Re-awakening dormant life history variation: stable isotopes indicate anadromy in bull trout following dam removal on the Elwha River,Washington

Migratory species take advantage of multiple habitats during their life cycle to optimize growth, survival, and reproduction. However, migration also makes them vulnerable to habitat degradation and exploitation in each habitat, and loss of connection between habitats. Partially migratory species (i.e., migration is facultative rather than obligate) can persist after loss of connectivity and may then resume migration after the habitats are reconnected. We analyzed stable isotopes of carbon and nitrogen to investigate the possible use of marine habitats for foraging by bull trout, Salvelinus confluentus, in years immediately after removal of impassable hydroelectric dams on the Elwha River, Washington State, USA. Juveniles in the Elwha River estuary were similar in δ¹⁵N and δ¹³C values to those in the estuary of the free-flowing Dungeness River nearby, and the values of fish from the estuaries were higher than those of juveniles collected in the river, consistent with use of marine food sources. Adult bull trout collected in each of the rivers had values indicating extensive reliance on marine prey - similar to those of adult Pacific salmon that had spent several years at sea. Taken together, these data demonstrate that the Elwha River bull trout, almost entirely landlocked for a century, are rapidly resuming anadromy and that the marine prey contribute substantially to their trophic ecology and likely their growth. More broadly, the results reveal the importance of connectivity for migratory fishes, their ability to resume anadromy once the connection between habitats is restored, and the population resilience that partial migration provides for them.