A recent article published by Ádám Lovas-Kiss, András Lukács and members of the DRI Wetland Ecology Research Group in the leading interdisciplinary journal PNAS points out how fish can appear, for example in a freshly dug garden pond isolated from other waters, without human intervention. The Hungarian ecologists have indicated the roe of fish eaten by waterfowl may be able to survive the path through the alimentary canal and hatch in water far from their origin.

Everyone has heard of the phenomenon of fish appearing in temporary waters, seemingly isolated from other waters (e.g. garden ponds, freshly dug artificial lakes, inland waters, etc.) within a short time, even without human intervention. It is therefore clear that the spread of fish is aided by some hitherto unknown process. The most likely natural form of this is dissemination by other animal species, namely species that move frequently between different water bodies and are able to travel long distances in a short period of time. It is understandable, therefore, that waterfowl were the “prime suspects” in the distribution of fish between water bodies. Previous research has considered the transfer of the sticky roe of fish attached to the feet of birds as the most likely means for carrying fish between water bodies. Although several studies have looked at mud and other substances stuck to the feet of birds, fish eggs have been found only on one or two occasions. At the same time, research over the past decade has demonstrated that birds do not carry organisms only on their feathers and feet. Recent studies have shown that resistant reproductive bodies of animals and plants (e.g. seeds, eggs, persistent cells) are likely to be discharged after being ingested by birds, often in very large quantities and several hours later as viable organisms, possibly at greater distances from the site they were eaten.

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The question therefore arises as to whether the soft, gelatinous eggs of the fish are able to survive the path through the birds’ alimentary canal and spread between water bodies in this way.

Waterfowl like to consume the roe of fish in huge quantities when available. A group from the Centre for Ecological Research investigated this issue: fertilized roe of carp and silver carp were fed to captive ducklings in the experiment. Carp and silver carp are considered invasive species that are growing rapidly around the world, and their natural spread is a frequently observed phenomenon. The researchers conducting the experiment fed each duck 500 silver carp and 500 carp eggs. Viable eggs were found in the faeces of six of the eight birds, but, overall, only 0.2% of the eggs consumed were able to pass through the ducks intact. Of the 18 eggs transferred, 12 contained viable embryos. One carp and two silver carp eggs were hatched from these. Eggs that were transferred successfully but did not hatch were killed by a fungal infection, which the researchers attributed to suboptimal hatching conditions. Because of the invasive ability of fish, it is important to note that only a single silver carp can form a new population (in the presence of other carp species) due to their ability to reproduce asexually (spontaneous gynogenesis). The results of the study show that dispersal by birds within the gastrointestinal tract (endozoochory) may play an important role in the spread of fish over greater distances.