| 11. |
- Dekker, Willem
(author)
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Assessment of the eel stock in Sweden, spring 2015 : second post-evaluation of the Swedish Eel Management Plan
- 2015
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Reports (other academic/artistic)abstract
- The population of the European eel Anguilla anguilla (L.) is in severe decline. In 2007, the European Union decided on a Regulation establishing measures for the recovery of the stock, which obliged Member States to implement a national Eel Management Plan by 2009. Sweden submitted its plan in 2008. According to the Regulation, Member States will report to the Commission every third year, on the implementation of their Eel Management Plans and the progress achieved in protection and restoration. The current report provides an assessment of the eel stock in Sweden as of spring 2015, intending to feed into the national reporting to the EU; this updates and extends the report by Dekker (2012). In this report, the impacts on the stock are assessed - of fishing, restocking and of the mortality related to hydropower generation. Other anthropogenic impacts (climate change, pollution, spread of parasites, disruption of migration by transport, and so forth) probably have an impact on the stock too, but these factors are hardly quantifiable and no management targets have been set. For that reason, and because these factors were not included in the EU Eel Regulation, these other factors are not included in this technical evaluation. Our focus is on the quantification of biomass of silver eel escaping (actual, potential and pristine) and mortality endured by those eels during their lifetime. The assessment is broken down on a regional basis, with different impacts dominating in different areas. In recent years, a break in the downward trend of the number of glass eel has been observed throughout Europe. Whether that relates to recent protective actions, or is due to other factors, is yet unclear. This report contributes to the required international assessment, but does not discuss that recent recruitment trend and the overall status of the stock. On the west coast, a fykenet fishery on yellow eel was overexploiting the stock, until this fishery was completely closed in spring 2012. Though research surveys using fykenets continued, insufficient information is currently available to assess the recovery of the stock. Obviously, current fishing mortality is zero, but no other stock indicators can be presented. It is recommended to develop a comprehensive plan for monitoring the recovery of the stock. In order to support the recovery of the stock, or to compensate for mortality elsewhere, young eel has been restocked on the west coast. No follow-up monitoring has been established. Noting the small expected effect – in comparison to the potential natural stock on the west coast, when recovered – it is recommended to reconsider this programme, or to set up adequate follow-up monitoring. For inland waters, this report presents a major update of the 2012 assessment. In the 2012 assessment, eel production estimates were based on information from past restocking, but natural recruitment and assisted migration were ignored; these have now been included. Additionally, the impact of hydropower is now assessed in a spatially explicit reconstruction. Based on 75 years of data on natural recruitment into 24 rivers, a statistical model is developed relating the number of immigrating young eel caught in traps to the location and size of each river, the distance from the trap to the river mouth, the mean age/size of the immigrating eel, and the year in which those eels recruited to continental waters as a glass eel (year class). Further into the Baltic, recruits are larger (exception: the 100 gr recruits in Mörrumsån, 56.4°N, where only 30 gr would be expected) and less numerous; distance upstream comes with less numerous recruits, but size is not related. Remarkably, the time trend differs for the various ages/sizes. Oldest recruits (age up to 7) declined already in the 1950s and 1960s, but remained stable since; youngest recruits (age 0) showed a steep decline in the 1980s and a little decrease before and after. In-between ages show in-between trends. Though this peculiar age-related pattern has been observed at other places in Europe too, the cause of this is still unclear. Using the results from the recruitment model, in combination with historical data on assisted migration (young eels transported upstream, across barriers) and restocking (imported young eels), the production of fully grown, silver eel is estimated for every lake and year separately. Subtracting the catch made by the fishery and down-sizing for the mortality incurred when passing hydropower stations, an estimate of the biomass of silver eel escaping from each river towards the sea is derived. Since 1960, the production of silver eel in inland waters has declined from 500 to 300t/a, and natural recruitment (assisted or not) has gradually been replaced by restocking for 90%. Fisheries have taken just over 30% of the silver eel, while the impact of hydropower has ranged from 20% to 60%. Escapement is estimated to have varied from 10% (35t) in the late 1990s, to 30% (100t) in the 2010s. The biomass of current escapement (including eels of restocked origin) is approx. 