Reading Rivers in Fish Fins: Tracing Bull Trout Movements at Mount Rainier

Bull trout in Klickitat Creek, Mount Rainier National Park. Image from the National Park Service’s page.

Bull trout, a federally threatened species, are native to several drainages in Mount Rainier National Park, including the Puyallup River basin and many of its tributaries. How they live and move there is variable and largely unknown. Little is known about their specific movements or habitat use. The best-known patterns are migratory forms, either river-bound (fluvial) or sea-running (anadromous), and presumably resident forms that stay in small headwater drainages. A project involving Mount Rainier National Park, the University of Washington and the U.S. Fish and Wildlife Service tries to read those movements from chemistry.

The idea

The chemical makeup of river water differs with the bedrock beneath it. Strontium isotope ratios (87Sr/86Sr) and trace elements such as strontium-to-calcium ratios in a fish’s bony structures often reflect the water it lived in, and have been used to follow fish movements and life histories. That is usually done on otoliths, small ear-stones, which means killing the fish, a poor choice for a listed species. The team instead takes sections of fin ray, a nonlethal method, to read an individual fish’s chemical record over its life.

First the water had to be tested. Samples show whether values differ enough between areas to tell habitats apart. If the ratios and trace-element concentrations are distinct geographically, fin-ray analyses can then be used to infer migratory patterns at a suitable scale.

Objectives

1. Measure 87Sr/86Sr ratios and concentrations of Sr, Mg, Zn, Mn and Ba in water samples from the Puyallup basin.
2. Decide whether those tracers can show bull trout habitat use and migratory patterns across the basin at a scale useful for management.
3. If so, use them with fin-ray microchemistry to infer habitat use and migration of bull trout.

A better picture of freshwater migration could let managers make better-informed habitat protection and restoration decisions. Connectivity matters especially for bull trout because they migrate far and use many habitats.

Methods

River water was collected at twenty locations, inside and outside the park, in early August. Each site gave duplicate, triplicate or quintuplicate samples through syringe filtration. The University of Utah’s Inductively Coupled Plasma Mass Spectrometry (ICPMS) Laboratory measured 87Sr/86Sr with a multi-collector instrument and the element concentrations with a single-collector one.

Fin rays were collected at the U.S. Army Corps of Engineers’ fish passage facility on the White River, downstream of the park boundary. Fish were dip-netted from the facility’s trap into a tank and anesthetized with MS-222. The ventral part of the second leading pectoral fin ray was cut out with a scalpel. Then each fish was moved by dip net to a tanker truck and returned to the White River upstream of Mud Mountain Dam.

Early water results

Preliminary results show that 87Sr/86Sr and the concentrations of Sr, Mg, Zn, Mn and Ba vary among sampling locations across the basin. The 87Sr/86Sr ratios differ significantly between the east (White River watershed) and the west (Carbon River watershed). In general, ratios are lower in the White River and upper Carbon River watersheds and higher in the Puyallup and lower Carbon River watersheds.

Scatterplot of average Mg/Ca (mmol/mol) and 87Sr/86Sr ratios for individual sampling locations, grouped by watershed. Image from the National Park Service’s page.

Mg/Ca ratios vary significantly within and among the three main sub-watersheds, and together with 87Sr/86Sr they should help tell streams in the White River watershed apart. The fin-ray data were still being processed, and based on the water results the team expects fin rays to reconstruct bull trout’s freshwater migration patterns at the sub-watershed scale.

Sources

• National Park Service, Mount Rainier National Park, Inferring Movements of Bull Trout Using Geochemical Signatures in Mount Rainier National Park; rewritten in hubnx's own words. The project lists contacts at Mount Rainier National Park, the University of Washington and the U.S. Fish and Wildlife Service. The page spells the anesthetic “tricane methanesulfonate”; MS-222 is used here.