Bull trout and estuary use in Puget Sound

For more information, view the original reports: The role of estuaries in the ecology of adult Pacific salmon and trout in Puget Sound and The role of estuaries in the ecology of juvenile Pacific salmon and trout in Puget Sound.

Overview

Bull trout, like coastal cutthroat trout, suffer from weak quantitative information because they cannot be legally retained in marine sport fisheries, and in rivers where they can legally be retained, no catch record is required. They are not the target of commercial fisheries, though some are taken as bycatch, for example in gillnet fisheries in the Hoh River on the Washington coast (Brenkman et al. 2007). The species is illegal to retain in sport fisheries in Puget Sound’s marine waters because of its status as Threatened under the ESA. Anglers are allowed to retain bull trout in some rivers and time periods but, unlike salmon and steelhead, no catch record is required. Consequently, there are no records in marine waters or rivers from commercial or recreational fisheries.

Bull trout in the Puget Sound basin are much less widely distributed than cutthroat trout; the main anadromous populations are in the large rivers flowing westward from the Cascades such as the Nooksack, Skagit, Snohomish, and Puyallup. Their distribution is much more limited than the other species, as the many small streams in Puget Sound are too warm for them. Bull trout smolts migrate downstream primarily in May and June in the Skagit River (Zimmerman and Kinsel 2010, Quinn and Losee 2022), but downstream migrants and estuarine residents there (Goetz et al. 2021) and in the Elwha River include a range of sizes (Lincoln et al. 2018). However, unlike salmon, adults survive after spawning and so downstream migrants include fish over a wide range of sizes including adults over 50 cm long.

Adults

Like cutthroat trout, bull trout do not die after their first spawning season but may move back and forth multiple times (up to 6 or more) between rivers and Puget Sound, as revealed by analysis of otoliths from Skagit River fish (Austin et al. 2019a). To further add to their complexity, some individuals do not migrate, and some do so intermittently, as shown by studies on the Hoh River (Brenkman and Corbett 2005, Brenkman et al. 2007) on the Washington coast, and the Elwha and Dungeness rivers on the Strait of Juan de Fuca (Ogg et al. 2008, Quinn et al. 2017, Brenkman et al. 2019). Bull trout in Puget Sound rivers are partially anadromous, meaning that some migrate to marine waters and some do not (Goetz 2016, Austin et al. 2019a), as are those on the Pacific Ocean coast of Washington (Brenkman and Corbett 2005, Brenkman et al. 2007), and the Strait of Juan de Fuca (Quinn et al. 2017, Brenkman et al. 2019). 

The timing of adult return migration can be inferred from two kinds of information: decreasing catches in systematic sampling with beach seines or similar scientific gear in estuaries, and by tracking individual fish. These data indicate that most bull trout that enter Puget Sound spend only a few months there, primarily from April to July (Figure 1A), before migrating back upriver where they hold in cool water before spawning in the fall, especially September and October (Austin et al. 2019b) (Figure 2B).  The lower reaches of rivers may get too warm for them in the late summer, so they pass up feeding opportunities (e.g., in Skagit Bay) to avoid high water temperatures, then spawn in the fall, feed, and return to salt water (or not) the following spring (Goetz et al. 2021).

However, these patterns show considerable variation among systems. Monthly sampling over many years by Anne Shaffer and the Coastal Watershed Institute at the mouth of the Elwha River revealed that bull trout were never abundant but present throughout the year, with highest catches of juveniles from December through May (Lincoln et al. 2018). Thus, variation among rivers and estuaries in physical conditions and biotic factors can result in different patterns.

In marine waters, bull trout stay near their home river and close to shore. For example, Skagit River bull trout with acoustic transmitters were mostly detected within 14 km of the river mouth (Hayes et al. 2011). This study also reported that most fish were located within 400 m of shore, in water shallower than 4 m, and tended not to cross open water. Their habitat use patterns thus resemble those of cutthroat trout in Puget Sound. Some bull trout spend at least part of the winter in marine waters, some do not spawn every year (Austin et al. 2019a, Goetz et al. 2021), and some move through marine waters and enter a non-natal river outside of the spawning season (Small and Martinez 2013). Thus, there is a lot more to be learned about this species, but anadromous Puget Sound bull trout depend strongly on estuary conditions. Because the anadromous and non-anadromous fish are part of the same populations, we can infer that they depend heavily on estuaries.

Juveniles

Bull trout smolts migrate downstream primarily in May and June in the Skagit River (Zimmerman and Kinsel 2010, Quinn and Losee 2022), but downstream migrants and estuarine residents there (Goetz et al. 2021) and in the Elwha River include a range of sizes (Lincoln et al. 2018). However, unlike salmon, adults survive after spawning and so downstream migrants include fish over a wide range of sizes including adults over 50 cm long.