1015% of the pristine level, that is 28% of the current potential. This biomass is below the 40% limit of the Eel Regulation, and anthropogenic mortality exceeds both the short-term limit establishing recovery (15%) and the ultimate limit (60% mortality, the complement of 40% survival). The temporal variation (in production, impacts and escapement) is largely the consequence of a differential spatial distribution of the restocked eel over the years. Natural (not assisted) recruits were far less impacted by hydropower, since they could not climb the hydropower dams when immigrating. Later, restocking has been practised in unobstructed lakes (primarily Lake Mälaren, 1990s), and is now concentrated in obstructed lakes (primarily Lake Vänern, to a lesser extent Lake Ringsjön, and many smaller ones). Trap & Transport of silver eel - from above barriers towards the sea - has added 1-6% of silver eel to the escapement. Without restocking, the biomass affected by fishery and/or hydropower would be only 10% of the currently impacted biomass, but the stock abundance would reduce from 10% to less than 2only 3% of the pristine biomass. It is recommended to reconsider the current action plans on inland waters, to take into account the results of the current, more comprehensive assessment. It is further recommended to ground-truth the current assessment on independent stock surveys. For the Baltic coast, the 2012 assessment has been updated, using information from re-continued mark-recapture experiments. Results indicate that the impact of the fishery is rapidly declining over the decades – even declining more rapidly towards the 2010s than before. The current impact of the Swedish silver eel fishery is estimated at 2%. However, this fishery is just one of the anthropogenic impacts (in other areas/countries) affecting the Baltic eel stock. Integration with the assessments in other countries has not been achieved. Current estimates of the abundance of silver eel (biomass) are in the order of a few thousand tonnes, but these estimates are highly uncertain due to the low values for catch and mortality (near-zero estimation problems). An integrated assessment for the whole Baltic will be required to ground-truth these estimates. It is recommended to develop an integrated assessment for the Baltic eel stock, and to coordinate protective measures with other range states. Considering the international context, the stock indicators – in as far as they could be assessed – fit the international assessment framework, but inconsistencies and interpretation differences at the international level complicate their usage. International coordination and standardisation of the tri-annual reporting is therefore recommended. Additionally, it is recommended to initiate international standardisation/inter-calibration of monitoring and assessment methodologies among countries, achieving a consistent and more cost-effective assessment across Europe.
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| 12. |
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| 13. |
- Dekker, Willem, et al.
(author)
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Assessment of the eel stock in Sweden, spring 2021 : fourth post-evaluation of the Swedish eel management
- 2021
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Reports (peer-reviewed)abstract
- For decades, the population of the European eel has been in severe decline. In 2007, the European Union decided on a Regulation establishing measures for the recovery of the stock, which obliged Member States to implement a national Eel Management Plan by 2009. Sweden submitted its plan in 2008. According to the Regulation, Member States shall report regularly to the EU-Commission, on the implementation of their Eel Management Plans and the progress achieved in protection and restoration. The current report provides an assessment of the eel stock in Sweden as of spring 2021, intending to feed into the national reporting to the EU later this year. This report updates and extends the reports by Dekker (2012, 2015) and Dekker et al. (2018).In this report, the impacts on the stock - of fishing, restocking and mortality related to hydropower generation - are assessed. Other anthropogenic impacts (climate change, pollution, increased impacts of predators, spread of parasites, disruption of migration due to disorientation after transport, and so forth) probably have an impact on the stock too, but these factors are hardly quantifiable and no management targets have been set. For that reason, and because most factors were not included in the EU Eel Regulation, these other factors are not included in this technical evaluation. Our focus is on the quantification of biomass of silver eel escaping from continental waters towards the ocean (current, current potential and pristine) and mortality risks endured by those eels during their whole lifetime. The assessment is broken down on a geographical basis, with different impacts dominating in different areas (west coast, inland waters, Baltic coast).In 2011, a break in the downward trend of the number of glass eel was observed throughout Europe, the trend since being upward, but erratic. Whether that relates to recent protective actions, or is due to other factors, is yet unclear. This report contributes to the required international assessment, but does not discuss the causing factors behind that recent trend and the overall status of the stock across Europe.
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| 14. |
- Dekker, Willem, et al.
(author)
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Assessment of the fishing impact on the silver eel stock in the Baltic using survival analysis
- 2013
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In: Canadian Journal of Fisheries and Aquatic Sciences. - : Canadian Science Publishing. - 0706-652X .- 1205-7533. ; 70:12, s. 1673-1684
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Journal article (peer-reviewed)abstract
- Restoration of the depleted stock of the European eel (Anguilla anguilla (L.)) requires anthropogenic impacts to bequantified, reduced, and controlled. In this article, we assess the impact of the silver eel fishery on the Baltic Coast in Sweden, applying survival analysis to 60 years of mark–recapture experiments, involving 8000 recaptures out of 18 000 releases. Thehazard of being recaptured (overall 46%) varies along the coast and declined substantially over the decades. But, most notably,the hazard for the individual diminishes strongly after the first kilometres en route. This individualized hazard disqualifies themore traditional mark–recapture methodology, which assumes random recaptures. We advocate the general use of survivalanalysis for conventional mark–recapture data. The result of our analysis indicates that the impact of the fishery just prior the2009 fishing restrictions was in the order of 10%—in itself well within sustainability limits, though only but one of the factors contributing to the mortality in the Baltic Sea.
