Can reduced predation offset negative effects of sea louse parasites for chum salmon?

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Stephanie Peacock


There has been a lot of interest in the effect of sea louse parasites on wild salmon in the Broughton Archipelago. Counts of salmon returning to spawn have suggested that outbreaks of sea lice on farmed and wild salmon have resulted in fewer pink salmon returning to the Broughton. But what about chum salmon?

Pink and chum migrate past farms together each spring on their way to the open Pacific, and tend to have similar levels of sea lice as juveniles. When kept in captivity, infected pink and chum salmon experience similar mortality. However, a recent study by Stephanie Peacock, Brendan Connors, Martin Krkosek and others has found that the number of chum salmon returning is relatively unaffected by sea lice, unlike pink salmon. Why are chum salmon different?

The study suggests that the negative effect of sea lice on chum salmon may be offset by reduced predation. How? Well, coho salmon predators prefer to eat pink salmon over chum salmon. Pink and chum salmon infected with sea lice are slower and easier for predators to spot than uninfected prey, and tend to be easier for predators to catch.  Therefore, when pink and chum are infected with sea lice, it might be easier for coho salmon predators to focus on their preferred prey, and leave the chum alone! Good news for chum salmon, but doubly bad news for pinks.

I am currently working to test this hypothesis with a series of experiments based at Salmon Coast.  This work will help unravel how parasites from farmed salmon affect juvenile salmon ecology, and ultimately, survival.

The article appeared in the Proceedings of the Royal Society B, and can be accessed online at


Doesn’t this result for chum salmon suggest that sea lice might actually not be a problem for any juvenile salmon, and analyses of pink salmon data may have been flawed?

No, the fact we didn't find an effect for chum does not discount previous results for both pink and coho.  Those correlations were highly significant and extremely unlikely to have occurred by chance. Conflicting reports had low power to detect an effect.  All of these analyses of the effect of sea lice on salmon populations were correlational, but there have been plenty of studies in Europe looking at survival of tagged salmon with and without protection from sea lice and survival is overall much lower when fish are vulnerable to sea lice. Together, this suggests that the effect for pink and coho was real, and there is something else going on with chum. Why there is a difference in the results is an interesting questions biologically, and not one to be ignored.

So predation might be important.  Why do you need so much math to conclude that?

Rather than just make a qualitative statement about what we think might be going on, using a mathematical model allowed us to take things we already know (e.g., direct mortality rates from sea lice, predation rates) and established ecological theory (e.g., functional responses) to figure out if reduced predation could offset the negative effects of sea lice for chum and under which conditions this might occur. The model also forced us to make our assumptions explicit, highlighting areas where future research should be focused.

More questions? Contact

Stephanie Peacock <stephanie dot peacock at ualberta dot ca>

    phone: (780) 218-6797

Brendan Connors <bconnors at sfu dot ca>

Martin Krkosek <martin dot krkosek at utoronto dot ca>

    phone: (416) 978-3839 

Mark Lewis <mark dot lewis at ualberta dot ca>

    phone: (780) 492-0197