Community Research and Development Information Service - CORDIS

Assessment impact hydropower & fisheries on silver eel

In the first two stretches tag loss occurred, which caused a reduction in ‘survival’ of 9.3% and 20.3%. The presence of Linne hydropower station and anchored stow net fishery, in combination reduced survival with 13.7%, whereas Alphen hydropower station and anchored stow net fishery, in combination reduced survival with 3.6%. Other fisheries show mortality rates of 0.4 to 2.5% per stretch between two detection stations. The estimation by MARK-model showed for Linne hydropower station up to 6.7 % direct mortality and for Alphen hydropower station up to 3.2 % direct mortality, based on the transponder experiment.

As estimated by Virtual Population Analysis (on total silver eel population) a total mortality of 63.3% was found. To this, hydropower contributes 15.8% (6.3% direct and 9.5% delayed mortality), fisheries contribute in total 22.2% (anchored stow net 5.1% and fykenet fisheries 17.1%) and unexplained mortality contributes 25.3%. The total mortality by the two hydropower stations is likely an overestimation, as of the eels that are considered to have passed the turbines and not being detected at downstream detection stations or are caught by fisheries, it is not sure if all are lethally injured. Fisheries mortality is likely an underestimation as there might be an underreporting of transponders by fisherman. Also, the unexplained mortality might be partially attributable to withdrawal by fisheries, which has not been recognised so far.

It can be concluded that the combined mortality by two hydropower stations is smaller than the combined mortality by commercial fisheries. Fykenet fishing is more intensive than the anchored stow net fisheries. Furthermore, fisheries mortalities based on recaptures are minimum estimates, and it is likely that the real mortalities are up to a factor 2 higher, when presuming an underreporting of recaptured tags, as indicated by the population estimates at Linne.

A tentative approximation solely holding for the disappearance and survival of silver eels that start migrating from upstream Linne has been made. The approximation is based on estimated population number at Linne, using the combined results and several assumptions to calculate the number of eel that hypothetically have reached the North Sea during the silver eel migration season of 2002.

For the river Meuse section from upstream Linne to the North Sea, an approximate maximum mortality of about 73% is calculated (on a total of 94,000 eels), i.e. a total of 25,321 specimens (about 27%) that tentatively reached the sea. Of the mortality, hydropower contributes about 21.7% (n = 20,385; for both Linne and Alphen), anchored stow net fisheries contributes 4.8% (n = 4,488) and fykenet fisheries + other cause contribute 46.6% (n = 43,806). This mortality solely holds for the eel population that started migration upstream Linne.

When approximated for the river Meuse section from upstream Alphen to the North sea, an approximate maximum mortality of about 49% can be calculated (on a total of 225,000 eels starting from upstream Alphen, including the eels that successfully passed Linne), i.e. a total of 114,379 specimens (about 51%) that hypothetically reached the sea. Of this mortality, hydropower contributes 7% (n = 15,660), anchored stow net fisheries contributes 5.8% (n = 12,948) and fykenet fisheries + other cause contribute 36.5% (n = 82,013).

To answer the question how many eels migrating downstream in the river Meuse, starting in the river section from upstream Linne to upstream Alphen, have hypothetically reached the North sea during the migration season of 2002, the approximate number of eels surviving from Alphen has to be used, as these also include the eels that have successfully passed Linne. Tentatively a survival rate of 51% is calculated of the total population starting from upstream Alphen of 225,000 specimens, which includes the number of eel that successively passed Linne. The total number of downstream migrating silver eels that tentatively have reached the North Sea amounts approximated 114,379 specimens. To what extent the lower section of the river Meuse downstream Alphen contributes to the total population is unknown.

Reported by

KEMA Power Generation & Sustainables
Utrechtseweg 310
6800 ET Arnhem
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