One big question remains however: how did fresh-water river crayfish arrive in The Netherlands from north America? Dutch scientists believe the bird migrations are responsible. However, there are no direct bird-migrations straight across the Atlantic ocean except between Scandinavia and Nova Scotia. And fresh-water crayfish cannot tolerate the salty Atlantic ocean.
The problem was also noted in Denmark and Spain.
Seven species of these exotic crayfish
, especially the species p.acutus
, found mainly in the Mississippi river basin, have now settled in permanently in the Netherlands and Denmark, with these low-lying nations' large networks of canals, channels, lakes and rivers, and in ponds in Spain. This is a puzzling development as the water temperatures in the Mississippi are generally higher than those in northern Europe.
Danish scientist Lars Briggs, who studies specific pools with fragile colonies of endangered species of salamanders, in attempts to save these little creatures from extinction, this year was deeply alarmed to find that the pools had been invaded by voracious, exotic north-American white river crayfish, and that these have practically wiped out the rare amphibians he had been studying and protecting.
And Dutch ecologists, who already warned eleven years ago that the country's amphibian population was on the verge of extinction, also are deeply alarmed about the way these exotic colonies of white river crayfish are destroying many of their carefully nurtured frog- and salamander populations. Similar alerts are also issued in Spain.
How did they get across the North Atlantic?
However, how did those north American fresh-water crayfish get to northern Europe? They can't survive in salt water and thus could not have marched across the Atlantic ocean by themselves. Dutch scientists say that the crayfish must have arrived via the feces of migrating birds which consume the eggs of the north-American fresh-water crayfish and deposit them in their droppings in Europe, where they hatch.
However, the worldwide bird-migration patterns map -- on the left -- shows that there is actually very little bird migration taking place between the north American and European continent - the vast number of birds migrating across the world each year, seem to stick to migrating north and south rather than taking the west and east routes across the oceans.
The only ocean-crossing migration pattern across the Atlantic occurs along the so-called "East Atlantic route'
which hugs the coastlines of West Africa, with the migration jumping across the Atlantic only via Greenland to Nova Scotia and into the rest of Canada - and then back again.
Crayfish wiping out the salamanders
The Ravon foundation, a group of Dutch scientists studying reptiles, amphibians and fishes in the Dutch eco-system, warn this week that the fresh-water crayfish are now wiping out the country's endangered salamanders, frogs and other amphibians at a huge rate. see
Especially the small, already endangered water salamanders in The Netherlands and Denmark are now being wiped out by large colonies of these invading exotic white river crayfish, Ravon warns.
These crayfish now have flourished so well, that they are turning into very 'high-density colonies' and are expected to expand rapidly now that they have reached these large concentrations. And they don't seem to be fished here as yet for food, either - so they have no enemies.
"These crayfish are causing devastation and mayhem amongst all the local amphibian populations, and this could become a serious problem in The Netherlands,' Ravon warns.
Dutch nature lovers and fishermen are asked to report these crayfish concentrations here
whenever they locate high concentrations of these invading crustaceans - not only to give them a clearer picture of their relentless march across our water-logged landscape, but also to try and destroy such colonies.
Even fresh-water fishermen complain that these crayfish are becoming a plague. The Sports Fishing Association of the Netherlands' spokesmen warn that the creatures are threatening to consume all the aquatic vegetation and the fish population too and are very aggressive. Crayfish populations also cause murky, oxygen-low water because they march across the floor of the rivers and streams and stir up the mud.
One official organisation, the water-management group "De Stichtste Rijnlanden
', has already applied for a 50,000 Euro subsidy from the State to try and hunt these colonies down and destroy them and more are expected to follow suit.
They want to specifically target one very aggressive species, the 15-cm long 'crested' American freshwater crayfish. "These colonies expand very rapidly are voracious, consuming fish-eggs and water plants at a very rapid rate once they settle into a riverine system," said a spokesman for De Stichtste Rijnlanden.
It has also been noticed by farmers that the creatures, as also do the eels, manage to march across the wet polderlands in The Netherlands very easily, thus reaching unexplored fresh-water populations rapidly.