Models to describe the stock dynamics of the endangered European eel, Anguilla anguilla, are needed on river system level to develop conservation management strategies. For these models, estimations ...of age and growth of local eel stocks are important prerequisites. To investigate the variation of these two parameters I collected 160 European silver eels grouped by 100mm size classes at capture from the lower part of the Elbe River in Germany during fall 2011. Length of collected eels ranged from 360mm to 957mm and age from 7 years to 23 years. Mean age and growth, estimated by otolith increments, were higher for females (14 years, growth 52mmyear−1) than for males (13 years, 35mmyear−1). Larger females were older and grew faster as compared to smaller females. Across all size classes at capture, the weighted mean age and annual length increment based on the percentage of eels per size class of the migrating silver eels were 13 years and 50mmyear−1, respectively. The parameters of the von Bertalanffy growth model L∞, k and t0 for silver eels are given, which can be used to model the dynamics of the eel stock of the Elbe River system.
Fish marking is an essential tool for fisheries management, especially for evaluating the stocking of endangered fish species to support conservation and sustainable use of fish stocks. Batch marking ...of young European eels Anguilla anguilla (L.) prior to stocking is recommended as the benefits of stocking for the spawning stock can be evaluated by recapturing marked fish over time, therefore mass marking of young eels with substances such as alizarin red S (ARS) is becoming increasingly important. To improve the marking method and reduce marking costs when immersing glass eels in an ARS solution, eight laboratory experiments under varying conditions (e.g., temperature, ARS concentration, immersion time, osmotic induction, fish density) and with ARS from different suppliers were carried out. The results show that optimal marking of glass eels can be carried out in the field or during transport by putting approximately 50 g of glass eels per liter in 150 mg L−1 ARS solution for 3 h at 10–15°C. Lower concentrations did not result in reliable marking. Water temperatures of 5°C and below can have a stunning effect on the eels and increase mortality significantly, regardless of the concentration of ARS. Glass eel densities below 50 g L−1 in the marking bath increase marking costs unnecessarily, while a higher density of 100 g L−1 resulted in significantly higher mortality and lower marking success. A somewhat more difficult but less expensive alternative is to bathe the fish in a saline solution of 1% (10 PSU) of 80 mg L−1 ARS for 3 h at 10°C. Costs can also be significantly reduced by choice of supplier for ARS, but care should be taken as the quality of the powder appears to vary (mean percentage of sufficiently marked eels ranged from 59% to 91% among suppliers in the present study) and can lead to marking failure. The optimal marking conditions can help ensure that stocked glass eels can be reliably identified in future studies to assess stocking benefits while reducing costs.
The critical status of the catadromous European eel is addressed with European wide conservation efforts. One option is conservation-oriented restocking where juvenile glass eels are caught upon ...their arrival at the European continent and transferred to waters with no or limited natural recruitment caused in part by fragmented migration routes. In light of the critical stock status, conservation oriented restocking activities needs to be conducted in a careful and informed manner. Due to warmer winters and better glass eel availability, glass eels have increasingly been restocked in Germany during winter in recent years, and not as usual in spring. However, it is not known if restocking in winter reduces glass eel survival due to colder temperatures and limited food availability compared to the traditional restocking approach. In this study, growth and survival rates for glass eels restocked in winter were estimated in five isolated lakes. The generated data were compared to values of a previous study where glass eels were restocked in the same lakes during spring. This comparison indicates that glass eels restocked in winter had similar survival rates (19–45%) and growth performance to those released in spring. Accordingly, restocking glass eels in the winter is a suitable alternative compared to traditional release in spring.
•Glass eels restocked in winter in small lakes show survival rates of 19–45% 3 – 4 years after restocking.•Glass eels restocked in winter had similar survival rates and growth performance to those released in spring.•Restocking glass eels in the winter is a suitable conservation alternative compared to traditional release in spring.
Recent developments in tracking technology resulted in the mapping of various marine spawning migration routes of the European eel (Anguilla anguilla). However, migration routes in the North Sea have ...rarely been studied, despite many large European rivers and hence potential eel growing habitat discharge into the North Sea. In this study, we present the most comprehensive map to date with migration routes by silver European eels in the North Sea and document for the first time successful eel migration through the English Channel. Migration tracks were reconstructed for 42 eels tagged in Belgium and 12 in Germany. Additionally, some eels moved up north to exit the North Sea over the British Isles, confirming the existence of two different routes, even for eels exiting from a single river catchment. Furthermore, we observed a wide range in migration speeds (6.8-45.2 km day
). We hypothesize that these are likely attributed to water currents, with eels migrating through the English Channel being significantly faster than eels migrating northward.
The spawning migration of the European eel (
L.) to the Sargasso Sea is one of the greatest animal migrations. However, the duration and route of the migration remain uncertain. Using fishery data ...from 20 rivers across Europe, we show that most eels begin their oceanic migration between August and December. We used electronic tagging techniques to map the oceanic migration from eels released from four regions in Europe. Of 707 eels tagged, we received 206 data sets. Many migrations ended soon after release because of predation events, but we were able to reconstruct in detail the migration routes of >80 eels. The route extended from western mainland Europe to the Azores region, more than 5000 km toward the Sargasso Sea. All eels exhibited diel vertical migrations, moving from deeper water during the day into shallower water at night. The range of migration speeds was 3 to 47 km day
. Using data from larval surveys in the Sargasso Sea, we show that spawning likely begins in December and peaks in February. Synthesizing these results, we show that the timing of autumn escapement and the rate of migration are inconsistent with the century-long held assumption that eels spawn as a single reproductive cohort in the springtime following their escapement. Instead, we suggest that European eels adopt a mixed migratory strategy, with some individuals able to achieve a rapid migration, whereas others arrive only in time for the following spawning season. Our results have consequences for eel management.
