On the lower Seine River, the Martot dam (currently destroyed) was constructed
in 1864 just upstream of Rouen, followed by the Poses dam a few kilometres
upstream. The latter has been operational since 1886, and, at that time, it was the
largest navigation dam in the basin at 4 m high; it had a major impact by reducing
the accessibility of a large part of the basin. This led, a few years later, to a collapse
of salmon and allis shad stocks, whose spawning grounds were located further
upstream, while species such as twaite shad and smelt, able to reproduce further
downstream, were less adversely affected (Fig. 2). As a result, the most vulnerable
species, i.e. European sturgeon, salmon and allis shad, became extinct in the Seine
River basin at the beginning of the twentieth century.
The degradation of water quality has also contributed significantly to the decline
of migratory fish. From the end of the nineteenth century (and probably well before),
downstream of Paris, several kilometres of the Seine River were affected by a low
oxygen concentration, due to wastewater spills directly into the river. With the
growth of the Paris urban area, this situation deteriorated gradually until the
early 1970s [17]. At that time, the entire lower Seine River stretch, downstream of
Paris, was depleted with dissolved oxygen, with particularly harsh conditions in its
estuarine course. In the estuary, a 125-km-long reach underwent severe hypoxia
(<3 mg/L) for 200–300 days every year, sometimes reaching periods of total anoxia,
particularly in summer and autumn [18, 19]. As a result, the populations of smelt
and twaite shad, two fish species that are able to reproduce in the lower parts of the
river and which had consequently been less affected by the establishment of dams,
collapsed during the 1950s until their assumed extinction throughout the basin
around 1965–1970.
Overall, the construction of dams on major rivers, in combination with the
considerable increase in pollution downstream of the Seine since the mid-nineteenth
century, has therefore caused a considerable decline of migratory fish communities,
until the gradual extinction of seven species. At the end of the 1970s, only four
diadromous species were still present in the Seine River basin, L. fluviatilis,
A. anguilla, Salmo trutta trutta and Platichthys flesus, but their populations were
very sparse at that time [20].
Since the mid-1970s, considerable efforts have been made to treat domestic and
industrial sewage and to prevent their direct discharge into watercourses. This
resulted in a gradual improvement in water quality along the Seine River, from the
Paris urban area to the estuary, which allowed the return of diadromous species [17].
The first significant returns concerned species that reproduce in the downstream part
of the river, such as twaite shad and smelt, whose catches have become regular since
1998 [21]. In the early 2000s, species migrating further upstream, such as salmon or
allis shad, began to recolonise the basin, supported by the gradual installation of fish
passes on navigation dams and by the improvement of water quality (in relation with
more efficient wastewater treatment), but without any restocking operations [22, 23]
(Fig. 3).
Since 2008, several tens of salmons and allis shads and several hundred sea
lampreys have been observed each year, at the fish pass installed at Poses (furthest
downstream dam, located about 150 km from the sea) [24]. For these three species,
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J. Belliard et al.
in 1864 just upstream of Rouen, followed by the Poses dam a few kilometres
upstream. The latter has been operational since 1886, and, at that time, it was the
largest navigation dam in the basin at 4 m high; it had a major impact by reducing
the accessibility of a large part of the basin. This led, a few years later, to a collapse
of salmon and allis shad stocks, whose spawning grounds were located further
upstream, while species such as twaite shad and smelt, able to reproduce further
downstream, were less adversely affected (Fig. 2). As a result, the most vulnerable
species, i.e. European sturgeon, salmon and allis shad, became extinct in the Seine
River basin at the beginning of the twentieth century.
The degradation of water quality has also contributed significantly to the decline
of migratory fish. From the end of the nineteenth century (and probably well before),
downstream of Paris, several kilometres of the Seine River were affected by a low
oxygen concentration, due to wastewater spills directly into the river. With the
growth of the Paris urban area, this situation deteriorated gradually until the
early 1970s [17]. At that time, the entire lower Seine River stretch, downstream of
Paris, was depleted with dissolved oxygen, with particularly harsh conditions in its
estuarine course. In the estuary, a 125-km-long reach underwent severe hypoxia
(<3 mg/L) for 200–300 days every year, sometimes reaching periods of total anoxia,
particularly in summer and autumn [18, 19]. As a result, the populations of smelt
and twaite shad, two fish species that are able to reproduce in the lower parts of the
river and which had consequently been less affected by the establishment of dams,
collapsed during the 1950s until their assumed extinction throughout the basin
around 1965–1970.
Overall, the construction of dams on major rivers, in combination with the
considerable increase in pollution downstream of the Seine since the mid-nineteenth
century, has therefore caused a considerable decline of migratory fish communities,
until the gradual extinction of seven species. At the end of the 1970s, only four
diadromous species were still present in the Seine River basin, L. fluviatilis,
A. anguilla, Salmo trutta trutta and Platichthys flesus, but their populations were
very sparse at that time [20].
Since the mid-1970s, considerable efforts have been made to treat domestic and
industrial sewage and to prevent their direct discharge into watercourses. This
resulted in a gradual improvement in water quality along the Seine River, from the
Paris urban area to the estuary, which allowed the return of diadromous species [17].
The first significant returns concerned species that reproduce in the downstream part
of the river, such as twaite shad and smelt, whose catches have become regular since
1998 [21]. In the early 2000s, species migrating further upstream, such as salmon or
allis shad, began to recolonise the basin, supported by the gradual installation of fish
passes on navigation dams and by the improvement of water quality (in relation with
more efficient wastewater treatment), but without any restocking operations [22, 23]
(Fig. 3).
Since 2008, several tens of salmons and allis shads and several hundred sea
lampreys have been observed each year, at the fish pass installed at Poses (furthest
downstream dam, located about 150 km from the sea) [24]. For these three species,
306
J. Belliard et al.