Deliverables Monitoring downstream migration of silver eel by telemetry The biotelemetry Nedap Trail System proved to be an appropriate system to monitor downstream migration of silver eel. The applicability of transponders was tested in a controlled tank-experiment with 20 tagged and 20 untagged silver eel. No differences in mortality and timing of activities were observed, but activity level was lower in tagged group. A pilot study with 10 silver eel in 2001 was conducted, of which 8 started their migration and 2 made it to the sea. In 2002 a field experiment with 150 silver eel was carried out. Of 125 started their migration of which at least 32, and most probably 38, reached the sea. 16 transponders were recaptured by fishermen. Most eel migrated downstream during end October - early November when river discharge increased. A second migration peak occurred during January. At hydropower stations 63 % passed during 19:00-24:00 hours, indicating that closing the turbines on the days with peak migrations during 5 hours yields a relatively high protection. 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. Monitoring migration events by Migromat During the investigation period of the EU Silver eel project, by identification of peaks of eel activity in the MM tanks and registration of subsequent warnings, it was possible to identify 24 days in Linne, and 17 days in Alphen during which downstream migration events have been predicted. This represents respectively 10 and 15% of the total period of investigation, i.e. the migration period of silver eel in the Dutch section of the river Meuse. During these few days 66% of the transpondered eels passed through Linne, and even 73% passed Alphen. In total, at Linne 20,445 eels and at Alphen 14.681 (in total 38,375 eels) have died due to hydropower. By means of the warnings provided by Migromat(Registered) during the migration season of 2002, the mortality would have been reduced to 6,951 (66% reduction) at Linne and to 3,964 (73% reduction) at Alphen (in total 10,915 specimens left that die due to hydropower). This implies a total reduction of the hydropower mortality by 71.6%. The results of the monitoring experiments have verified that the Migromat (Registered) system accurately registers the pre-migratory restlessness of eels, thereby predicting the downstream migration events of silver eels with high precision. The prediction of this early warning system enables an eel-friendly operating management of hydropower facilities. Hence, a high percentage of downstream migrating eels can be saved without installation of cost intensive protection and bypass systems. Turbine management Human impact on downstream migrating silver eel in European inland waters is caused by commercial fisheries and by mortality of eel passing the turbines of hydropower stations. These human activities are widespread in many European rivers and might have detrimental effects on the population level of the European eel. Therefore it is important to know to what extent damage to eel caused by hydroelectric power stations as well as the impact of withdrawal of eel by commercial fisheries are a threat to the population. Furthermore, there is the need to develop and optimise technical countermeasures to protect downstream migrating eels at hydropower stations. Countermeasures to protect downstream migrating fish at hydroelectric power stations are under research. One optional countermeasure is turbine management i.e. to close down the turbines during migrating periods of silver eel and to offer them a save passage over the weirs. Assessment of turbine passage & mortality of silver eel In the current project, silver eel passage and turbine-related injuries at Linne hydropower station has been estimated in two different ways: The first method is the conventional method by netting eels directly at the outlet of turbine 4 of Linne hydropower station. The second method is a novel telemetry method based on the transponder technique (Nedap Trail System (Registered). The results of the telemetry method provide valuable information on the cumulative damage due to passage of two hydropower stations at Linne and Alphen. The results are summarised in another result section of this document. In the turbine netting experiments, a total 1196 eel were captured during 17 samplings, most of the silver eel occurred in the length class of 60 69 cm. The number of yellow eel passing the turbines is very low, counting 16 individuals (1.34 % of total catch). The increase of the number of silver eels passing the turbine starts late September and correspond to an increase of the river discharge. However, not each increase of river discharge implicated a migration event. The migratory activity of eel, as shown by checking the nocturnal partitioning of turbine passage, started between 19:00 and 20:00, just after the darkness fell and less than 50% of the migrating silver eels passed the turbines before midnight. After midnight the number of eel per sub-sample decreased again. These results fully correspond to the observations made by the telemetry experiment. It is expected that during the night of October 26 27, when breakage of the net occurred due to an excess of turbine flow, the amount of eel through Linne hydropower station has been the largest, considering both the detection rate of transpondered eels and the result of the Migromat (Registered). The total of injured eels (non-lethal + lethal) is 34.0 % of the total of eel that passed the turbine. The total of lethal injuries amounts 24% of the total of eel that passed the turbine, however, this is likely to be an overestimation as part of the injured eels that have been defined as lethally injured still were alive and might survive the injury. The non-lethally injured eels correspond to 10% of the total of eel that passed the turbine. These specimens are likely to be able to, although with an unknown delay, continue their migration. The mortality is clearly related to turbine flow and the mortality rates found at 30 and 50 m3xs-1 correspond to the results of earlier investigations at Linne hydropower station. A clear relation was also found between the mortality and the length of the eel at 30 m3xs-1: larger eel showing higher mortality than smaller eel. At turbine flows of 50 and 100 m3xs-1 only a small length-effect was found. Overall, highest mortality rates are found for the 70 79 cm length class. Monitoring silver eel fisheries in the Dutch section of the river Meuse Commercial fisheries in the River Meuse have been monitored at two locations in 2001 and at three locations in 2002 to determine the timing of silver eel migrations for comparison with the transponder-experiments and the Migromat early warning systems. In both years catches of silver eel started July-August and peaked in September-October. On average in 20 days about 50 % of all silver eels pass, implying that implementation of an early warning system appears promising. Moreover, these data and monitoring can be used in catch recapture analyses to estimate total population size. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
