Mortality Of Silver Eels Research Articles

When “safe” dams kill: Analyzing combination of impacts of overflow dams on the migration of silver eels

1.The drastic decline in European eel Anguilla anguilla stock is now widely recognized. However, while various causes for this decline have been identified, the relative importance of each cause remains unclear.2.During the catadromous migration of silver eels, the negative impact of dams is frequently highlighted, but mainly for powered dams (with turbines) or where connectivity is completely ruptured. Mechanical impact due to turbine blades is often considered the most important cause of mortality of silver eels during downstream migration. Consequently, non-powered dams equipped with spillways are often considered safe for the passage of migrating silver eels.3.We hypothesized that, to understand the negative impacts of dams, a much wider context must be considered than turbine mortality alone. Using an acoustic telemetry survey of silver eels, we demonstrated the negative effects of non-powered dams on downstream migration.4.Five main impacts on eel populations were highlighted: (i) the attenuation or loss of triggering factors, leading to an absence of or delay in migration; (ii) extra delays and extra distances travelled when crossing the dam; (iii) extra energetic costs of the additional distance travelled as result of exploring the dam and the reservoir to find other escape passages; (iv) the selection of a more risky behavioral phenotype, i.e., bold eels; and (v) direct blocking once migration has started. Mortality was evaluated as a supplementary impact. Some of these effects (attenuation of triggers, extra delays to cross the dam) might be more important than the same effect from powered dams, probably due to the constant high water discharge required for turbines that facilitate the passage of eels.5.As these “safe” dams are very widespread, they must be considered a potential threat to effective eel conservation.

The impact of an Archimedes screw hydropower turbine on fish migration in a lowland river

There has been an increase in the abundance of small hydropower (<10 MW) installations. These tend to be ‘run-of-river’, thereby reducing or averting the need for impoundment and water storage, and so are considered to have lower environmental impact. The Archimedes screw turbine (AST) has been described as ‘fish friendly’ based on magnitude of observed first order impacts, i.e. low rates of direct damage and mortality due to blade strike. However, potential second order effects, such as altered fish behaviour prior to or after passage, and consequences for long-term survival and fitness, have been largely ignored. This two-year study employed acoustic and Passive Integrated Transponder (PIT) telemetry and dual frequency imaging sonar to investigate the influence of an AST and associated fish passes (Larinier super-active baffle pass and upstream eel ladders) on the movement patterns and behaviour of five species of potadromous fish and European eel (Anguilla anguilla). Seaward-migrating adult (silver) eels successfully descended the AST, but under high flows most individuals took an alternative route via an overshot weir which resulted in faster passage rates. There was no immediate silver eel mortality as a result of passage through the turbine and no effect on subsequent migration behaviour through the lower freshwater catchment and estuary. Some eels were delayed at the complex and exhibited frequent rejection and milling on their approach to the AST and fish passes. Upstream passage rate (%) at the Larinier pass varied among species with highest for dace (81%) and the lowest for roach (10%). Fish that aggregated up- and downstream of the AST rapidly dispersed on turbine start-up. Although ASTs can be considered a potential downstream passage route for both eels and potadromous species, adult eels milled and rejected at the upstream entrance, while other fish exhibited startle response on turbine start-up. Potential delay to seaward migration of silver eel and the energetic costs of observed behaviours should be considered during future impact assessments, particularly where there is no alternative migration route or there are multiple such facilities in a watercourse.

Mortality of silver eels migrating through different types of hydropower turbines in Lithuania

AbstractHydropower plants (HPP) are considered to be one of the major threats to the survival of European eels when they migrate downstream along inland water bodies during the early part of their annual journey to the spawning area in the Sargasso Sea. There are 98 HPPs in Lithuania and thousands throughout Europe. Numerous studies describe HPP induced mortality rates among European eels as variable depending on local, environmental, and technical factors. This heterogeneity in effect complicates theoretical extrapolation to eel mortality arising from specific types of HPP, necessary for effective management of local stocks. Silver eel mortality was estimated for 4 different HPPs in Lithuania. Mortality was estimated using RFID (Radio Frequency Identification, passive integrated transponders) tags and acoustic telemetry in a large HPP (&gt;100 MW) with Kaplan turbines, a small HPP (&lt;1 MW) with a Kaplan turbine and a fish passage, and for the first time in two small HPPs (&lt;1 MW) with CINK turbines. The results supported a hypothesis that the mortality rate of migrating eels depends mainly on the type and size of the turbine. HPP induced mortality varied from 100% in a small CINK turbine down to 25% in the large HPP with Kaplan turbines. The importance of simple mitigation measures was highlighted by 34% of all tagged eels bypassing one of the HPP via an adjacent fish passage constructed for upstream migration of salmonids. The observed differences in mortality provide essential information for long term strategies designed to restore depleted eel populations in Lithuania and other European countries.

