How can isolated lakes, deprived of access to a watercourse, teem with fish?

FRANCE MEDIA AGENCY
Wednesday, March 22, 2023 3:04 p.m.

UPDATE Wednesday, March 22, 2023 3:04 p.m.

A true stowaway, the common perch, a freshwater fish very common in Europe, colonizes lakes with the transport of its eggs by ducks, according to a study published in Biology Letters on Wednesday.

How can isolated lakes, deprived of access to a watercourse, still teem with fish? Charles Darwin had suggested a clue when he noticed that mollusc larvae attached themselves to the legs of a duck, before assuming that they could survive the flight leading them to a new body of water to colonize.

Avian zoochory

Experiments, most of them recent, have explored the process of avian zoochory, by which living organisms play stowaways from one place to another, on the feathers or even in the stomach of a bird.


The study carried out by doctoral student Flavien Garcia and his colleagues from the Evolution and Biological Diversity Laboratory at the University of Toulouse III, with the help of an American professor of aquatic biology, is the first to seek proof of this in the field.

More precisely in a set of gravel lakes in Haute-Garonne, in the south-west of France. Typically, these flooded quarries are operated by companies and closed strictly to the public. Once their resources are exhausted, after ten or fifteen years, they are then generally opened to him.

Biologists examined 37 of them, a third of which were still closed and inaccessible to anglers. All these lakes had a population mainly composed of common perch.

The study first ruled out a possible source of “colonization” of these bodies of water through the habit of angling enthusiasts to populate them with fish, to better hook them.

Gravel pit managers have ruled out any introduction of fish into their operation. As for lakes open to the public, fishermen who confessed to wild releases of fry admitted to doing so with more sporty species, such as trout perch or carp.

Fish roe as an appetizer

Another observation excluding human intervention is based on the genetic analysis of more than 500 perch. The artificial introduction of perch should result in greater genetic diversity of the species in lakes open to fishing... However, it is approximately equal to that of gravel pits closed to the public.

Other “lines of evidence” support the role of birds in colonization, particularly the mallard duck. “There is a synchrony between the time of perch spawning and a period of high duck abundance,” notes Flavien Garcia.

The mallard and the Coot, a moorhen, populate the lakes until the end of their wintering period, in February. Precisely during the reproduction period of the common perch, which needs very cold water for its spawning, between 8 and 10 degrees Celsius.

Its eggs, as tiny as they are innumerable, extend into long gelatinous ribbons that can reach up to five feet. Adhering, at water level, to plants and stones, they can easily stick to the legs or feathers of ducks. Or even end up as an aperitif in their throats.

However, recent experiments have shown that fish eggs can survive the intestinal transit of their host...

Genetic analysis provides another clue, with a link between the geographical proximity of the lakes and the genetic proximity of the perch that live there. Researchers have even identified “first generation migrants,” explains Mr. Garcia. That is to say “perch whose genotype belongs to that of the population of another lake”.

Furthermore, half of lake colonizations take place over a distance of less than 2 km. The same one that ducks usually cover.

The only missing evidence is the ability of the perch egg to survive the digestion of the mallard. It would require a “practically and ethically complicated” experiment, says Mr. Garcia, including sacrificing the animals to examine their digestive tract.