Timing and pattern of annual silver eel migration in two European watersheds are determined by similar cues

Sandlund, Odd Terje; Diserud, Ola Håvard; Poole, Russell; Bergesen, Knut Aanestad; Dillane, Mary; Rogan, Gerard; Durif, Caroline; Thorstad, Eva B.; Vøllestad, Leif Asbjørn

doi:10.1002/ece3.3099

Cited by 42 publications

(50 citation statements)

References 40 publications

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“…Eels using the spillway gate passed the power station at a faster speed than those using other routes, which indicates that eels can be motivated or reluctant to pass barriers dependent on the physical and environmental conditions, as pointed out by Piper et al (). The eels seemed stimulated to migrate during floods, as also shown in other studies (Durif, Elie, Gosset, Rives, & Travade, ; Marohn et al, ; Sandlund et al, ; Trancart et al, ). However, there was large individual variation in migration timing, and some individuals spent a much longer time than the others to pass.…”

Section: Discussionsupporting

confidence: 79%

Mortality of downstream migrating European eel at power stations can be low when turbine mortality is eliminated by protection measures and safe bypass routes are available

Økland

Havn

Thorstad

et al. 2019

Internat Rev Hydrobiol

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The abundance of the European eel has seriously declined during recent decades. Hydropower production is one of the main threats, and solutions at power stations are needed to reduce the mortality of the downstream migrating silver eel. We examined the mortality, migration routes, and behavior of silver eel at a power station in Germany, after the power station was rebuilt to reduce the mortality of downstream migrating fish. Of 270 eels implanted with radio transmitters and released upstream of the power station, 222 eels passed the power station, primarily in October and November, although some descended during winter and spring. Most eels followed the main flow and passed over the spillway gate (59% and 49% in the 2 study years) or followed the route toward the bar racks in front of the turbines (24% and 27%), where they were guided to a route outside the turbines via the flushing channel. Some eels used the vertical slot fish passage (12% and 8%), whereas few used a nature‐like fishway, canoe pass, or custom‐made bypasses for eel. The eels showed large individual variation in migration timing, migration speeds, and choice of bypass. No eels were killed in the turbines, as none passed through them, likely due to the narrow bar spacing of the racks (10 mm). The results demonstrated that the mortality of eel passing power stations can be low (0–4% and 0–8% in the 2 study years) when the turbine intake is covered by racks hindering eels from entering turbines and safe bypass routes are available. Mortality estimates are given as ranges because the fate of 4% and 8% of the individuals could not be determined. Potential mortality could have been related to injuries in the bypass routes or increased predation risk, but there were no indications of injuries caused by installations in any of the bypass routes.

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Section: Discussionsupporting

confidence: 79%

Mortality of downstream migrating European eel at power stations can be low when turbine mortality is eliminated by protection measures and safe bypass routes are available

Økland

Havn

Thorstad

et al. 2019

Internat Rev Hydrobiol

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“…Will the effect of increased density be increased mortality, due to more competition for resources or more predation from older eels, or reduced growth due to displacement to lower quality habitats? At very low densities, other effects like Allee effects, depensatory mechanisms, changing sex ratios or life‐history strategies can also obscure the relationship (see references in Poole et al, 2018; Sandlund et al, 2017). One should also note that developing river‐wise stock‐recruitment models for European eel is not possible.…”

Section: Discussionmentioning

confidence: 99%

“…At some of the monitoring stations, like in the river Imsa, Norway, both upstream ascending elvers and downstream migrating silver eels are trapped and counted (Sandlund et al, 2017). The Imsa reference time series was started in the 1970s, and the age distribution and the population dynamics in this catchment were especially studied in the 1980s and 1990s, yielding fundamental knowledge on the ecology of European eels (Vøllestad & Jonsson, 1986, 1988; Vøllestad, Jonsson, Hvidsten, & Næsje, 1994; Vøllestad et al, 1986).…”

