small population of fathead minnow Pimephales promelas was discovered in a
stream near Paris (C. Houeix, pers. com.). This North American minnow was used
as bait a few decades ago, and populations are already established in several rivers in
Belgium [48]. These two examples illustrate the diversity of the origin of alien
species likely to establish in the Seine basin and may be the first signs of larger-scale
colonisation. The recent record, in 2015, of the round goby N. melanostomus in a
gravel pit connected to the lower Seine and the subsequent spread of its distribution
[49] deserve particular attention. This event is in line with a general process of
extending distribution of several Ponto-Caspian gobiid species, facilitated by inland
navigation and establishment of canals between rivers previously disconnected
[48, 50]. Thus, three other species, Proterorhinus semilunaris, Ponticola kessleri
and Neogobius fluviatilis, are present in Northeastern France and could colonise the
Seine basin in the near future. Unlike the other non-native species already present in
the Seine River, these species, in particular N. melanostomus, are highly invasive.
For instance, on several sites in the Rhine basin, although recently colonised, gobiid
species (mainly N. melanostomus) generally account for 60–90% of the fishes, and
they seem to compete with several native species, particularly benthic species [49].
If the forthcoming colonisation of the Seine basin by these species is confirmed, in
the coming decades, it could lead to a profound change in fish communities on most
watercourses.
The return of migratory species in the Seine River basin over the past two decades
is a remarkable event, especially since, unlike other major European rivers, this
recolonisation has taken place naturally, from nearby populations, without any use
of restocking operations [23]. This return is a direct response to improvements in
water quality, but the establishment of fish passes on some navigation dams has
also contributed to this recovery. In the future, it is planned to pursue the establishment of fish passes on the basin’s main watercourses, which should reinforce the
existing populations and extend their distribution area. The high potential of the
basin for the recovery of migratory fish with the restoration of river continuity can be
exemplified by the case of the Carandeau navigation dam located on the Aisne River
(a sub-tributary of the Seine River), about 400 km from the sea. It was equipped with
a fish pass in 2016, which in the subsequent months resulted in the passage of several
salmon and shad individuals, when these species had no longer been observed on
this axis since the nineteenth century.
Over the longer term, however, global warming could compromise the return of
some migratory species. Modelling approaches suggest that the thermal conditions
predicted by the end of the twenty-first century could jeopardise the sustainability of
the smelt stock [20], while it has recovered a flourishing level in recent years in the
estuary. The question of a negative impact of global warming also arises for salmon
and sea trout, two cold water migratory species [51].
Beyond migratory species, climate change is expected to affect all freshwater
fish species. At the scale of France, and more locally in the Seine basin, prospective
approaches evaluating different warming scenarios predict a contraction of the
distribution ranges of cold water species [39, 52, 53]. This contraction of distribution
areas is reported to be particularly pronounced in the case of brown trout and
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J. Belliard et al.
stream near Paris (C. Houeix, pers. com.). This North American minnow was used
as bait a few decades ago, and populations are already established in several rivers in
Belgium [48]. These two examples illustrate the diversity of the origin of alien
species likely to establish in the Seine basin and may be the first signs of larger-scale
colonisation. The recent record, in 2015, of the round goby N. melanostomus in a
gravel pit connected to the lower Seine and the subsequent spread of its distribution
[49] deserve particular attention. This event is in line with a general process of
extending distribution of several Ponto-Caspian gobiid species, facilitated by inland
navigation and establishment of canals between rivers previously disconnected
[48, 50]. Thus, three other species, Proterorhinus semilunaris, Ponticola kessleri
and Neogobius fluviatilis, are present in Northeastern France and could colonise the
Seine basin in the near future. Unlike the other non-native species already present in
the Seine River, these species, in particular N. melanostomus, are highly invasive.
For instance, on several sites in the Rhine basin, although recently colonised, gobiid
species (mainly N. melanostomus) generally account for 60–90% of the fishes, and
they seem to compete with several native species, particularly benthic species [49].
If the forthcoming colonisation of the Seine basin by these species is confirmed, in
the coming decades, it could lead to a profound change in fish communities on most
watercourses.
The return of migratory species in the Seine River basin over the past two decades
is a remarkable event, especially since, unlike other major European rivers, this
recolonisation has taken place naturally, from nearby populations, without any use
of restocking operations [23]. This return is a direct response to improvements in
water quality, but the establishment of fish passes on some navigation dams has
also contributed to this recovery. In the future, it is planned to pursue the establishment of fish passes on the basin’s main watercourses, which should reinforce the
existing populations and extend their distribution area. The high potential of the
basin for the recovery of migratory fish with the restoration of river continuity can be
exemplified by the case of the Carandeau navigation dam located on the Aisne River
(a sub-tributary of the Seine River), about 400 km from the sea. It was equipped with
a fish pass in 2016, which in the subsequent months resulted in the passage of several
salmon and shad individuals, when these species had no longer been observed on
this axis since the nineteenth century.
Over the longer term, however, global warming could compromise the return of
some migratory species. Modelling approaches suggest that the thermal conditions
predicted by the end of the twenty-first century could jeopardise the sustainability of
the smelt stock [20], while it has recovered a flourishing level in recent years in the
estuary. The question of a negative impact of global warming also arises for salmon
and sea trout, two cold water migratory species [51].
Beyond migratory species, climate change is expected to affect all freshwater
fish species. At the scale of France, and more locally in the Seine basin, prospective
approaches evaluating different warming scenarios predict a contraction of the
distribution ranges of cold water species [39, 52, 53]. This contraction of distribution
areas is reported to be particularly pronounced in the case of brown trout and
318
J. Belliard et al.