Our lab has long been involved in monitoring the colonization of sub-Antarctic Kerguelen Islands by introduced salmonids: decades of data and samples, which we try to mine and perpetuate. There is a wealth of questions that can be investigated using such extraordinary framework. A very common question is « how will these fish fare in an unknown environment ? ». In more scientific terms, are they ready to survive there, or do they need to adapt, can they adapt, and how will they do it ?

Our long term monitoring indicates that all species did not fare evenly (Lecomte et al. 2013). Some of them disappeared, others persisted, and one became invasive: the nefarious brown trout (Labonne et al. 2013). The partial migration strategy of this species seems to have fitted perfectly in the sub-Antarctic environment of Kerguelen, blotted with fjords, lakes and lagoons. The very first natural generation produced sea trout – although their genitors originated from tenths of generations reared and isolated in fish farms in Europe. During the first generations, our lab maintained a tight monitoring of the dynamics in the one of the two first populations. We benefited from this work, data and samples, and explored at what speed these fish were growing at sea, depending on their age of departure from freshwater, on their sex and on their birth date (cohort effect).

Our results are somewhat surprising (Jarry et al. 2018): these fish possibly never fared better than in this far corner of the world, at least regarding their life at sea. We found growth rates among the fastest we know about, for both sexes (see figure below). We also found that their reproductive investment was rather high, and did not differ between males and females. In other terms, their fitness seems nearly stellar, and growth did not seem strongly limited by reproductive investment, or vice versa. Although we did not have yet estimated the survival rates of these sea trout, we suspect it has been extremely high during the first steps of colonization (Jarry et al. 1998). We even have found very old individuals among sea trout (Labonne et al. 2013). Of course, not everything is bright for our fish, and some stages of their early life in freshwater might be especially taxing since no other fish lived in these freshwater before these introductions, and brown trout is a known fish predator. We recently found for instance that juveniles tended to adapt their feeding behaviour to carbohydrates consumption, which may provoke negative consequences through physiological disorders (Marandel et al. 2017).

Yet they keep on colonizing, and currently try to settle in very eutrophic rivers. How do they choose their next eldorado ? Well, we thought you might want to know, so we are now deploying a monitoring protocol on the colonization front, thanks to our colleagues from OTN, University of Dalhousie , and NTNU, which will allow the acoustic tracking of sea trout, right on the spot where new virgin rivers are available. More details here.

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All this work is founded and supported by the French Polar Institute.