Invasive alien species in Switzerland - Schweizer ...

3 > Crustaceans – Crustacea 67
ballast tanks and through hull-fouling). Many Ponto–Caspian crustaceans were also
transplanted between water bodies as food for native fish to stimulate fish production
within the former USSR.
Although the exact time and place of the first introductions of many species are not
documented, the data available suggest that many of them have invaded Switzerland
fairly recently. Thus, further spread and increase in species densities in the short or
long term are very likely. Moreover, other species are expected to arrive via river
systems.
With the exception of the crayfish species, almost all the crustacean invasions of
Switzerland have been facilitated by the construction of canals and ship traffic. In most
cases it is difficult to prove the exact pathways for the various species. Natural migration
of some mobile species along rivers and canals is likely to be important over short
distances. However, the rapid expansion of most species and some isolated records
indicate spread by ship traffic, either in ballast tanks or on hulls. The crayfish species,
however, were deliberately introduced as a food resource for human consumption.
Additionally, escapees from aquaculture facilities have founded several populations. A
third pathway is the importation of live crayfish for consumption. Crayfish are a delicacy
and have to be added to boiling water alive. Thus, they are imported alive and
some may have escaped. Moreover, some specimens escape from aquariums and
garden ponds, or are released by their owners, who no longer want to keep them.
The impacts of the species, especially the six high-risk species, are detailed in the Fact
Sheets and in Table 3.1. The three North American crayfish species are of greatest
concern with regard to the survival of the native crayfish populations, because they act
as a vector of the devastating crayfish plague. The two amphipod species D. villosus
and C. curvispinum alter the invaded habitats by predation, competition and causing
changes to the substrate. Demonstrating impacts of alien species on native biodiversity
and ecosystems is always difficult owing to the complex interactions. In conclusion,
species thought to be harmless may turn out to have some, as yet undetected, negative
impacts. This is especially true for freshwater ecosystems. Many of the alien species
occur in enormous abundance. This must have some impact on the ecosystem, since
each individual uses some resources and is a resource for others. Rey and Ortlepp
(2002) found that the biomass in the Rhine near Basel was dominated by alien species
– 97 % of the animal biomass and more than 90 % of the individuals were of alien
origin. Thus, the original character of the Rhine has vanished; in this area it is not the
typical Rhine anymore. Most of the alien species have invaded this stretch of the Rhine
within the past five years. This suggests that their distribution will further expand and
their dominance in other areas will increase. The dominance of alien species in European
(and other continental) inland waters is dramatic.
In most cases, eradication of established problematic species is not feasible. Control
has limited success in open freshwater systems, such as rivers. Some species, in particular
the crayfish, have economic value and intensive fishing could reduce their
numbers. However, a reduction in the crayfish population might not be a sufficient
goal, given the great threat they pose to native crayfish. One crayfish plague-infested