In a recent research paper, authors Cordoleani, Phillis, and Sturrock describe what they call a “rare” life history of spring-run Chinook salmon in Mill Creek and Deer Creek, tributaries to the Sacramento River. The authors suggest that this life history is becoming increasingly important in our warming climate.¹ For more discussion of this topic, see also:  https://fishbio.com/worth-waiting-for-the-advantages-of-late-migrating-spring-run-chinook/. The authors’ abstract for the paper provides the following summary:

ABSTRACT: Rare phenotypes and behaviours within a population are often overlooked, yet they may serve a heightened role for species imperilled by rapid warming. In threatened spring-run Chinook salmon spawning at the southern edge of the species range, we show late-migrating juveniles are critical to cohort success in years characterized by droughts and ocean heatwaves. Late migrants rely on cool river temperatures over summer, increasingly rare due to the combined effects of warming and impassable dams. Despite the dominance of late migrants, other strategies played an important role in many years. Our results suggest that further loss of phenotypic diversity will have critical impacts on population persistence in a warming climate. Predicted thermally suitable river conditions for late migrants will shrink rapidly in the future and will be largely relegated above impassable dams. Reconnecting diverse habitat mosaics to support phenotypic diversity will be integral to the long-term persistence of this species.

What the authors of this study are noting is the two dominant life history patterns of Chinook salmon: subyearling and yearling smolt production, or “ocean” type vs. “river” type Chinook. One type or the other often dominates in a particular river system, but often both types exist, providing for a diversity of life history that protects the species from extinction.

The main difference is that the subyearling or ocean type leaves for the ocean, estuaries, and coastal waters in late winter or spring, whereas the river type over-summers in rivers before emigrating to the ocean in the following fall or winter.

Technically speaking, neither “type,” “behavior,” or “strategy” is “rare” (or “overlooked”). The ocean type occurs in many river systems, especially in the Chinook salmon’s southern range, which provides conditions for rapid winter growth that allows young salmon to reach smolt size by spring – “early” outmigrants. Slow growth, more common in the colder northern range of Chinook, often requires young salmon to “over-summer” in rivers to reach smolt size to migrate to the ocean.

Spring-run Chinook have adapted to colder, higher elevation streams, especially in their southern range in California’s Central Valley. In contrast, fall-run Chinook spawn in lower elevation streams or lower portions of coastal, Central Valley, and Klamath-Trinity rivers. The spring-run tend to be more river type because of the colder water and longer journeys, whereas fall-run are faced with warmer water and shorter journeys. Fall-run also tend to rear in estuaries.

In Central Valley rivers, most of the historical populations of spring-run Chinook have been cut off from the higher elevation spawning reaches. They are forced to spawn below rim dams, and populations specific to many rivers (such as the American) have not survived. For populations that survive downstream of rim dams, the ocean type strategy pre-dominates, though in the coldest tailwaters of rim dams, some over-summering is possible, and the river type life history occurs for both spring-run and fall-run Chinook. In drought years, tailwaters may become too warm, and river type smolt production suffers.

In Central Valley rivers where no major dams occur, such as Mill Creek and Deer Creek, both life history strategies occur. The authors document that more than half of the adult spring-run sampled over a 12-year period that returned to Mill and Deer creeks had emigrated from their natal streams using a river type life history. The trend was more pronounced for drought years.

The authors emphasize the importance of cold-water habitats in higher-elevation rivers and the river type life history for spring-run Chinook as the climate warms. Yet there are additional factors that should be considered in evaluating why the spring-run populations in Mill and Deer Creek are so heavily dependent on the river type life history. These other factors are related to lack of rearing habitat in the lower reaches of these streams and to the dependence of their outmigrating juvenile salmon on flows, including in the Sacramento River.

A look at the CDFW Grand Tab for escapement of Central Valley spring-run Chinook (pp. 8-9) shows Mill Creek and Deer Creek haven’t reached 1000 adult fish returning to either stream since 2006. In contrast, returns to Butte Creek are perennially in the thousands. and in four years since 2006 topped 10,000. In large part, this is because there is abundant rearing habitat in the Butte Sink and Sutter Bypass complex for Butte Creek spring-run juveniles. Mill Creek and Deer Creek don’t have substantial low-elevation rearing habitat accessible to juvenile spring-run.

So while the river type life history appears to be a viable strategy to help save the spring-run populations in Mill Creek and Deer Creek from extinction, it has not yet shown itself to be a viable strategy for recovery comparable to the elements present on Butte Creek.

Reports by the California Department of Fish and Wildlife (formerly Fish and Game ) document the relative success of the river type life history for spring-run juveniles in Butte Creek.² However, successful returns of river type spring-run in Butte Creek (measured in single digits) are grossly overshadowed by overall escapement.

A major common trait of spring-run Chinook that survive to escapement, both among river type spring-run in Mill Creek and Deer Creek, and among ocean type and river type spring-run in Butte Creek, is that they are well positioned to emigrate to the Delta and Bay as large smolts in the December-March time period. This is the most likely time for flows in the Sacramento River that have sufficient magnitude to allow successful downstream migration and rearing in the Sacramento and the Delta. It is also necessary in all three of these tributaries to the Sacramento, because agricultural diversions ramp up substantially in mid-April, and flows on the valley floor in these streams then become much more difficult to navigate than before April.

In summary, the authors’ characterization of the “river” strategy as “rare,” especially for spring-run Chinook, is not accurate. Nonetheless, the difficulty of maintaining the river type life history strategy because of drought and global warming for Central Valley spring-run and fall-run Chinook is increasing on the valley floor. Furthermore, the “ocean” strategy for both spring-run and fall-run Chinook suffers the most from drought and global warming as the rearing and emigration windows of the lower rivers and estuary shrink.

Improved access to thermally suitable higher elevation streams, including habitat upstream of rim dams, is going to be essential under a warming climate in the future. A river type life history may play an increasing part. However, the contrasts in escapement of spring-run Chinook between Mill and Deer Creeks on the one hand, and Butte Creek on the other, show the importance of also establishing and maintaining connectivity to quality rearing habitats on or near the valley floor. Migration habitat, or sufficient flows in the lower reaches of the Sacramento River and its tributaries, is a third key element of recovering spring-run and other runs of Central Valley Chinook salmon.