How extractive fishing pressure can influence natural selection and the evolution of our salmonid populations.
A few months ago, while having a few beers with a friend who had just returned from a trip to the Galápagos Islands, and as I attentively looked through the photographic report he had brought back from that earthly paradise in the Pacific archipelago, I was struck by how the species that live there are completely unafraid of human presence. To such an extent that one can take a nap just a few centimeters from a sea lion, or bring the lens of a camera within a hand’s breadth of a pelican’s prominent beak.
The explanation is quite simple, and more than 150 years ago Charles Darwin, after his visit to these magical islands, articulated in his book On the Origin of Species the theory that would make him one of the fathers of modern biology: natural selection as a means to understand the evolution of species.
Indeed, the absence of a selective factor such as predation can trigger not only certain phenotypic characteristics, but also a series of behaviors that would, without question, be completely impossible in the presence of predators. And let us not deceive ourselves: human beings have been, and continue to be, one of the most influential predators or mechanisms of selection through environmental alteration that exist.
Let us leave the Galápagos behind and return to our own latitudes to analyze a few facts through a Darwinian lens. Of course, this is merely an exercise in abstract reflection; it does not aim to lay down dogma, nor is it based on objective data. It simply seeks to draw some conclusions from the subjectivity granted by observation and reflection. If along the way we manage to analyze certain facts from a different perspective, the exercise will undoubtedly have been enriching.
Let the following examples serve as the backbone upon which to structure this hypothesis:
The Evolution of Atlantic Salmon
That the situation of Atlantic salmon in our waters is—being very generous—worrying, escapes no one. In recent decades, populations have suffered a severe decline across all our river basins, but there are not only quantitative values to be analyzed.
Among the reasons for this brutal regression we can list: habitat degradation, disease, overfishing both in marine feeding grounds and in rivers, among others. And we are probably leaving many out. The purpose of this article is not to dissect the causes of the current situation.
This scenario has given rise, in some of our most weakened river basins, to various phenomena that clearly exemplify the health status of Atlantic salmon.
For instance, in many of our rivers it is increasingly common for the bulk of the population to enter in autumn, while the populations that used to run in spring are now far fewer than the autumn fish. As for winter salmon, they are largely extinct in most of our basins—if not all of them.
What is the reason behind these events? Of course, there may be other variables, such as the deterioration of the hydrological regime in some basins, with pronounced low-water periods that encourage fish to spend as little time as possible in an increasingly hostile environment. But… why not consider other possibilities? Why not attribute a shaping role in this population trajectory to humans and their extractive fishing pressure?
If we analyze the facts, we find ourselves facing dozens—perhaps hundreds—of generations of fish that have directly endured the traditional fishing season, which extends or once extended from March to July. It is therefore not trivial that, if we attribute a genetic imprint to the migratory behavior of fish, we might consider that the population decline resulting from extractive fishing pressure has led to a reduced stock of spawners capable of transmitting that “spring-run” genetic load. Consequently, over the years and reproductive cycles, this would result in a decrease in winter and spring fish returns.
Viewed differently, we have positively selected autumn-run populations—on which we exerted little to no pressure—producing a predominance of these fish over the populations that we did fish.
Another observable fact, especially in the most degraded populations, is the reduction in the average size of returning fish. This could be attributed to deterioration of the marine environment and reduced food availability, but this hypothesis may be challenged when we observe that in other, healthier populations, fish size remains unchanged.
Of course, we must understand that variations in size and weight exist among fish from the same year class, and these do not necessarily correlate with greater or lesser food intake. As with humans, some fish are simply more efficient at extracting value from their diet than others.
Therefore, it is plausible that humans, in their extractive zeal, have focused their efforts on larger specimens, while simultaneously protecting smaller individuals through minimum size limits. This inevitably leads to a greater proportional weight of these smaller fish in spawning, and in the long term, to a reduction in the average size of the population.
Behavior of Trout Populations
Who has not observed how, over the years, a lure that was once infallible begins to lose effectiveness until it becomes just another option?
We all know that fish learn. We have all experienced it—sometimes painfully—firsthand. That much is clear.
However… what if there were something more? Let us start from the premise that certain fish have a preference for a particular type of lure. Let us even assume that within the same population there are fish governed by different behavioral patterns. Suppose those behaviors become a “target” for a specific lure. For example, highly aggressive trout may be particularly vulnerable to streamers, spinners, and the like.
Why not consider that, throughout our history as predators, we have selected for certain behaviors? Behaviors that, naturally, are more resistant to our fishing techniques.
Conclusions
Is extractive fishing pressure truly a selective factor capable of influencing the evolution of salmonid populations and their behavior?
It is undoubtedly difficult to prove, but we must bear in mind that, without question, everything carries a genetic imprint that goes beyond a fish simply being a salmon or a trout; that there are genetic factors defining a population far beyond these purely physical traits.
That the predator–prey dynamic has played—and continues to play—a crucial role in natural selection and in shaping populations, both prey and predators alike, favoring those characteristics that make individuals more fit, allowing them to reproduce and perpetuate their genetic load, in which such traits are reflected.
That the smaller the reproductive stock, the greater the relative weight each individual has on population dynamics, and therefore the greater the inevitable impact of our actions.
That our pressure affects populations in ways beyond the merely quantitative; that we can shape a population in the long term in ways that, from our typically short-term perspective, we may not even be able to imagine.
In short, that every action has its consequence.
Text: Álvaro G. Santillán
Photos: Author and Aitor Coterón