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| 15. |
- Dekker, Willem
(author)
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Chinese eel products in EU markets imply the effectiveness of trade regulations but expose fraudulent labelling
- 2021
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In: Marine Policy. - : Elsevier BV. - 0308-597X. ; 132
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Journal article (peer-reviewed)abstract
- The stock of the European eel (Anguilla anguilla) is in a multi-decadal decline. Therefore, trade in European eel is now restricted by EU law and the listing in CITES Appendix II. EU law prohibits the trade of European eel across the EU’s outer border and CITES regulates the global trade elsewhere. In November and December 2019, we purchased 108 eel products in 21 cities in five major eel importing countries in Europe (Germany, Belgium, Netherlands, Great Britain, France) and three online shops. All were imported from China and 73 samples were genetically identified as American eel (A. rostrata), 33 as Japanese eel (A. japonica), and a single sample each as European eel and Indian shortfin eel (A. bicolor pacifica). The one European eel sample violated the EU trade ban and CITES trade regulations. However, 28.7 % of the product labels violated EU Regulation (EU) No 1169/2011 on the provision of food information to consumers (FIC). Our results imply that Chinese exporters sell their European eel products outside the EU market and therefore avoid violating EU law. However, fraudulent labelling point at inadequate existing EU labelling requirements for prepared and preserved products and ongoing molecular genetic control of eel commodities entering the EU from China.
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| 16. |
- Dekker, Willem
(author)
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Climbing back up what slippery slope? Dynamics of the European eel stock and its management in historical perspective
- 2016
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In: ICES Journal of Marine Science. - : Oxford University Press (OUP). - 1054-3139 .- 1095-9289. ; 73, s. 5-13
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Journal article (peer-reviewed)abstract
- Fewfish stocks are as influenced by (intentional and inadvertent) human impacts as the European eel, all across the continent. The dynamics of this stock, however, are poorly understood—neither the causes of the historically lowabundance, nor minimal protection levels, are beyond discussion. Rather than analysing contemporary processes, this article turns back in time—two centuries or more—unravelling historical abundances and distribution patterns; reviewing historical actions and objectives; and discussing technical developments and scientific advice—picturing the slippery slope the eel stock has come down from. The first claim, that the continental stock was in decline, dates from the early 1800s; stock-enhancement actions were initiated shortly after. Diffuse objectives, technical innovations, eternal optimism, and—above all—no quantification impede the exact evaluation of historical reports. After 1950, when quantification improved, a slow but consistent decline was observed, but it is only two decades after the crash in glass eel recruitment (in 1980), that protection plans addressed the bad status of the stock. A slippery slope, full of pitfalls—yet, we now observe several years of increasing recruitment.
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| 17. |
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| 18. |
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| 19. |
- Dekker, Willem
(author)
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Faire mieux que la nature? The History of Eel Restocking in Europe
- 2016
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In: Environment and History. - 0967-3407. ; 22, s. 255-300
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Journal article (peer-reviewed)abstract
- Young eel, recruiting from the ocean towards Europe, are most abundant along the Atlantic coast of France. Since 1840, attempts have been made to redistribute them from the areas of highest abundance to other countries and farther inland. This 'restocking' has been troubled by technical constraints (e.g. mode of transport and maximum distance eel can be shipped alive), wars (e.g. the Franco Prussian War and World Wars One and Two) and, in recent decades, by shortage of supply due to the general decline of the eel stock all across Europe. Though objectives and procedures have changed considerably over the years, the recurring aim has been to increase production and, in that way, to 'faire mieux que la nature'. We document the historical development of these efforts from their inception, and contrast the achievements to the objectives. Except for the 1952-1990 period in Eastern Europe, restocking has probably added only slightly to the natural production. As successful as restocking might have been locally, it has not markedly changed the overall trends and distribution patterns or halted the general decline of the stock and fishery. Poor post-evaluation, frequent technical innovation and a constant renewal of the countries and people involved have kept the promise of a better future alive for 175 years.
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| 20. |
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