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”.
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.
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.
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.
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.