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

Local extinctions and population declines of bull trout (Salvelinus confluentus) led the United States Fish and Wildlife Service to list this species as threatened under the Endangered Species Act. Coterminous listing of all five Distinct Population Segments (DPS) in the continental United States was announced in 1999. Due to their diverse life histories and iteroparous spawning, the health of bull trout populations is particularly difficult to assess. Bull trout in the Coastal-Puget Sound DPS use four different life history strategies: resident, fluvial, adfluvial, and anadromous. The anadromous form is unique to this recovery region and perhaps the least understood of all the life history strategies. The goal of this report is to characterize anadromous juvenile bull trout migrations and associated environmental variables in the Skagit River. The Skagit River watershed contains 26 of the 57 local bull trout populations in the Puget Sound Management Unit and all four life history strategies. A juvenile fish trap, operated near Mount Vernon, has collected biological information on juvenile salmonid migrants since 1990. This report (1) summarizes bull trout life history from Skagit juvenile trap collections, (2) evaluates whether juvenile trap catches are a valid index of abundance, (3) determines whether catch expansions to a total abundance estimate are warranted, and (4) identifies the contributions of spawner abundance, rearing temperatures, and food availability to anadromous juvenile bull trout.

Catches of juvenile bull trout were assumed to be anadromous based on the unimodal pattern of the outmigration and the corresponding seasonality of catches in an estuary monitoring program conducted by the Skagit River Systems Cooperative (SRSC). Catch of juvenile bull trout in the Skagit juvenile trap has averaged 186 fish per season with no apparent trend over time. Downstream movements occurred primarily at night. Migration occurred between April and mid-July with peak catches in late May. Catch was determined to be a valid index of total abundance; however, we concluded that expansion of catch to a total abundance estimate was not defensible because existing trap data violated two assumptions necessary for this estimation.

Lengths of anadromous juvenile bull trout have ranged from 90 to 290-mm fork length (FL). Average annual lengths ranged from 124.8 to 143.7-mm FL. Catches in the downstream trap underrepresented catches of larger bull trout when compared with juvenile size distributions compiled from estuary collections and from scale back-calculations of anadromous spawners. Non-detection of larger bull trout appeared to miss a substantial (31%) portion of the population in only one of the five years that size distributions were compared.

Correlations between environmental variables and anadromous juvenile bull trout catches were based on the assumption that the majority of migrants were age-2 fish. This assumption was supported by existing bull trout age and length data from the Skagit River and elsewhere but remains to be directly validated. Lengths of anadromous juvenile bull trout were predicted by a combination of rearing temperature and spawner carcasses. Results indicated that chronically low escapements of pink and chum salmon combined with high stream temperatures may limit early growth of anadromous bull trout. Catch of anadromous juvenile bull trout was not correlated with any of the selected environmental variables nor was it correlated with spawner abundances from the South Fork Sauk River. Variables that influence anadromous bull trout production and survival will only be detectable as downstream catch if each life history strategy is a constant proportion from year to year. However, the mechanisms that influence the anadromous component of a given brood year are unknown.

Initial comparison of juvenile anadromy in the Skagit River with a previous study on the Hoh River indicates that both age structure and migration timing differs between these river systems. Further work in the Skagit River is needed to resolve the relative contributions of source populations to the anadromous life history form and validate assumptions regarding the age structure of juvenile migrants.