How can isolated lakes, deprived of access to a watercourse, still teem with fish? Charles Darwin had raised a trail by noticing that mollusc larvae attached themselves to the legs of a duck, before speculating that they could survive the flight to a new body of water to colonize.
Experiments, most of them recent, have explored the process of avian zoochosty, whereby living organisms play stowaways from one place to another, on the feathers or even in the stomach of a bird.
The study conducted by PhD student Flavien Garcia and his colleagues at the Laboratory of Evolution and Biological Diversity at the University of Toulouse III, with the help of an American professor of aquatic biology, is the first to look for evidence in the field.
More precisely in a set of gravel pit 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 usually open to him.
Biologists examined 37 of them, a third of which are still closed, and inaccessible to anglers. All these lakes had a population composed mainly of common 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, to better shoe them there.
The gravel pits have ruled out any introduction of fish into their farm. As for lakes open to the public, fishermen who confessed to wild releases of fry confessed to doing so with more sporty species, such as trout perch or carp.
Fish eggs as an aperitif
Another observation ruling out human intervention is based on genetic analysis of more than 500 perch. The artificial introduction of perch should result in a greater genetic diversity of the species in lakes open to fishing... However, 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 laying of perch and a period of high abundance of ducks," notes Flavien Garcia.
Mallard ducks land on a pond in Blackwater US Nature Park, Maryland, January 25, 2023 © Jim WATSON / AFP
Mallard and Coot, a moorhen, inhabit the lakes until the end of their wintering period in February. Precisely in the breeding period of the common perch, which needs for its spawning very cold water, between 8 and 10 degrees Celsius.
Its eggs, as tiny as they are countless, extend over long gelatinous ribbons that can reach up to one meter fifty. Adhering, flush with water, to plants and pebbles, they can easily stick to the legs or feathers of ducks. Or even finish as an aperitif in their throat.
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," Garcia says. That is, "perch 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 as the one usually covered by ducks.
The only missing evidence is the ability of the perch egg to survive mallard's digestion. It would require an experiment that would be "practically and ethically complicated," Garcia says, including sacrificing the animals to examine their digestive tracts.
© 2023 AFP