Young salmon outsmart predators on their journey to the sea

Young salmon swimming through the San Joaquin River make vital adjustments in their route and timing to improve their odds of surviving a perilous journey.

Recent research has uncovered how these young fish exploit different parts of the river at specific times of day to avoid predators and cut down on energy costs.

The study was led by Mike Gil, an assistant professor in the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder. The findings reveal the dynamics behind the adaptive behaviors of juvenile salmon. 

A decline in salmon numbers

“The salmon fishery in the San Joaquin River delta area is on the verge of collapsing,” said Gil, the paper’s first author.

“We know these juvenile salmon are getting wiped out on their migration to sea. We need to know why and how this is happening, and if there are opportunities to leverage conservation practices.” 

Many of these salmon endure their first year in a portion of the river where they were born, and then migrate hundreds of miles to reach the ocean’s richer feeding grounds. 

Later, as adults, they retrace their path upstream to spawn, perpetuating a cycle that has long supported both the fish population and local fisheries.

According to the California Department of Fish and Wildlife, the number of Chinook salmon migrating in the fall through the Sacramento and San Joaquin river systems has plummeted from 872,669 in 2002 to 79,985 in 2022, reflecting an alarming 90% decline in just twenty years.

While elevated water temperatures, poor water quality, and the presence of dams all pose significant risks, humans have also introduced non-native predators – namely striped and largemouth bass – that claim a large percentage of juvenile salmon. 

Tracking the journey of young salmon

In an effort to discern how these fish respond to their numerous threats, Gil’s team placed tracking devices in 424 juvenile Chinook salmon, along with 23 striped bass and 17 largemouth bass. 

They lined the riverbanks with detection gear to monitor the fish’s activities over two months, capturing a detailed view of predator attacks, salmon movement patterns, and how these two forces interact.

From this data, the researchers confirmed that salmon move farther at night than during daytime – a pattern that had previously been noted but not deeply explained. 

Young salmon modify their routines

The team’s tracking revealed the reason behind this nocturnal tendency. By day, the predatory bass prefer to patrol the middle of the river, which also happens to be where salmon can ride the strong current downstream and conserve the most energy. 

Unfortunately, the fish also face greater danger there when the sun is up, as hungry bass congregate in that mid-river channel to intercept them. 

Faced with that threat, young salmon have modified their routine: they cover greater distances mid-river only when it’s dark, taking advantage of lower predator activity. 

During the day, these vulnerable fish swim closer to the riverbanks – even though the weaker currents in shallow water mean they expend more than twice as much energy to make progress. 

“Intuitively, one would think these fish should just be taxiing right down the middle of the river all the time, so they can get out to the ocean and get away from all these terrifying predators as fast as possible. But that’s not what we saw,” said Gil. 

“Our study suggests that bass activities are forcing these fish to adopt a different strategy.”

Facing predators at dawn and dusk 

Another noteworthy finding was that predator attacks peaked around dawn and dusk. “These fish seem to really pick up on changes in ambient lighting,” Gil said. 

Because striped bass have relatively large eyes, they maintain a visual edge in dim conditions, while salmon, with smaller eyes, struggle to see well in twilight. 

As a result, these early-morning and late-evening hours become particularly dangerous for juvenile salmon seeking to slip by their hunters.

Insights for salmon conservation

Protecting salmon in the San Joaquin River is paramount, especially given the historic collapse in salmon populations that is unfolding. 

Understanding the fish’s nuanced responses to predator behavior can help direct conservation efforts. 

Even small adjustments in human management strategies, such as limiting light pollution around the river corridor at night, could offer salmon a stronger advantage in moving under cover of darkness. 

“We as humans are quite limited in our understanding of how animals in the wild behave. By better understanding this, we can make the most informed decisions about how to keep these species around,” Gil said.

Complex picture of coping strategies 

One avenue for improvement lies in examining whether minor changes in water flow or releases from dams can shift the timetable of salmon migration, potentially helping the fish slip through the delta under conditions that reduce bass predation. 

Resource managers and conservationists can also investigate the impact of removing or reducing invasive predators in targeted areas. Each approach, if carefully assessed, could raise survival rates enough to keep the population from collapsing further.

By unraveling the timings and tactics underlying salmon migration, this study paints a more complex picture of how an imperiled species copes with multiple stressors. 

The fish must contend not only with environmental hurdles – such as rising temperatures – but also with an altered predator landscape shaped by human intervention. 

Their strategy of traveling under the cover of night is a stark example of adaptation, but it also illuminates just how fragile the balance between predator and prey has become. 

With salmon runs hanging in the balance, measures to synchronize conservation goals with the fish’s finely tuned behaviors may be the key to reversing the ongoing decline.

The study is published in the journal Ecology Letters.

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