Assessment of the eel stock in Sweden, spring 2015

• Main Author: Dekker, Willem
• Format: Report
• Language: English
• Published: 2015
• Subjects: Fish and Aquacultural Science; Anguilla anguilla
• Online Access: https://pub.epsilon.slu.se/12446/; https://pub.epsilon.slu.se/12446/7/dekker_w_150722.pdf

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.