166
The Migrant Biota
Let us now discuss the eight cases mentioned above, in the order of their
increasing similarity with Case No. 5-the Lessepsian Suez Canal.
The Corinth Canal connects two areas of sea inhabited to the extent of our
knowledge by a similar marine fauna. However, there must have been differences
in the biota of the eastern and western side of the Isthmus of Corinth; these have
never been surveyed. An equalization and a mixing most probably occurred
through the few kilometers-Iong Canal. Nothing is known about this either.
The WeIland Canal which was opened in 1829, was a final addition to a Canal
system started in 1819, connecting the upper reaches of the Hudson River, a
tributary of the Atlantie, with the Great Lakes, bypassing the obstacle of the
Niagara falls. Aron and Smith (1971) have summed up the changes in the fish
fauna of the Great Lakes, following the opening of the WeIland Canal. The
changes were catastrophic, since two euryhaline estuarine species, the alewife
Alosa pseudoharengus and the sea-Iamprey Petromyzon marinus invaded the
Lakes. Through food competition ( Alosa) and direct predation (Petromyzon) the
invaders nearly exterminated several important commercial fishes of the Great
Lakes, such as the Atlantie salmon (Salmo salar), the lake trout (Salvelinus
namaycush), the lake herring (Leucichthys artedi), etc. Another species whieh
entered the lakes from the sea, the white perch (M orone americana) did not
become a pest.
The WeIland Canal case is a typieal case of invasion into a confined environment (the Great Lakes) oftwo successful invaders. It may be better compared to
the deep disturbances produced by accidental or anthropochorie invaders on
oceanic islands like Hawaii or St. Helena. That this is the case is especially evident
in the competition between Alosa and the autochthonous grazer fishes: according
to Aron and Smith (1971) "it became clear that the alewife used only a fraction of
the niehes occupied by the species that it displaced, thus reducing total fish
productivity". Such a situation-extermination and resulting under-exploitation
of the trophic basis-cannot occur in an open sea. A closed end of a sea may be
ecologically undersaturated with species but this happens not because of a competitive hazard, but because of extreme worsening in the environmental abiotie
factors. If this occurs then the impoverishment (or conversely, the penetration of
new elements) manifests itself at all the levels ofthe food web and the whole range
of the taxonomie diversity. In the Great Lakes, the invasion seems to have been
restrieted and impressive only on the level of the fish fauna, and not the whole
aquatie flora and fauna.
The Nord-Ostsee Canal did not shorten the way for a faunal interchange
between the two seas. The western (North Sea) entrance to the Canal carries the
freshwaters ofthe EIbe and the salinity within the Canal increases eastwards until
it reaches 15-20%0 in the Bay of Kiel. In fact, this is a paradoxieal situation: the
North Sea has anormal oceanie salinity of around 35%0, while the natural transition to the Baltic through the Danish Sounds leads through a range of decreasing
salinities. As far as reported, no Baltie brackish species has used the opportunity
to migrate through the Canal westward-a phenomenon which is logieally quite
possible. The advance of the North Sea-Atlantie fauna into the Baltie indeed
occurs through the natural contact ofthe Sounds (see below) and not through the
freshwater baITier ofthe Nord-Ostsee Canal.
The Migrant Biota
Let us now discuss the eight cases mentioned above, in the order of their
increasing similarity with Case No. 5-the Lessepsian Suez Canal.
The Corinth Canal connects two areas of sea inhabited to the extent of our
knowledge by a similar marine fauna. However, there must have been differences
in the biota of the eastern and western side of the Isthmus of Corinth; these have
never been surveyed. An equalization and a mixing most probably occurred
through the few kilometers-Iong Canal. Nothing is known about this either.
The WeIland Canal which was opened in 1829, was a final addition to a Canal
system started in 1819, connecting the upper reaches of the Hudson River, a
tributary of the Atlantie, with the Great Lakes, bypassing the obstacle of the
Niagara falls. Aron and Smith (1971) have summed up the changes in the fish
fauna of the Great Lakes, following the opening of the WeIland Canal. The
changes were catastrophic, since two euryhaline estuarine species, the alewife
Alosa pseudoharengus and the sea-Iamprey Petromyzon marinus invaded the
Lakes. Through food competition ( Alosa) and direct predation (Petromyzon) the
invaders nearly exterminated several important commercial fishes of the Great
Lakes, such as the Atlantie salmon (Salmo salar), the lake trout (Salvelinus
namaycush), the lake herring (Leucichthys artedi), etc. Another species whieh
entered the lakes from the sea, the white perch (M orone americana) did not
become a pest.
The WeIland Canal case is a typieal case of invasion into a confined environment (the Great Lakes) oftwo successful invaders. It may be better compared to
the deep disturbances produced by accidental or anthropochorie invaders on
oceanic islands like Hawaii or St. Helena. That this is the case is especially evident
in the competition between Alosa and the autochthonous grazer fishes: according
to Aron and Smith (1971) "it became clear that the alewife used only a fraction of
the niehes occupied by the species that it displaced, thus reducing total fish
productivity". Such a situation-extermination and resulting under-exploitation
of the trophic basis-cannot occur in an open sea. A closed end of a sea may be
ecologically undersaturated with species but this happens not because of a competitive hazard, but because of extreme worsening in the environmental abiotie
factors. If this occurs then the impoverishment (or conversely, the penetration of
new elements) manifests itself at all the levels ofthe food web and the whole range
of the taxonomie diversity. In the Great Lakes, the invasion seems to have been
restrieted and impressive only on the level of the fish fauna, and not the whole
aquatie flora and fauna.
The Nord-Ostsee Canal did not shorten the way for a faunal interchange
between the two seas. The western (North Sea) entrance to the Canal carries the
freshwaters ofthe EIbe and the salinity within the Canal increases eastwards until
it reaches 15-20%0 in the Bay of Kiel. In fact, this is a paradoxieal situation: the
North Sea has anormal oceanie salinity of around 35%0, while the natural transition to the Baltic through the Danish Sounds leads through a range of decreasing
salinities. As far as reported, no Baltie brackish species has used the opportunity
to migrate through the Canal westward-a phenomenon which is logieally quite
possible. The advance of the North Sea-Atlantie fauna into the Baltie indeed
occurs through the natural contact ofthe Sounds (see below) and not through the
freshwater baITier ofthe Nord-Ostsee Canal.