Simon, J. 2007. Age, growth, and condition of European eel (Anguilla anguilla) from six lakes in the River Havel system (Germany). – ICES Journal of Marine Science, 64: 1414–1422. A total of 199 ...female yellow European eels (Anguilla anguilla), 21.6–66.2 cm long and 3–14 years old, was collected by electro-fishing from six lakes in the River Havel system (Germany) in spring 2001. The condition and the growth rate, estimated by otolith increments, varied between eels within single lakes and between lakes. Fulton's condition factor ranged from 0.10 to 0.24 and the gross energy content varied between 4.3 and 15.3 MJ kg−1. There were no significant differences in mean condition factor (0.16–0.18) or gross energy content (6.5–9.3 MJ kg−1) between lakes. Fastest growth was in Lake Blankensee (mean 5.3 cm year−1), and the slowest in Lake Sacrow (mean 4.0 cm year−1). For all lakes combined, the overall mean annual increment was estimated to be 4.5 cm year−1. The biggest annual increment on the otoliths was generally laid down during the first and second years in fresh water, when the growth rate was 6.1–8.5 cm year−1. Then, in the subsequent 12 years, the annual increment remained almost constant or decreased slightly (with lake-dependent values of between 1.6 and 6.8 cm year−1). In the River Havel system, the time between stocking of the lakes with glass eels and the recapture of eels at 45 cm body length was 7–10 years. The physiologically possible maximum length (L∞ values) of eels lay in the range 50–130 cm. In comparison with previous investigations (between the 1950s and the 1970s), the only difference observed was a trend towards slower growth.
European eels Anguilla anguilla stocked as wild‐sourced glass eels showed a better overall performance of growth and survival compared with farm‐sourced eels after stocking in five isolated lakes ...within a 7‐year study period. Eels stocked as farm eels lost their initial size advantage over eels stocked as glass eels within 3–5 years after stocking. Population sizes estimated for consecutive stocking batches indicated that 8–17% of eels stocked as farm eels survived 3–6 years after stocking compared with 5–45% of eels stocked as glass eels. This study coupled with results of previous studies suggests that stocking of farm eels may have no advantage in growth and survival compared with stocking of glass eels if stocking occurs at an optimal time in spring. In addition, the use of relatively expensive farm eels may provide no general advantage over stocking of glass eels. However, if glass eels are only available for stocking purposes very early in the year, lower survival rates than obtained in the present study can be assumed and stocking with relatively more expensive farm eels could possibly be a better option.
•Mark retention of alizarin red S in European eels was above 95% after 1–14 years.•Tag retention of coded wire tags in European eels was above 95% after 1–14 years.•Both marking methods are suitable ...for lifelong marking and tagging of eels.
Durable marking of fish is an important prerequisite in many fish-ecological studies. In the course of a mark-recapture experiment with small European eels stocked in lakes, marking success rates above 95% were observed for coded wire tags and for alizarin red S marks 1–14 years after stocking. Both marking methods are therefore considered suitable for lifelong marking and tagging of eels, and application in long-term mark-recapture experiments.
Gravel pit lakes are common across Europe. These novel ecosystems serve as model systems to study human‐induced and natural colonisation of isolated lakes by fish. Fisheries‐management activities can ...quickly spread species over large distances, possibly homogenising fish communities across ecosystems, while fostering local fish diversity.
Our objective was to evaluate the effects of lake genesis (gravel pit lakes < 100 years old vs. natural lakes of glacial genesis ~10,000 years old) and fisheries management (fish stocking activities present vs. absent) on the fish community in small lakes, while controlling for key environmental variables known to affect lake fish communities.
We sampled fish communities by electrofishing and multimesh gillnetting in 47 isolated lakes managed for fisheries, and 19 unmanaged and isolated lakes of both natural and artificial origin in northern Germany. Unmanaged lakes were used as reference to assess fisheries‐management impacts in small natural and artificial lakes.
We caught 178,506 fish from 30 species and found that the accumulation of native lake fish species in lakes was associated with fisheries management, which increased local species richness (α‐diversity) and number of predatory species, and reduced among‐lake variation in fish community composition (β‐diversity; i.e., homogenisation). The homogenisation‐effect associated with fisheries happened with introduced native fish species, whereas non‐native species were rarely detected.
In unmanaged gravel pit lakes, the littoral fish community composition was substantially different to the communities present in both types of managed lakes and unmanaged natural lakes. Therefore, the relatively young unmanaged gravel pit lakes revealed evidence of ongoing, stochastic colonisation processes that resulted in comparatively species‐poor fish communities.
We concluded that fisheries management by anglers speeds up the colonisation of gravel pit lakes with native fish species in the study area. For planning initial fish introductions in newly created gravel pit lakes, it is recommended that fish communities from ecologically similar natural lakes within the same geographical region are used as references to maintain the biotic integrity of newly created fish communities.
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