Wednesday, June 15, 2011

Anadromous Juvenile Bull Trout in the Skagit River, 1990-2009

Juvenile bull trout              (Photo: WDFW)
Bull trout are a member of the salmonid family, which includes salmon, trout, whitefish, char, and grayling. Salmonids are particularly known for their migratory nature. Anadromous salmon are an extreme case, spending most of their lives in the ocean but returning to headwater streams to spawn. Resident trout are at the opposite end of the spectrum, spending generation after generation in one stream. Between these extremes are migratory fish that never reach salt water, including adfluvial fish which spawn in streams but live in lakes, and fluvial fish which spawn in headwater streams but live downstream in larger rivers. Bull trout in Washington exhibit all ranges of this spectrum. Many are resident to a single stream, while migratory bull trout spawn in tributary streams where juvenile fish rear for 1 to 4 years before migrating to either a lake (adfluvial form), river (fluvial form), or salt water (anadromous) to rear as subadults or to live as adults. Bull trout are further unique in that individuals within a population can be anadromous while others stay in a river their entire life. Within one population, some fish may reside in tributaries, others migrate into lakes, and still others are anadromous.

Bull trout love cold, clean, complex, and connected streams and other aquatic habitat. Habitat degradation and fragmentation, blockage of migratory corridors, poor water quality, the effects of climate change and past fisheries management practices, including the introduction of non-native species such as brown, lake and brook trout, have resulted in local extinctions and population declines of bull trout (Salvelinus confluentus). This has led to the listing of bull trout as threatened under the Endangered Species Act in 1999. Within the Puget Sound Bull Trout Management Unit, there are eight core areas with a total of 57 local populations identified. Bull trout are found in the Chilliwack, Nooksack, lower and upper Skagit, Snohomish-Skykomish, Stillaguamish, upper Cedar (Chester Morse Lake system), and Puyallup River basins. With the exception of the Chilliwack and upper Cedar River systems, these basins all support anadromous bull trout that use Puget Sound marine waters for foraging and migration.

A high priority goal for the Puget Sound Management Unit is to acquire more complete information on the current distribution and abundance of bull trout within each core area. Additional information is needed on bull trout use of and distribution in estuarine and marine waters of Puget Sound. The anadromous form is unique to this recovery region and perhaps the least understood of all the life history strategies.

The goal of a joint effort between the U.S. Fish and Wildlife Service and Washington Department of Fish and Wildlife (WDFW) is to characterize anadromous juvenile bull trout migration and associated environmental variables in the Skagit River. The Skagit River watershed contains 26 of the 57 local bull trout populations and all four life history strategies. A juvenile fish trap operated by WDFW near Mount Vernon (see map below), has collected biological information on juvenile salmonid migrants since 1990. However, the data on bull trout has never been summarized, so we provided funds to WDFW to analyze the 19 years of data they had collected. WDFW recently produced a report of their findings, which includes:

  • On average, 186 juvenile bull trout were caught per season (see graph below).
  • The fish ranged in length from 90-290 mm, averaging 125-144 mm.
  • These fish move downstream primarily at night
  • Migration occurs between April and mid-July, with peak catches in late May.
  • The trap catch can be used as an index of abundance; however, the trap catch cannot be expanded to a total abundance estimate because it under-represents the larger fish.
  • Relationships may exist between anadromous juvenile bull trout and spawner abundance, rearing temperatures, and food availability. Low spawner abundance of pink and chum salmon combined with high stream temperatures may limit early growth of anadromous bull trout.

Bull trout are a diverse species with specific habitat requirements. Successful conservation and management of this species will require an accurate understanding of which habitats are important for bull trout growth and survival, as well as migratory corridors that connect critical habitats. Further understanding the anadromous life history strategy will help us conserve this unique component of the Puget Sound Bull Trout Management Unit.

Download the full report at:
Learn more about bull trout biology, critical habitat, and conservation measures at

Skagit River downstream trap and areas of known
bull trout spawning (WDFW)

Catch of juvenile bull trout in trap (WDFW)