Written by T. Lagarrigue and A. Frey
Eels are an amphihaline migrating species classified as vulnerable and inscribed on the Red List at a national level. Considering the alarming decline in their population at the three continental phases of their biological cycle, the European Community has set a framework for the protection and the sustainable exploitation of the European eel stock through its September 2007 regulation (EC) n°1100/2007. This regulation especially provides for each member state to implement, as fast as possible, adapted measures to decrease the eel mortality resulting from factors unrelated to fishery, such as hydroelectric turbines. The installation of so-called “fish-friendly” turbines, to replace existing turbines or on new developments, is one possible solutions.
MJ2 Technologies has embarked on this path, and has been developing and marketing the very low head turbine, VLH, designed to comply with the main “fish-friendliness” criteria relative to the passing of fish through turbines. To evaluate the actual efficiency of the compliance with these theoretical criteria, the VLH has been submitted to 2 series of in-situ tests. The first tests have been carried out on the first VLH commissioned at the Troussy site, on the Tarn river closely upstream of Millau
(ECOGEA, 2007, 2008a, and 2008b). For an operation at full opening and full power, general mortality rates of 7.7% for adult silver eels (from 356 to 1045 mm; average size 846 mm) and 3.1% for Atlantic salmon smolts have thus been observed. These results already ranked the VLH as less penalizing than conventional Kaplan turbines. However, significant prospects of improvement of the “fishfriendliness” of the VLH had been brought to light during these first tests (mortalities mainly located at
the level of an area where the fish were “pinched” between the blade ends and the discharge ring). Following these conclusions, MJ2 Technologies has decided to modify the hydraulic contour of its new VLHs (spherical contour at the discharge ring level) to attempt further decreasing the mortality caused to downstream-migrating fish. This new VLH has been installed in Frouard and submitted to this second series of tests, the main results of which are discussed in the present report.
The “lock” plant (centrale “de l’écluse”) is implanted on the Moselle river, in Frouard, close to Nancy, and comprises the new VLH 4500 turbine provided with a spherical discharge ring. This turbine has 8 blades and develops a 400-kW maximum electric power, limited by the energy sales agreement. It discharges 22 m3/s, with a 2.4-m net head.
The preferential downstream migration period of eels in France generally lasts from October to January, the migration essentially occurring by night, during environmental windows most often corresponding to flow increases (“water rushes”), along with a temperature drop and a turbidity increase. We have thus chosen to carry out the tests with a turbine operation very close to that occurring in high discharge conditions, that is, almost at full opening (95% of the nominal opening) and at full power (limited to 400 kW by the energy sales agreement), with a 38-cpm rotation speed.
The 244 eels used for this test originate from a professional fishery on the German side of the Rhine. Their size ranges between 610 and 1002 mm (average: 761 mm) and their weight varies from 557 to 1963 g (average: 843 g). They have been divided into 2 size groups (“Large individuals” of total length > 775 mm and “Small individuals” of total length ≤ 775 mm), to form 8 test batches of 25 individuals each (4 injection points x 2 size groups). They have been housed for the entire test period in large
circular tanks permanently supplied with water from the Moselle by submersible pumps.
The system used for the test has enabled to inject eels at 4 points of the machine, as close as possible to the guide blades, and then to recover them in the tail race by filtering of the entire turbine discharge, by means of a net (6 m x 3.5 m opening; 14 m long; 3 degressive 27-mm, 15-mm, and 10- mm meshes) emerging into a semi-submerged fish box supported by a floating pontoon.
During the tests, the recapture rates have varied from 88% to 100% (93% average), for the “large individual” group and from 72% to 100% (84% average), for the “small individual” group. For this last size group, in 3 out of the 4 injected batches, 2 live eels have been seen escaping from the net, at the level of a pocket formed by the largest mesh, as the net was being lifted by the crane. Thus, given that the recapture rates had not been total for all batches, a batch of 25 dead eels has been injected next
to the hub. All the dead eels having been recaptured, we have been able to make the assumption that the individuals that have not been recaptured are live individuals, capable of actively searching for a hole in the net to escape from it.
Test for evaluating the injuries suffered by downstream-migrating eels in their transiting through the new spherical discharge ring VLH turbogenerator unit installed on the Moselle river in Frouard.
No direct mortality has been observed on the total 200 injected eels (8 batches of 25 individuals each). Among the 177 eels recaptured in the net after their passing through the VLH, a close examination of their external aspect (no autopsy) has revealed, in 4 individuals, the presence of external injuries, which were not lethal in the short term (no mortality after from 24 to 48 hours of observation in the storage tanks).
The tests show that the rate of immediately lethal injuries is extremely low, and even zero, and that the rate of injuries not lethal in the short term (from 24h to 48h) is close to 2%. Conversely to the tests carried out in Troussy, no effect of the injection point on the injury rates can be observed. Similarly, conversely to what can be observed on other types of turbines, no effect of the size of individuals on the injury rate can be observed.
During the different tests carried out in Frouard, individuals of other species than the injected eels have been captured in the net: 195 European perches (size from 66 to 185 mm), 8 ruffes (size from 112 mm to 120 mm), 1 roach (76 mm), 1 common bream (70 mm), and 3 non-indigenous crayfish (Orconectes limosus). No direct mortality by severing has been observed in these other species (however, the ratio of individuals having really crossed the VLH in operation to those having crossed the VLH while stopped or having been trapped in the tail race by the installation of the recovery device is not known).
As a conclusion, the rate of immediately lethal injuries of adult eels (from 60 cm to 1 m) transiting through the new spherical discharge ring VLH installed in Frouard, running at full opening and full power, is extremely low, and even zero, and the rate of injuries not lethal in the short term (from 24 to 48 h) is low, since it approaches 2%.
Regarding the short-term mortality, these results rank the new-generation VLH with a spherical discharge ring, running at full power and full opening, as a turbine with a very light impact on the downstream migration of silver eels (from 60 cm to 1 m). However, the tests carried out in Frouard do not enable to assess the deferred mortaliy, which is always possible for eels transiting through a turbine, or the possible mortality generated by a VLH running at reduced opening and power.
It would eventually be useful to complete these trials with tests on other smaller fish species, in particular on Atlantic salmon smolts, which are also particularly affected by the injuries caused by hydroelectric turbines during their downstream migration.