Mortality of silver eel (Anguilla anguilla) migrating downstream through a small hydroelectric plant on the Drawa River in northern Poland

Abstract The European eel, Anguilla anguilla L., is an endangered species. Barriers to its downstream spawning migration are one of the greatest threats this species faces. There are hundreds of hydroelectric plants (HEP) on rivers in Poland (&gt; 600), and thousands throughout Europe. Eel that pass through HEP turbines as they migrate downstream suffer high mortality, but this depends mainly on local and technical conditions. Silver eel mortality was estimated and the possibility of the fish bypassing the turbines was studied between November 2013 and June 2014 at a typical HEP in northern Poland. Two telemetry methods were used with 49 eel: passive integrated transponder (PIT) system and acoustic telemetry. Fifty five percent of eel migrated downstream in fall 2013, soon after their release, and 45% migrated the next spring. The eel did not use the fish passes designed for upstream migration; thus, they were forced to go through the turbines, which resulted in 55% mortality. HEPs cause interruptions and delays in eel spawning migrations and are responsible for high eel mortality. This can make implementing an eel restitution plan difficult or even impossible in river systems with many barriers.

Forecasting animal migration using SARIMAX: an efficient means of reducing silver eel mortality caused by turbines

Hydroelectric power plants are often considered a major cause of mortality for migra- tory fish. The endangered status of European eels Anguilla anguilla forces managers to make efforts to reduce this mortality. Among the mitigation measures used, turbine shutdowns appear to be an efficient method, but they involve a substantial financial loss for hydropower producers. To optimise the use of turbine shutdowns, the present study aimed to provide a precise but simple forecast of the migration peaks of migrant silver eels. We first developed a model to forecast silver eel migration using SARIMAX models and real biological data of silver eel migration from 2 fish- ing sites in Brittany (north-western France). This model combines exogenous covariates and past biological data to forecast future migrations. We then evaluated this model with several years of biological data from the same trap sites, based on the decision criterion that turbines should be shut down when the number of silver eels forecasted by our model represents >10% of the mean annual number of migrants. This model would have served to save 90 and 70% of eels on the Oir and Scorff Rivers, respectively, with only 3.3 to 5.6 turbine shutdowns per year on average. Initial shutdowns would have lasted 1 wk, but this study also showed that the duration of turbine shut- downs could be reduced to 2 d without a significant decrease in efficiency. SARIMAX models reduce the influence of exogenous factors in forecasting outcomes, these being the most variable factors in the current context of climate change. This model appears to be a powerful tool for eco- logical forecasting of endangered species in the context of global warming.

Spawner escapement of European eel (Anguilla anguilla) from the River Erne, Ireland

AbstractKnowledge of silver eel production and escapement is essential for conservation of the European eel (Anguilla anguilla) stock. Silver eel trap and transport, being undertaken (2009–2012) to mitigate for negative impacts of hydropower on eels in some Irish rivers, was therefore monitored in the River Erne (catchment area 4375 km2). Eel population analyses at an experimental fishing weir involved mark–recapture experiments and sonar (DIDSON) surveys. Silver eel mortality rates at two hydropower stations were assessed by acoustic telemetry. The combined results of these studies enabled estimation of silver eel production and escapement for two migration seasons. For the 2010 season, production and escapement estimates were 42.5 t (1.62 kg·ha−1) and 38.8 t (1.48 kg·ha−1), respectively. For the 2011 season, production and escapement estimates were 44.6 t (1.70 kg·ha−1) and 41.8 t (1.59 kg·ha−1), respectively. Closure of all commercial eel fisheries and initiation of silver eel conservation fishing in 2009 resulted in 9.36, 19.33 and 25.25 t of silver eels being transported downstream of the hydropower dams in 2009, 2010 and 2011, respectively. The protocols developed in this study can be used to test models being used for eel population management in Ireland.

