June 8, 2023
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A real 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 Wednesday.

How can isolated lakes with no access to a watercourse still be teeming with fish? Charles Darwin had raised a trail by noting 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

Most recent experiments have explored the process of avian zoochory, by which living organisms stowaway from one place to another, on the feathers or even in the stomach of a bird.

The study conducted 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 have been exhausted, after ten or fifteen years, they are then generally open to him.

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

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

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

Fish eggs as an appetizer

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

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

The mallard and the coot, a water hen, inhabit 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 in 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 finish 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 perches that evolve there. Researchers have even identified “first-generation migrants,” says Garcia. That is to say “perches whose genotype belongs to that of the population of another lake”.

In addition, half of lake colonizations occur 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 mallard digestion. It would require a “practically and ethically complicated” experiment, says Garcia, including sacrificing the animals to examine their digestive tracts.

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