NEWS EXCLUSIVE
Ever wondered how hidden pools and isolated lakes become stocked with fish? One of the ways nature lends a hand might just surprise you
New research reveals the truth behind mystery fish movements
THE resilience of our coarse fish has been underlined in a fascinating scientific study that reveals how lakes can become stocked without human intervention.
For the first time, researchers have proved carp eggs can be eaten by ducks, digested and excreted, yet still survive and hatch into fry. One of the eggs in the experiment hatched after between four and six hours in the bird’s gut, during which time it could theoretically have flown 220 miles.
The newly released report, published by the Proceedings of National Academy of Sciences in the USA, was the result of work by Hungarian scientists.
HOW DO THEY DO IT?
“Fish have somehow colonised isolated water bodies all over the world without human assistance,” they write.
“It has long been speculated that these colonisation events are assisted by water birds transporting fish eggs attached to their feet and feathers, yet empirical support for this is lacking.”
Citing a recent theory that birds actually eat the eggs and excrete some of them alive, the report explains how the scientists decided to test this, and what they found.
THE EXPERIMENT
The team collected eggs from common carp and Prussian carp (similar to a crucian) and fed approximately 500 eggs to each of eight mallards in a controlled environment.
The birds’ faeces were collected at timed intervals and soaked in filtered river water. All recovered eggs were then moved to tanks for incubation.
In total, eight common carp eggs and 10 Prussian carp eggs were recovered, and all but one were excreted in the first hour after ingestion. One common carp egg was found in faeces between four and six hours after it had been eaten.
Fungal infection killed many of the viable embryos – as it did with the control eggs which had not been ingested by the birds – but three fish did hatch into fry.
NATURAL STOCKING
The study found just 0.2 per cent of eggs survived passage through the mallards, but when you consider a single carp can lay up to 1.5 million eggs during each spawning event, and the huge population of water birds in this country, such a survival rate could still lead to successful natural stocking.
Many of our waterfowl are also very fond of eating fish eggs, as they contain useful protein and fats. The phenomenon of eggs surviving being eaten by birds is described in the report as “likely to be frequent in nature, given the frequent feeding by birds on fish roe”.
HOW FAR DO THE EGGS SPREAD?
The report adds that although most of the viable eggs passed through the birds within an hour, the average mallard could have travelled over 30 miles during that time.
“How often such dispersal events lead to the successful establishment of new populations of invasive freshwater fish is a critical question for future research,” concludes the report.
The full study can be viewed at https:// tinyurl.com/carpeggs
ALTERNATIVE NATURAL STOCKING
FLOODING: When rivers and lakes burst their banks, fish can move between watercourses. Summer flooding, when the fish are most active, will have the biggest impact. The July floods of 2007 washed many Thames Valley gravel-pit carp into the river.
BIRDS OF PREY: Fish-eating birds such as ospreys can pluck their prey from one water and accidentally drop them into another during flight.
STICKING TO BIRDS: Although, as mentioned in this new report, scientific evidence is scarce, it is safe to assume some fish eggs become attached to birds that move from water to water. Most coarse fish that spawn in weed produce naturally sticky eggs. WATER SPOUTS: Highly unusual in the UK, but fish have been known to be plucked from the sea off our coast by swirling water spouts and dumped inland