EUROPEAN SILVER EEL MIGRATION AND FISHERIES‐INDUCED MORTALITY IN THE HAVEL RIVER SYSTEM (GERMANY)

ABSTRACTBy using acoustic telemetry, the behaviour and fishery mortality of silver eels Anguilla anguilla (L.) were studied during the early phase of the spawning migration through the Havel River downstream to the river Elbe. A total of 99 female silver eels were implanted with acoustic transmitters and released at two locations in the Havel River system in autumn 2007. Tag recaptures by fisheries were used to assess fishery mortality. Most eels continued their downstream migration immediately after release during October–November in the year of release. However, some eels continued their downstream migration the following spring or autumn; only a few eels migrated in winter and summer. Tagged eels reached the river Elbe from 2 days to more than 1 year after tagging. Fishing efficiency was very high, and by the end of December 2008, only 8% of the tagged eels had reached the river Elbe, whereas 25% were reported caught by commercial fishers and 2% by recreational fishers. The remaining 65% of the tagged eels were unaccounted for at the end of the study period. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Survival and progression rates of large European silver eel Anguilla anguilla in late freshwater and early marine phases

The population of European silver eel Anguilla anguilla has declined tremendously in the last decades. The cause of this decline is unknown, and it is necessary to investigate the migra- tory behaviour and survival rates of silver eels during the reproductive migration in order to under- stand if the decline is related to factors acting during that migration. We estimated survival and pro- gression rates of European silver eel migrating in the lower part of the River Gudenaa and during the first phase of the marine migration in the Randers Fjord in Denmark. Fifty migrating silver eel (total body length: 56 to 84 cm) were captured, and each was equipped with an acoustic transmitter. Their migration was subsequently monitored using an array of automatic listening stations, and progres- sion rate and mortality in the river, inner part of the fjord and outer part of the fjord were estimated. Survival was high in fresh water. However, 60% of eels were lost in the inner and outer fjord, sup- porting the hypothesis that mortality is large in the early phase of the marine migration and that fish- ing may be a major cause of mortality of silver eels. There was no indication that the slowest-migrat- ing individuals were more prone to fishing mortality than the faster-migrating individuals. Progression rate increased as the eels proceeded downriver and out of the fjord. The migration was predominantly nocturnal, both in the river and fjord. Based on the available evidence, a considerable increase in eel survival in the river-fjord system will be needed in order to fulfil the goals in the Euro- pean Union recovery plan for eels.

Just go with the flow? Route selection and mortality during downstream migration of silver eels in relation to river discharge

Abstract Jansen, H. M., Winter, H. V., Bruijs, M. C. M., and Polman, H. J. G. 2007. Just go with the flow? Route selection and mortality during downstream migration of silver eels in relation to river discharge. – ICES Journal of Marine Science, 64: 1437–1443. The European eel (Anguilla anguilla) has been in steep decline for several decades. Fisheries and hydropower-induced mortality presumably play an important role during the downstream migration of silver eels, and downstream-migrating silver eels must make various navigation and route-selection decisions to reach the sea. We examined the influence of river discharge on route selection of silver eels. To quantify the impact of hydropower and fisheries on silver eel mortality, radio-telemetry experiments were performed in the River Meuse in 2002 and 2004, surgically implanting 300 silver eels with Nedap-transponders. Route selection and passage behaviour near detection stations was assessed. Silver eels were distributed over the alternative migration routes in the river in proportion to the discharge until the silver eels reached the entrance to the turbines. The eels altered their behaviour when approaching the turbines of hydropower plants and showed stationary and recurrent behaviour. We discuss the consequences of this on route selection and mortality rates caused by hydropower facilities and fisheries.

Silver eel mortality during downstream migration in the River Meuse, from a population perspective

Abstract Winter, H. V., Jansen, H. M., and Breukelaar, A. W. 2007. Silver eel mortality during downstream migration in the River Meuse, from a population perspective. – ICES Journal of Marine Science, 64: 1444–1449. The European eel (Anguilla anguilla) population has decreased sharply over the past few decades owing to a combination of many factors. To determine the impact of hydropower and fisheries during the downstream migration of silver eel in the River Meuse, telemetry experiments were performed during the years 2002–2006, using 18 detection stations (NedapTrail-System®) in the river and two at the entrance to the hydropower turbines. Recaptures in fisheries were used to assess fisheries mortality. In all, 300 silver eels were surgically implanted with Nedap-transponders. For each stretch between subsequent stations, mortality rates were assessed and related to the different factors. However, to determine the overall effect on the escapement of silver eels from the River Meuse, insight into the distribution of silver eels in the entire catchment of the River Meuse is required. At two locations, mark-recapture experiments in 2002 revealed that the estimated number of migrating silver eels increased strongly in a downstream direction, suggesting that a large proportion of silver eels start their migration from the downstream stretches and tributaries of the River Meuse. Approaches and monitoring requirements that can be used to determine the impact on silver eel populations in a river basin are discussed.