The resilience of Atlantic salmon populations is lessened by Climate Change.

Density-dependence is a fundamental principle in ecology: it states that the growth, the survival, the fitness of individuals is directly related to local density. This is so because trophic resources are limited, a point stated by Malthus in 1798 that inspired Darwin’s theory of natural selection. Malthus had indeed predicted that demographic parameters should change with density. One interesting consequence of density dependence is that it tends to promote homeostatic dynamics: when density is low, survival is increased so to reach quickly an equilibrium point; once reached, the population size will not increase greatly simply because survival decreases due to high density. In a nutshell, this is the concept of population “resilience”.

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Figure 1: juveniles of Atlantic salmon (Salmo salar).

Fishes, and especially salmonids, are no exception to this natural law. When resources per capita change, then individual fitness changes accordingly. Of course, if resources, or access to resources, are controlled by environmental variation, then environmental variation controls density dependent mechanisms in salmonid populations. There is a wealth of papers describing this density dependence in natural or experimental environments.

Ranking among one of the most potent environmental change, rainfall variation shapes many aspects of salmon life history. It controls for trophic resources by affecting the availability of preys, but it also determines local density for salmon themselves, by changing water discharge in rivers. Climate change reshuffling our knowledge of rainfall patterns, it becomes paramount to investigate how this parameter can affect the resilience of salmon populations.

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Figure 2: A view of the semi-natural channel before the experiment, and its setup for our experimental design:  High Flow (HF) and Low Flow (LF) conditions, at either High Density (HD) or Low Density (LD).

Our lab set up an experiment in a semi-natural channel, where we introduced wild Atlantic salmon juveniles from known parents. In this channel, we created several replicates for a simple design combining two density levels (2.5 and 5 fish.m²) and two water discharge levels (Low Flow =70 m3.h-1 and High Flow = 110 m3.h-1, see Figure 2). 4 replicates were created for each condition, totalizing 960 juveniles originating from 7 families. We monitored individual growth and survival in each experimental condition especially during the first summer. The data indicate that at High Flow, survival and growth are strongly controlled by density: this was the expected mechanism at work, which fosters population resilience. But at Low Flow, this density dependent effect nearly disappeared, on both survival and growth. Environmental change, through river flow dynamics in summer, would impact negatively one of the fundamental mechanisms that govern the persistence and stability of salmon populations.

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Figure 2: Growth and survival, in High Flow (HF) and Low Flow conditions, at either High Density (HD) or Low Density (LD). For both growth and survival, the differences due to density contrast are greatly reduced when flow is low.

Although this pattern itself is already interesting, because it teaches us that the dynamics of our resources may be less resilient than it used to be, it also shadows a number of possible explanations that are probably not mutually exclusive. You can discover more about this experiment: family effects, standard metabolism, and expression of nutritional metabolism related genes, it is all here.

References:

Bardonnet A., Lepais O., 2015. Interactions and effects of density, environment and parental origin on Y-O-Y Atlantic salmon survival, growth and early maturation. IV International symposium on « Advances in the population ecology of stream salmonids”, May 25-29, Girona, Spain.

Bardonnet A., Lepais O., Bolliet V., Panserat S., Salvado J.-C., Prévost, E., 2017. Impact of low flow on young-of-year Atlantic salmon: density-dependent and density-independent factors interact to decrease population resilience. 50th Anniversary Symposium of the Fisheries Society of the British Isles, 3-7 July, Exeter, UK.

 

When brown trout invade far places of the world.

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 ?

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The archipelago is located on the circumpolar current.

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).

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A view of a Kerguelen hydrosystem.
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Most of travelling in Kerguelen involves trekking.

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).

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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.