Fish Don’t Leave Footprints: Steelhead Tracking 101
Understanding the decline of Puget Sound steelhead is no easy task. Try explaining the unexpected death of one of your children’s cherished goldfish in a scientifically defensible way and you’ll only begin to understand this daunting challenge. LLTK and our partners investigating marine survival devote countless hours developing strategies that allow us to understand one key piece to the puzzle – where steelhead die.
To identify where steelhead die, scientists need to characterize typical steelhead migration routes. They can do this by tagging a sample of steelhead and extrapolating the data gathered to the larger population. Steelhead maintain a relatively consistent speed and trajectory as they race to the Pacific Ocean. When tracking data show that they’ve deviated from that path or speed, it means they’ve likely encountered some sort of obstacle during their migration. The obstacle could be a physical barrier, an encounter with a predator, a decline in health related to disease, and/or a variety of other things. When scientists observe these changes in behavior, we know to investigate further in order to pinpoint the problem.
Acoustic telemetry is a relatively new and precise way to understand steelhead behavior. The system utilizes a tiny, pill-sized transmitter, or “tag”, inserted into a juvenile steelhead’s belly that communicates with an array of receivers, placed on the seafloor in strategic locations along the migration path. The transmitters are about the size of a pill capsule and the receivers no larger than a Gatorade cooler. There are over 125 receivers deployed in the Puget Sound and Strait of Juan de Fuca today. This includes a tight network of receivers around the Hood Canal Bridge. High steelhead mortality is occurring at the bridge, and we are now using the receivers to triangulate exactly where the steelhead are dying.
Acoustic receivers can detect tagged fish from 200 meters away! The properties of water make acoustic systems especially effective. Molecules in a liquid are more densely packed than those of a gas. This higher molecule density means less sound energy is lost over the same distance in water relative to air. If you’ve ever swum in a lake and heard a distant motorboat while underwater, you’ve experienced this property of water. Whales take advantage of the same principle, communicating great distances using echolocation.
When receivers “hear” a tag, they catalog the precise movement of the fish. When multiple acoustic receivers hear a tag at the same point in time, researchers are able to triangulate the location of the fish based on the relative intensity of the fish’s signal at each receiver. Some types of acoustic tags contain a pressure sensor which records information about the fish’s depth in the water column.
Acoustic tags can also tell scientists about predator-prey interactions. In Puget Sound and Hood Canal, scientists attach a “backpack” containing an acoustic receiver to the backs of harbor seals. While the seals swim around the Sound, they collect data on every tagged fish they encounter. When the seals molt (shed its fur) in the fall, the backpack falls off and scientists can retrieve the data. Scientists are currently using this technique, combined with analyzing seal diets, in attempt to determine how big of an impact seals have on steelhead survival.
Now, an astute reader may be thinking, “Doesn’t tagging a fish increase its chances of mortality and bias the results?” Many studies have been done to test this question. In these experiments, fish are tagged and held in large tanks for several months or more. Throughout the experiment, scientists monitor fish health to see whether the tags have any effects on growth or survival. Thankfully, all the studies have concluded that the tags don’t adversely affect the fish.
Scientists working on the Salish Sea Marine Survival Project have also tested whether tags act as a dinner bell, calling seals to the tagged juvenile steelhead. This study is performed by pausing the “ping” a tag emits when the fish pass through the areas seals frequent, and the tags resume “pinging” before the fish reach the Pacific Ocean. We compare the fish whose tags were paused, to fish with tags that “pinged” continuously. The survival of these two groups is not different, and therefore, showing no evidence of a dinner bell effect in Puget Sound.