A true stowaway, the perch, a freshwater fish common in Europe, colonizes lakes with ducks transporting its eggs, according to a study published in biology letters Wednesday.
How can remote lakes with no access to a watercourse still be teeming with fish? Charles Darwin had gotten a lead by noting that mollusc larvae would attach themselves to a duck’s legs before he thought they could survive the flight that would take them to a new body of water to settle.
zoochory of birds
Recent experiments have examined the process of avian zoochory, in which living organisms stow from one place to another, on a bird’s feathers or even in its stomach.
The study carried out by doctoral student Flavien Garcia and his colleagues from the Laboratory of Evolution and Biodiversity at Toulouse III University, with the help of an American professor of aquatic biology, is the first to aim to demonstrate this in practice.
More specifically, in a series of gravel lakes in the Haute-Garonne region of south-west France. Typically, these flooded quarries are corporate operated and strictly closed to the public. When their resources are exhausted, after ten or fifteen years, they are then usually open to him.
Biologists have studied 37 of them, a third of which are still sealed and inaccessible to anglers. All of these lakes had a population consisting mostly of European perch.
The study first ruled out a possible source of ‘colonization’ of these waters through anglers’ habit of stocking them with fish in order to get them more hooked.
Those responsible for the gravel pits have ruled out the use of fish in their operations. As for the lakes open to the public, fishermen admitted to releasing juvenile fish into the wild and admitted to doing so with more athletic species such as largemouth bass or carp.
Fish eggs as a starter
Another observation that rules out human intervention is based on genetic analysis of more than 500 Poles. Artificial introduction of perch is said to result in greater genetic diversity of species in fished lakes… but is essentially on par with that of gravel pits that are closed to the public.
Other “lines of evidence” support the role of birds in colonization, particularly the mallard. “There is a synchronicity between the time of perch spawning and a period of high duck abundance,” notes Flavien Garcia.
The mallard and the coot, a coot, inhabit the lakes until the end of their wintering season in February. Especially during the breeding season of the perch, which needs very cold water between 8 and 10 degrees Celsius to spawn.
Its eggs, which are as tiny as they are innumerable, stretch out in long, gelatinous ribbons that can reach 1.50 m in height. They attach to plants and rocks at water level and can easily become attached to ducks’ legs or feathers. Or even finish as an aperitif in their throats.
However, recent experiments have shown that fish eggs can survive their host’s intestinal transit…
Genetic analysis provides another clue, linking the geographic proximity of the lakes to the genetic proximity of the perch that develop there. Researchers have even identified “first-generation migrants,” Garcia says. So “perch whose genotype belongs to the population of another lake”.
In addition, half of lake settlements occur over a distance of less than 2 km. The same that ducks usually cover.
The only missing evidence is the leech’s ability to survive mallard digestion. It would require a “practically and ethically complicated” experiment, Garcia says, including sacrificing the animals to study their digestive tracts.