Like cutthroat trout, bull trout feed in estuaries as juveniles and as adults, but bull trout get much larger, commonly over 50 cm long whereas coastal cutthroat trout of that size are rare (Losee et al. 2024). In Puget Sound, bull trout eat almost exclusively fish, and especially surf smelt, though Pacific herring, shiner perch, juvenile salmonids, and sand lance are also important. Goetz et al. (2004) gathered numerous data sets on Puget Sound bull trout ecology and concluded that “Together, more than 99% of the total prey biomass (pooled data for all locations) in bull trout stomach samples was comprised of fish species.” [p. 110]. This conclusion is consistent with analysis of tissue samples of bull trout from the Elwha River; stable isotopes indicated a diet similar to that of Chinook salmon (Quinn et al. 2017). Because they seem to remain in the nearshore area rather than migrate to the ocean or to offshore waters of Puget Sound, we can think of bull trout as highly dependent on the estuarine environment, or at least the component of their populations that migrates to salt water.

References

Austin, C. S., M. H. Bond, J. M. Smith, E. D. Lowery, and T. P. Quinn. 2019a. Otolith microchemistry reveals partial migration and life history variation in a facultatively anadromous, iteroparous salmonid, bull trout (Salvelinus confluentus). Environmental Biology of Fishes 102:95-104.

Austin, C. S., T. E. Essington, and T. P. Quinn. 2019b. Spawning and emergence phenology of bull trout Salvelinus confluentus under differing thermal regimes. Journal of Fish Biology 94:191-195.

Brenkman, S. J. and S. C. Corbett. 2005. Extent of anadromy in bull trout and implications for conservation of a threatened species. North American Journal of Fisheries Management 25:1073-1081.

Brenkman, S. J., S. C. Corbett, and E. C. Volk. 2007. Use of otolith chemistry and radiotelemetry to determine age-specific migratory patterns of anadromous bull trout in the Hoh River, Washington. Transactions of the American Fisheries Society 136:1-11.

Brenkman, S. J., R. J. Peters, R. A. Tabor, J. J. Geffre, and K. T. Sutton. 2019. Rapid recolonization and life history responses of Bull Trout following dam removal in Washington's Elwha River. North American Journal of Fisheries Management 39:560-573.

Goetz, F. A. 2016. Migration and residence patterns of salmonids in Puget Sound, Washington. University of Washington, Seattle.

Goetz, F. A., E. Beamer, E. J. Connor, E. Jeanes, C. Kinsel, J. W. Chamberlin, C. Morello, and T. P. Quinn. 2021. The timing of anadromous bull trout migrations in estuarine and marine waters of Puget Sound, Washington. Environmental Biology of Fishes 104:1073-1088.

Goetz, F. A., E. Jeanes, and E. Beamer. 2004. Bull trout in the nearshore. US Army Corps of Engineers, Seattle.

Hayes, M. C., S. P. Rubin, R. R. Reisenbichler, F. A. Goetz, E. Jeanes, and A. McBride. 2011. Marine habitat use by anadromous bull trout from the Skagit River, Washington. Marine and Coastal Fisheries 3:394-410.

Lincoln, A. E., J. A. Shaffer, and T. P. Quinn. 2018. Opportunistic use of estuarine habitat by juvenile bull trout, Salvelinus confluentus, from the Elwha River before, during, and after dam removal. Environmental Biology of Fishes 101:1559-1569.

Losee, J. P., D. Palm, A. Claiborne, G. Madel, L. Persson, T. P. Quinn, T. Brodin, and G. Hellström. 2024. Anadromous trout from opposite sides of the globe: Biology, ocean ecology, and management of anadromous brown and cutthroat trout. Reviews in Fish Biology and Fisheries 34:461-490.

Ogg, L., S. Spalding, and M. McHenry. 2008. Dungeness River basin bull trout study 2003--2006. United States Department of Agriculture - Forest Service, Olympia, WA.

Quinn, T. P., M. H. Bond, S. J. Brenkman, R. Paradis, and R. J. Peters. 2017. Re-awakening dormant life history variation: stable isotopes indicate anadromy in bull trout following dam removal on the Elwha River, Washington. Environmental Biology of Fishes 100:1659-1671.

Quinn, T. P. and J. P. Losee. 2022. Diverse and changing use of the Salish Sea by Pacific salmon, trout, and char. Canadian Journal of Fisheries and Aquatic Sciences 79:1003-1021.

Small, M. P. and E. Martinez. 2013. Genetic analysis of native char collected at the screw trap in the Nisqually River. Washington Department of Fish and Wildlife, Olympia, WA.

Zimmerman, M. S. and C. Kinsel. 2010. Migration of anadromous juvenile bull trout in the Skagit River, 1990 - 2009. Washington Department of Fish and Wildlife, Olympia, WA.