Section: Introductionmentioning

confidence: 99%

Age of European silver eels during a period of declining abundance in Norway

Durif

Diserud

Sandlund

et al. 2020

Ecology and Evolution

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The European eel (Anguilla anguilla) is critically endangered throughout its range. Knowledge about age distribution of future spawners (silver eels) is essential to monitor the status and contribute to the recovery of this species. Determination of age in anguillid eels is challenging, especially in eels from the northern part of the distribution area where growth is slow and age at maturation can be up to 30 years or more. Eels from the river Imsa in Norway have been monitored since 1975, and this reference time series has been used to assess the stock at the European level. Population dynamics in this catchment were analyzed during the late 1980s by estimating ages on whole cleared otoliths. However, techniques for revealing annual increments on otoliths have evolved over the years sometimes yielding significant differences in age estimates. In this study, the historical otolith data were reanalyzed using a grinding and polishing method rather than reading the whole otolith. The new age estimates were considerably higher than the previous ones, sometimes by up to 29 years. Since the 1980s, mean age of silver eels only slightly increased (from 19 to 21 years in the 2010s). This was mainly due to the disappearance of younger silver eels (<15 years) in the 2010s. The new age estimates agreed with the steep decline in recruitment which occurred in the late 1980s in the Imsa catchment. Mean growth (30 mm/year, min–max: 16–64 mm/year) has not changed since the 1980s, although density in the catchment has decreased. Revealing and reading age of slow‐growing eels remain a challenge but adding a measure of otolith reading uncertainty may improve age data collection and contribute to recovery measures for this species.

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“…In addition, the importance of environmental variables may change over time and space, further complicating the understanding of their relative impacts. Recent findings (Sandlund et al, 2017) concluded that the onset of migratory behaviour in two catchments from northwest Europe is triggered by the same environmental variables. However, there is no evidence that the variables implicated in this migration are the same in the southern part of the eel's range, especially due to differences in climate between regions and to the prevalence of females in most studies.…”

mentioning

confidence: 99%

Migration and escapement of silver eel males, Anguilla anguilla, from a southwestern European river

Monteiro

Domingos

Almeida

et al. 2020

Ecology of Freshwater Fish

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The catadromous life cycle of the European eel encompasses a spawning migration to the Sargasso Sea. Prior to their migration, eels develop a silver coloration being referred to as “silver eels.” Due to the dramatic European eel recruitment decline, it is crucial to quantify silver eel escapement to evaluate the success of measures taken under Eel Management Plans (Regulation EC (1100/2007)). Thus, the percentage of silver eels escaping from the River Mondego (Portugal) was estimated during two consecutive spawning seasons (2014–2016) and their migratory behaviour was studied. Thirty‐six male silver eels were tagged with acoustic transmitters and their downstream migration was tracked using automatic receivers deployed at four locations along the river course, from the first impassable obstacle up to the river mouth (67 km). Among the tagged eels, 15 (42%) escaped to the sea. In both years, individuals displayed faster migration speed in the downstream reaches of the river. BRT (“Boosted Regression Trees”) models were used to explain downstream migration and escapement of silver eels. The analyses revealed that the downstream migration is triggered by several environmental variables with water conductivity as the most influential predictor. Escapement of eels from the estuary occurred a few hours after their arrival and was primarily explained by the offshore wind intensity, which promotes a current in the coast that may favour their transport to the open sea. These findings contribute to a better understanding of silver eel migration in southern latitudes and highlight and reinforce the need to monitor spawner escapement.

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Timing and pattern of annual silver eel migration in two European watersheds are determined by similar cues

Cited by 42 publications

References 40 publications

Mortality of downstream migrating European eel at power stations can be low when turbine mortality is eliminated by protection measures and safe bypass routes are available

Mortality of downstream migrating European eel at power stations can be low when turbine mortality is eliminated by protection measures and safe bypass routes are available

Age of European silver eels during a period of declining abundance in Norway

Migration and escapement of silver eel males, Anguilla anguilla, from a southwestern European river

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