Late-Migration Strategy, Although Rare, May Be Key

To the Future for Chinook Salmon in California’s Central Valley

In late October, a study spotlighting the migration patterns, or life-history strategies, of the spring-run Chinook salmon in California’s Central Valley showed that juveniles that migrate later in the year may have higher survival rates during years with unfavorable living conditions, such as drought. This late-migration strategy, once thought to be a rare behavior among spring-run Chinook salmon, may in fact prove pivotal for the fish population facing an increasing warming climate.

“Chinook salmon are born in the freshwater tributaries of the Sacramento River in the Central Valley, where they will stay and grow before they migrate to the ocean,” said Flora Cordoleani, project scientist with the University of California Santa Cruz and lead author of the study. “They’ll stay in the ocean for two to three years and then come back to spawn in the river where they were originally born.”

After the adults spawn, the juveniles have been known to begin their migration to the ocean at varying points in time. Cordoleani’s study, published in Nature Climate Change, categorized these juveniles into three behavior groups, each based on how soon they began their migration after emerging. 

The first group, categorized as early migrants, begin migrating to the ocean during the late fall and winter, shortly after being born. The early migrants, often between just 30-50 millimeters in size, will then spend more time in the lower watersheds of the Sacramento River and continue growing before reaching the ocean. 

Scientists examined otoliths, the ear bones of salmon, to understand the migration timing of fish that survived drought and poor ocean conditions – Image: George Whitman and Kimberly Evans/UC Davis

The second group of the intermediate migrants begin their journey in the spring, at a size of 60-80 millimeters in length. 

But it was the third behavior group, those that didn’t begin migrating until nearly a full year later, that was the focus of the study. 

“The late-migrant or yearling size class first emerge in the winter, but will stay in the tributary over the spring and summer to grow,” Cordoleani said. “Then they will migrate in the fall, after staying for almost a year in the tributary habitat and coming out much larger, around 90-110 millimeters.” 

The late-migration strategy, up until this point, has been considered particularly rare across the four species of Chinook salmon in Central Valley: the spring-run, fall-run, late-fall run and winter run. This study shows that, although rare, this strategy actually plays a significant role in their survival.

A late-migrating spring-run Chinook salmon from one of the few Northern California creeks that remain without dams – Image: Jeremy Notch/UC Santa Cruz.

“In the juvenile tracking data [from California Department of Fish and Wildlife], the late-migrants represented about 10% of the whole migrating juveniles,” Cordoleani said. “But then if you look at the adult otoliths, we saw that late-migrants were representing 90-100% of the returning population in some years. So even though we thought that [late-migration] was a rare behavior, the study showed that this behavior was way more important than what we thought before.”

Consequently, Cordoleani equates this variety in migration strategies to what’s known as the ‘portfolio effect.’

“The portfolio effect spreads the risk so that if in one year, there are conditions that are bad for one strategy, there is another strategy that can alleviate the [population] losses,” Cordoleani said. “So those late-migrants are really like an insurance policy, where they can save the whole cohort during the drought years when the other strategies were really performing poorly.” 

That’s not to say, however, that the late migrants will always have the best survival rates in a given year.

“In some years where it’s wetter conditions, you will have an equal proportion of the three strategies that survive,” Cordoleani said. “So while late-migrants won’t always be a winner, in some conditions those late-migrants help them face the different climates they’re going to experience in their life. That way it’s good to spread the risk so that at least one of the strategies will make it and will save the population from local extinction.”

The ear bones of salmon show growth rings similar to trees that reveal their life histories. Scientists used clues from the otoliths to determine which salmon best survived difficult conditions – Image: George Whitman/UC Davis

Cordoleani has hypothesized a few reasons why late-migrants may have better survival rates in drier, warmer, and harsher conditions. 

“Our assumption is that during drought, between the warmer water temperatures and lower water levels, their survival significantly decreases because they have less of a chance to get to the ocean compared to that of a wetter or cooler year,” Cordoleani said. “But those late-migrants, because they stay in the tributary during the spring and summer, avoid this risk and bad conditions they would have experienced by migrating in the spring. This way they still have a high-elevation, coldwater habitat where they can have this refuge in the summer as they grow. Then they leave in the fall where maybe the conditions in the river are much better [for migration] because they have bypassed the dry and warm conditions of the spring. Consequently, they migrate in a time where it’s more hospitable in the watershed and downstream habitats.”

Another influencing factor to consider, however, is the larger size of the late migrants as a possible correlation to their increased survival rates in harsher conditions. 

“Because they migrate at larger sizes, they get to the ocean at larger sizes than the other strategies,” Cordoleani said. “So in the ocean, they may have better competition rates, avoid predators better, find better food sources and they could also have an advantage when they get to the ocean compared to the other strategies that get to the ocean at smaller sizes.”

As climate change continues to accelerate and generate harsher conditions for the spring-run Chinook salmon, the fish species might become increasingly more reliant on this rare late-migration strategy. 

Consequently, Cordoleani is working with NOAA Fisheries to incorporate these findings in their reintroduction and conservation efforts for the spring-run Chinook salmon moving forward. 

“When we think about conservation [of spring-run Chinook salmon], we tend to try to conserve the most common behaviors in the bulk of a population, as though they all migrate in the spring,” Cordoleani said. “But what this shows is that we can’t just think about the main strategy. We need to protect those other strategies that seem to be rare, but will have an increasingly important impact when harsher climate conditions are observed. We must consider this late-migrating strategy in conservation efforts because not only is it important for their procreation, but without this strategy, we could have lost an entire cohort of spring-run in some years.”

Scott King writes about science and the environment for the Sierra Nevada Ally. Support his work.

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