Ready, set, go! Racing salmon to learn about migration and predator avoidance

A guest blog by Megan Sabal

Imagine you are a young salmon. You hatched from a small egg last winter, and competed with your siblings to feed on insects to grow big. Now, at four months old, a variety of signals (warming water, longer days, changing hormones) are telling you it is time to leave your home in the river, and embark on an epic journey—to travel downstream to the Pacific Ocean where food and space are plentiful. The journey will be perilous as fish predators hide behind river bends waiting to gobble you up, but there is no turning back…it’s in your genes, the ocean is calling, and you must go!

How do young salmon avoid predators while on their migration journey to the ocean? Salmon are amazing animals and are important economically, recreationally, and culturally. Populations in California are in decline, and many salmon die while they migrate to the ocean, which is why it is important to learn how young salmon avoid being eaten by predators. This is the question motivating my research. To address this, I ran an experiment where I timed young salmon swimming downstream through an obstacle course with and without a predator in their way. I wondered… will salmon change their speed when faced with a predator? Slow down to be cautious? Speed up to get past quickly? Not change speed at all?

THE OBSTACLE COURSE

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The obstacle course was a 7-foot long tunnel which emptied into a net pen at the river’s downstream end. I performed surgery on young salmon to insert electronic tags into their bodies. These tags were detected by circular antennae as the salmon swam down the tunnel. Each salmon was timed swimming through the tunnel without a predator, recaptured and then timed with a predator in the way. The predator? A plastic replica of a largemouth bass.

THE CONTESTANTS: 3 types of juvenile Chinook salmon

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I raced three types of salmon contestants through my obstacle course. All three groups were different shapes and sizes, and had different life experiences growing up.

  1. Hatchery salmon were born and raised in a fish hatchery.
  2. Wild-migrating salmon were born and raised in the river, but were caught while already migrating downstream.
  3. Wild-rearing salmon were also born and raised in the river, but had not started migrating yet and were still living on the floodplain.

THE RESULTS

Wild salmon slowed down when faced with a fake predator, but hatchery salmon stayed the same speed!

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Growing up in the river, wild salmon are better educated about predators. This may be why they react more strongly to the largemouth bass in the obstacle course than hatchery salmon, which have never seen a real predator! In nature, slowing down might be beneficial if salmon hide in habitat cover, become less conspicuous, or allow time to assess the predation threat.

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Wild-migrating salmon were fastest overall!

Wild-migrating salmon were the biggest, strongest, and were ready to move! As salmon grow up, signals tell the young salmon it is time to leave their home in the river and move to the ocean. These wild-migrating salmon were already on their journey to the ocean. This could be why they were the quickest.

Why does this matter?

This experiment showed that predators may affect salmon migration speed. Since salmon need to reach the ocean when food abundance peaks in late spring, changes in migration speed, and hence arrival time, may affect salmon survival. Also, hatchery salmon may be more vulnerable to predation because they do not react to predators in the same way that wild salmon do. Wild salmon were faster overall, but slowed down when faced with a predator suggesting they are more strategic with their downstream movements. Ultimately, salmon reactions to predators during migration and the interactions between hatchery and wild salmon can affect the survival of juveniles and consequently population sizes of adults. The more we understand about these amazing fish, the better we can protect them!

Megan Sabal is a PhD student in Ecology and Evolutionary Biology at UC Santa Cruz.