Should I stay or should I go?

'Can I migrate?' is a question that we at WTT often raise on behalf of fish. In most instances, this question is associated with two physical factors. One is whether there is sufficient water in the river for fish to move through; a problem exacerbated in the southern parts of the UK by abstraction pressures. The other is whether there are any barriers or obstacles to free fish passage - and there are usually plenty! But 'Should I migrate or not?'  is an interesting one that does not get asked very often. Luckily I know someone who does ask such questions! With great pleasure, I hand over to Kim Birnie-Gauvin from the National Institute for Aquatic Resources at the Technical University of Denmark who is conducting her PhD research within the AMBER project.

Physiology & partial migration: from the free radical theory of ageing to residancy and migration in brown trout....

As many of you probably already know, brown trout (Salmo trutta) is a partially migrant species. This means that within a single population, some individuals will migrate to sea and become ‘sea trout’ while others will stay put and become ‘residents’. Many hypotheses have been formulated in an attempt to explain the evolution of such patterns, but today we still have a poor understanding of the mechanisms at play in choosing residency or migration.

Figure 1. Adult resident (A) and migrant (B) brown trout, Salmo trutta capture in a Danish stream (Images by Kim Birnie-Gauvin).

As an animal physiologist by training who has joined the field of fish ecology in recent years, my interests lay in understanding the physiological mechanisms that lead to ecological patterns as well as how fish cope in a human-dominated world. Currently, I am investigating the physiology that underlies residency and migration choices in brown trout. What makes an individual decide to stay in a river its whole life? What makes a seemingly identical individual decide to migrate to marine environments?

With growing evidence suggesting that oxidative stress processes were linked to life-history strategies, I investigated the role of these processes in brown trout partial migration. If you are not familiar with the term, oxidative stress is essentially what makes you wrinkle (also termed the free radical theory of aging) – it is a slow process which takes place when pro-oxidants (‘bad stuff’) are present in greater amounts than antioxidants (‘good stuff’), and damages your proteins, lipids and DNA. You might be wondering how wrinkling might have anything to do with migration… It’s a little more complicated than that, but it essentially has to do with how well a fish can cope with the demands of migration. Hence, you would expect that fish with higher antioxidants have a greater capacity to cope with migration, and may therefore migrate. Similarly, a fish with a lesser ability to cope with the demands of migration (i.e. lower antioxidant) may stay and assume residency.

This is exactly what my study showed.  Here’s what I did: I captured over 500 juvenile brown trout, obtained a blood sample from each of them and tagged them using PIT tags (passive integrated transponder tags). These are small tags that allow us to identify each individual with a unique identification. I then used the red blood cells to evaluate antioxidant capacity (which essentially requires a lot of time in a lab). 

Figure 2. Study design: electrofishing (A), 23mm PIT tag (B) and blood sampling (C) (Images by Kim Birnie-Gauvin).

The analysis showed the migrants had a higher antioxidant capacity, and perhaps enhance these antioxidants as part of the smoltification process in preparation for migration. We also found that within migrant individuals, fish that migrated earlier had higher antioxidants than those that migrated later – this may reflect a fish’s readiness to migrate. Do brown trout plan ahead?

Figure 3. Antioxidant capacity in resident and migrant brown trout (Salmo trutta).

So far, this has been the first evidence suggesting a link between oxidative stress processes and partial migration, not only in fish but in all animals. Though this is only the first step, my team and I are excited for what will come of this.

If you have any questions, please don’t hesitate to contact me by email at

Kim (@kbg_conserv)