species such as trout (S. trutta) and European bullhead (Cottus perifretum) and
the establishment of eurytopic and warm water species. Compared with the streams
mentioned above, located on the periphery of the Seine River basin, this site
has undergone a completely different change in its environment, shifting in a
few decades from a rural catchment to an urbanised area with a high human
population density. This increase in human pressures has led to deterioration in
water quality and transformations in the morphology and hydrology of the watercourse, which most likely triggered the major changes observed in the fish assemblage composition.
The large rivers currently used for commercial navigation and located upstream
from the Paris conurbation (e.g. lower Yonne, Marne, Oise and Aisne rivers) show
fairly homogenous long-term changes characterised by a generally modest increase
in the proportion of limnophilic, eurytopic/phytophilic and warm water species.
For these watercourses, the oldest historical sources used to describe fish communities date back to the late nineteenth century. At that time, the main infrastructures
for navigation (dams and locks) were already in place, and the canalisation of
river courses was already nearly completed. This situation could explain the small
magnitude of long-term fish community changes detected in such rivers.
In contrast with the other navigated waterways, the lower Seine River, from
Paris to the estuary, experienced much more pronounced and complex changes in
fish communities. In this part of the Seine River, river sections for which historical
data are available show very similar trajectories. But it is in the Seine section of Paris
and immediately downstream that fish community changes can be reconstructed
with the greatest precision, due to the greater richness of the historical archives
(Fig. 5). Fish communities deteriorated continuously from the late nineteenth
century to the 1970s or 1980s (depending on sectors and data availability). This
deterioration was evidenced in particular by an increase in the proportion of eurytopic species at the expense of rheophilic species, an increase in the proportion of
species tolerant to oxygen depletion and an increase in the proportion of omnivorous
species at the expense of specialist species (invertivorous and piscivorous fish). This
trend is consistent with the previously reported deterioration in water quality that
affected the whole lower Seine River during this period. During the late 1960s and
early 1970s, the level of pollution reached its maximum [17], and the Seine downstream of Paris experienced widespread and repeated periods of severe anoxia. As a
consequence, only a small number of species, tolerant to low dissolved oxygen
contents, remained [31]. This deterioration of fish assemblages was probably also
accentuated by the development of the waterway during this period, which was
marked in part by an increase in the river canalisation (removal of islands and side
arms, deepening of the navigation channel) and the gradual increase in the height of
dams and locks. The 1990s and subsequent decades have seen a shift in this trend,
which is illustrated by a return of specialised and intolerant species (rheophilic,
oxyphilic, invertivorous and piscivorous fish) (Fig. 5). As already reported for
diadromous fish, this trend reversal, which affected the entire fish community,
coincided with the improvement of water quality and in particular the decrease in
the level of organic pollution resulting in a dissolved oxygen increase. However, the
Changes in Fish Communities of the Seine Basin over a Long-Term Perspective
311
the establishment of eurytopic and warm water species. Compared with the streams
mentioned above, located on the periphery of the Seine River basin, this site
has undergone a completely different change in its environment, shifting in a
few decades from a rural catchment to an urbanised area with a high human
population density. This increase in human pressures has led to deterioration in
water quality and transformations in the morphology and hydrology of the watercourse, which most likely triggered the major changes observed in the fish assemblage composition.
The large rivers currently used for commercial navigation and located upstream
from the Paris conurbation (e.g. lower Yonne, Marne, Oise and Aisne rivers) show
fairly homogenous long-term changes characterised by a generally modest increase
in the proportion of limnophilic, eurytopic/phytophilic and warm water species.
For these watercourses, the oldest historical sources used to describe fish communities date back to the late nineteenth century. At that time, the main infrastructures
for navigation (dams and locks) were already in place, and the canalisation of
river courses was already nearly completed. This situation could explain the small
magnitude of long-term fish community changes detected in such rivers.
In contrast with the other navigated waterways, the lower Seine River, from
Paris to the estuary, experienced much more pronounced and complex changes in
fish communities. In this part of the Seine River, river sections for which historical
data are available show very similar trajectories. But it is in the Seine section of Paris
and immediately downstream that fish community changes can be reconstructed
with the greatest precision, due to the greater richness of the historical archives
(Fig. 5). Fish communities deteriorated continuously from the late nineteenth
century to the 1970s or 1980s (depending on sectors and data availability). This
deterioration was evidenced in particular by an increase in the proportion of eurytopic species at the expense of rheophilic species, an increase in the proportion of
species tolerant to oxygen depletion and an increase in the proportion of omnivorous
species at the expense of specialist species (invertivorous and piscivorous fish). This
trend is consistent with the previously reported deterioration in water quality that
affected the whole lower Seine River during this period. During the late 1960s and
early 1970s, the level of pollution reached its maximum [17], and the Seine downstream of Paris experienced widespread and repeated periods of severe anoxia. As a
consequence, only a small number of species, tolerant to low dissolved oxygen
contents, remained [31]. This deterioration of fish assemblages was probably also
accentuated by the development of the waterway during this period, which was
marked in part by an increase in the river canalisation (removal of islands and side
arms, deepening of the navigation channel) and the gradual increase in the height of
dams and locks. The 1990s and subsequent decades have seen a shift in this trend,
which is illustrated by a return of specialised and intolerant species (rheophilic,
oxyphilic, invertivorous and piscivorous fish) (Fig. 5). As already reported for
diadromous fish, this trend reversal, which affected the entire fish community,
coincided with the improvement of water quality and in particular the decrease in
the level of organic pollution resulting in a dissolved oxygen increase. However, the
Changes in Fish Communities of the Seine Basin over a Long-Term Perspective
311