Geoscience Reference
In-Depth Information
Danube and some of its tributaries. It was introduced to many European rivers by the Ro-
mans about 2,000 years ago after large numbers of legionnaires developed a taste for wild
carp while stationed along the Danube - then the northern boundary of the Roman Empire
- in the province of Pannonia.
This was how the common carp became the first species to be introduced into the Seine in
France, for instance. It was followed in the Middle Ages by other species, including tench
and rudd, which escaped from fish-farming ponds kept by noblemen and religious com-
munities. In the late 19th century, further invaders (nase and pikeperch) arrived in the Seine
from rivers further east via canals. These were followed at the end of the century by de-
liberate introductions of North American species: rainbow trout, black bass, pumpkinseed,
and black bullhead.
Native fish began to disappear from the Seine in the 20th century as the construction of
weirs and locks made it impossible for migratory species to reach their upstream spawning
grounds. With the exception of the eel, all of the Seine's migratory species became extinct:
sturgeon, salmon, sea lamprey, sea trout, European smelt, and shad. The original fish fauna
of the Seine probably consisted of about 30 species. Today, the river has 46 species, but
only 24 of them are native.
The catalogue of human impacts on the fish biology of the Seine is fairly typical of many
rivers in the more economically developed parts of the world. Biological invasions gen-
erally are widely acknowledged to be one of the major threats to biodiversity across the
world in rivers as well as other ecosystems. A study of the global patterns of freshwater fish
invasions in more than 1,000 river basins covering more than 80% of the Earth's continent-
al surface identified western and southern Europe as one of six global invasion hotspots,
where non-native species represent more than one-quarter of the total number of species
per basin. These hotspots also have the highest proportion of threatened fish species.
The human impact was found to be the most important determinant of this situation, par-
ticularly the level of economic activity, expressed by the gross domestic product, in a giv-
en river basin. The pattern can probably be explained in several ways. Regions that are
economically prosperous are more prone to habitat disturbances (e.g. dams and reservoirs
modifying river flows) that are known to assist the establishment of non-native species.
High rates of economic activity are also likely to increase the chances of invading species
arriving via aquaculture, sport fishing, and the ornamental trade. A higher demand for im-
ported products associated with economic development also increases the likelihood of un-
intentional introductions occurring when imports are made.
