Despite recent regulations to reduce or prohibit introduction of alien species,
their proportion in the whole Seine basin is constantly increasing, and, since the
beginning of the twenty-first century, the rate of implantation of new species has
reached nearly three species per decade (Fig. 1).
Interestingly, there are discrepancies between written scientific sources and
archaeological sources in terms of species introductions [8]. According to scientific
and written sources, wels catfish and pikeperch were established in the Seine basin
in the second half of the twentieth century, although attempts to introduce them
were already mentioned as early as the nineteenth century. However, archaeological
remains of these two species, dating from the late Middle Ages, have been discovered in the centre of the basin. This apparent contradiction may in fact reveal a
possible limitation of archaeological sources, as mentioned above (uncertainty
regarding the location of catches). However, these archaeological remains may
also reflect an earlier presence than previously assumed, suggesting very old and
repeated introduction attempts, before these two species really became established in
the twentieth century.
4 Migratory Fish: A Dramatic Decline Followed by a Still
Limited Recovery
Historically, 11 diadromous fish species occurred in the Seine River basin: eel
(Anguilla anguilla), thinlip mullet (Liza ramada), flounder (Platichthys flesus), sea
lamprey (Petromyzon marinus), river lamprey (Lampetra fluviatilis), European
sturgeon (Acipenser sturio), Atlantic salmon (Salmo salar), sea trout (Salmo trutta),
smelt (Osmerus eperlanus), allis shad (Alosa alosa) and twaite shad (Alosa fallax).
For some of them, several authors have mentioned an early decline in Northwestern
Europe as from the Middle Ages [14]. For example, in the medieval period, a sharp
decline of salmon populations coincided with the widespread establishment of water
mills, which prevented access to spawning grounds but also presumably encouraged
overfishing [15].
However, the nineteenth century was a turning point for migratory fish in
the Seine River basin, due to (1) the widespread development of waterways for
commercial navigation and (2) the significant increase in populations and human
activities, particularly around Paris. The building of weirs and locks to facilitate
navigation began in the middle of the nineteenth century in the centre of the basin
and gradually spread, over the following decades, on the Seine and its major
tributaries, involving increasingly higher structures. This has led to a gradual
increase in barriers to fish migration, which has been reinforced for some species
by the simultaneous construction of dams in headwater catchments in order to
regulate river flows. For example, the construction of the Settons Dam on the Cure
River (19 m high), finished in 1858, definitively closed off access to the best
upstream salmon spawning grounds [16].
Changes in Fish Communities of the Seine Basin over a Long-Term Perspective
305
their proportion in the whole Seine basin is constantly increasing, and, since the
beginning of the twenty-first century, the rate of implantation of new species has
reached nearly three species per decade (Fig. 1).
Interestingly, there are discrepancies between written scientific sources and
archaeological sources in terms of species introductions [8]. According to scientific
and written sources, wels catfish and pikeperch were established in the Seine basin
in the second half of the twentieth century, although attempts to introduce them
were already mentioned as early as the nineteenth century. However, archaeological
remains of these two species, dating from the late Middle Ages, have been discovered in the centre of the basin. This apparent contradiction may in fact reveal a
possible limitation of archaeological sources, as mentioned above (uncertainty
regarding the location of catches). However, these archaeological remains may
also reflect an earlier presence than previously assumed, suggesting very old and
repeated introduction attempts, before these two species really became established in
the twentieth century.
4 Migratory Fish: A Dramatic Decline Followed by a Still
Limited Recovery
Historically, 11 diadromous fish species occurred in the Seine River basin: eel
(Anguilla anguilla), thinlip mullet (Liza ramada), flounder (Platichthys flesus), sea
lamprey (Petromyzon marinus), river lamprey (Lampetra fluviatilis), European
sturgeon (Acipenser sturio), Atlantic salmon (Salmo salar), sea trout (Salmo trutta),
smelt (Osmerus eperlanus), allis shad (Alosa alosa) and twaite shad (Alosa fallax).
For some of them, several authors have mentioned an early decline in Northwestern
Europe as from the Middle Ages [14]. For example, in the medieval period, a sharp
decline of salmon populations coincided with the widespread establishment of water
mills, which prevented access to spawning grounds but also presumably encouraged
overfishing [15].
However, the nineteenth century was a turning point for migratory fish in
the Seine River basin, due to (1) the widespread development of waterways for
commercial navigation and (2) the significant increase in populations and human
activities, particularly around Paris. The building of weirs and locks to facilitate
navigation began in the middle of the nineteenth century in the centre of the basin
and gradually spread, over the following decades, on the Seine and its major
tributaries, involving increasingly higher structures. This has led to a gradual
increase in barriers to fish migration, which has been reinforced for some species
by the simultaneous construction of dams in headwater catchments in order to
regulate river flows. For example, the construction of the Settons Dam on the Cure
River (19 m high), finished in 1858, definitively closed off access to the best
upstream salmon spawning grounds [16].
Changes in Fish Communities of the Seine Basin over a Long-Term Perspective
305