new research from Oregon State University.

These fish have an important magnetic field surrounding them that serves as their "map sense," and these metals distort their ability to gain their directional bearings during migration.

"The better fish navigate, the higher their survival rate," lead researcher David Putman said in a news release. "When their magnetic field is altered, the fish get confused."

Putman and his team conducted their research at the Oregon Hatchery Research Center in the Alsea River basin last year, discovering a correlation between the oceanic migration patterns of salmon and drift of the Earth's magnetic field.

Subtle differences in the magnetic environment within hatcheries could help explain why some hatchery fish do better than others when they are released into the wild, Putman said. By using alternative methods for hatchery construction and stabilizing these magnetic fields, these fish may be able to get back on track.

"It's not a hopeless problem," he said. "You can fix these kinds of things. Retrofitting hatcheries with non-magnetic materials might be worth doing if it leads to making better fish."

In the new study, researchers separated a population of 400 steelhead trout into two cylindrical tanks about three feet wide and tall. One was a tank typical of those found in hatcheries - fiberglass sitting on a rebar-reinforced concrete slab with water-bearing iron pipes. The other was also fiberglass, but instead had an aluminum water pipe and was located at least 15 meters from any iron or steel.

After four months in confinement, researchers tested the fish's navigation skills using buckets equipped with magnetic coils that simulated the magnetic intensity at either the northern or southern most extent of steelhead trout's natural oceanic range.

As expected, the fish in the steel and iron hatcheries got confused, and faced the opposite direction than what the magnetic field indicated.

The research was funded by Oregon Sea Grant and the Oregon Department of Fish and Wildlife. The findings were published in the journal Biology Letters.