Flora and fauna use a variety of means to spread to new habitats, such as seeds being carried aloft on the breeze or hitching a ride on an animal’s fur. But scientists have long puzzled how fish, such as carp, manage to colonize isolated water bodies that are often hundreds of kilometres apart. One suspected possibility was that fish eggs could be transported from one waterway to another inside birds’ stomachs, a process known as endozoochory, emerging as viable eggs in the birds’ feces.
According to a recent report in the Proceedings of the National Academy of Sciences, lab testing has proven that fish eggs can survive the passage through a duck’s digestive system.
For the experiment, mallards were fed eggs from common carp and Prussian carp. “We chose carp eggs because they are extremely widespread and they are a causing a lot of trouble due to their invasiveness. We used mallards because they are present in very large numbers in Eurasia and America,” says Dr. Orsolya Vincze, one of the co-authors of the paper.
Between one and six hours after eating the birds excreted their meal. Their feces were collected and inspected for viable eggs. The researchers found 18, which were then incubated. Of those eggs, three (one common carp and two Prussian carp) actually hatched.
While only 0.2 percent of common carp eggs and 0.25 percent of Prussian carp eggs survived the journey from beak to butt, when you consider the sheer number of fish eggs that are consumed by ducks and other waterfowl, it’s entirely possible that this is a viable method of dispersal in the wild. (That doesn’t let us off the hook, though—cottagers and users of waterways need to prevent the spread of invasive species, for instance with proper use and disposal of live bait and ballast.)
The common carp is an introduced European species that’s widespread throughout the Great Lakes watershed. As noted in the report, a single carp can lay up to 1.5 million eggs at a time. And birds love to eat protein- and lupid-rich roe when they’re available, in some cases making up 100 percent of their stomach contents.
“During the spawning of these fish most birds are in their northward migration, so fish eggs are likely to be transported by them northward,” says Vincze. Based on a maximum flight speed of 60 km/h for mallards, the researchers calculated that eggs could be consumed and then excreted in another lake 60 km to 360 km away.
“It remains to be determined how often this happens in the field and whether eggs hatch in natural waters,” says Vincze. “We are working on these questions.”