Skip to main content
SLU publication database (SLUpub)
Report, 2021

Assessment of the eel stock in Sweden, spring 2021 : fourth post-evaluation of the Swedish eel management

Dekker, Willem; Van Gemert, Rob; Bryhn, Andreas; Sjöberg, Niklas; Wickström, Håkan

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. For the different assessment areas, results summarise as follows: On the west coast, a fyke net fishery on yellow eel was exploiting the stock, until this fishery was completely closed in spring 2012. A fishery-based assessment no longer being achievable, we present trends from research surveys (fyke nets), as in 2018. Insufficient information is currently available to assess the recovery of the stock in absolute terms. Obviously, current fishing mortality is zero (disregarding the currently unquantifiable effect of illegal fishing), but none of the other requested stock indicators (current, current potential and pristine biomass) can be presented. After years of decline, the research surveys now indicate a break in the decline of the stock. The formerly exploited size-classes of the stock show a recovery in abundance after the closure of the commercial fishery, and the smaller size classes show a break in their decline and a slight increasing trend in abundance in line with the recent trend of glass eel recruitment. In order to support the recovery of the stock, or to compensate for anthropogenic mortality in inland waters, young eel has been restocked on the Swedish west coast since 2010. Noting the quantity of restocking involved, the expected effect (ca. 50 t silver eel) is small, and hard to verify – in comparison to the potential natural stock on the west coast (an order of 1000 t). However, for the currently depleted stock, the contribution will constitute a larger share, and it might contribute to the recovery of the west coast stock. Executive summary For inland waters, this report updates the 2018 assessment, not making substantial changes in methodology. The assessment for the inland waters relies on a reconstruction of the stock from information on the youngest eels in our waters (natural recruits, assisted migration, restocking). Based on 75 years of data on natural recruitment into 22 rivers, a statistical model is applied which relates 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). The further into the Baltic, the larger and less numerous recruits are (with the exception of Mörrumsån, 56.4°N: 100 gr, where only 30 gr would be expected). Distance upstream comes with less numerous recruits, but size is unrelated. Remarkably, the time trend differs for the various ages/sizes. The oldest recruits (age up to 7 years) declined in abundance already in the 1950s and 1960s, but remained relatively stable since. The youngest recruits (age 0) showed a steep decline in abundance in the 1980s and little decrease before and after. In-between ages show in-between trends. Though this peculiar agerelated pattern has been observed elsewhere in Europe too, the cause of this is still unclear. Using the results from the above recruitment analysis, in combination with historical data on assisted migration (young eels transported upstream within a drainage area, across barriers) and restocking (young eels imported into a river system), we have a complete overview of how many young eels recruited to Swedish inland waters. From this, the production of fully grown, silver eel is estimated for every lake and year separately, based on best estimates of growth and natural mortality rates. Subtracting the catch made by the fishery (as recorded) and down-sizing for the mortality incurred when passing hydropower stations (percentwise, as recorded or using a default percentage), an estimate of the biomass of silver eel escaping from each river towards the sea is derived. Results indicate, that since 1960, the production of silver eel in inland waters has declined from over 500 to below 300 tonnes per annum (t/a). The production of naturally recruited eels is still falling; following the increase in restocking since 2010, an increase in restocking-based production is expected to occur in the near future. Gradually, restocking has replaced natural recruitment (assisted and fully natural), now making 90 % of the inland stock. Fisheries have taken 20-30 % of the silver eel (since the mid-1980s), while the impact of hydropower has ranged from 20 % to 60 %, depending on the year. Escapement is estimated to have varied from 25 % of the pristine level (100 t) in the late 1990s, to 50 % (200 t) in the early 2000s. The biomass of current escapement (including eels of restocked origin) is approximately 15 % of the pristine level (incl. restocked), or almost 30 % of the current potential biomass (incl. restocked). This is below the 40 % biomass limit of the Eel Regulation, and anthropogenic mortality (nearly 70 % over the entire life span in continental waters) exceeds the limit implied in the Eel Regulation (60 % mortality, the complement of 40 % survival). Mortality being that high, Swedish inland waters currently do not contribute to the recovery of the stock. The temporal variation (in production, impacts and escapement) is largely the consequence of a differential spatial distribution of the restocking of eel over the years. The original natural (not assisted) recruits were far less impacted by hydropower, since they could not climb the hydropower dams when immigrating. Until about 2009, restocking has been practised in freely accessible lakes (primarily Lake Mälaren, 1990s), but is since 2010 concentrated to drainage areas falling to the Kattegat-Skagerrak, thus also including 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 1-5 % of silver eel to the escapement biomass. Without restocking, the biomass affected by fishery and/or hydropower would be only 10-15 % of the currently impacted biomass, but the stock abundance would reduce from 20 % to less than 5 % of the pristine biomass. In summary: the inland eel stock biomass is below the minimum target, anthropogenic impacts exceed the minimum limit that would allow recovery, and those impacts are currently increasing. It is therefore recommended to reconsider the current action plans on inland waters, taking into account the results of the current, comprehensive assessment. For the Baltic coast, the 2018 assessment has been updated without major changes in methodology. Minor changes include the censoring of foreign-recaptured tagged eel (treating them as though they were not captured) so as to only describe the impact of the Swedish eel fishery, and complementing the decadal estimates with triannual estimates. Results indicate that the impact of the fishery continues to decline over the decades – even declining more rapidly within the 2010s than before. The current impact of the Swedish silver eel fishery on the escapement of silver eel along the Baltic Sea coast is estimated at 1 %. However, this fishery is just one of the anthropogenic impacts (in other areas/countries) affecting the eel stock in the Baltic, including all types of impacts, on all life stages and all habitats anywhere in the Baltic. Integration with the assessments in other countries has not been achieved. Current estimates of the abundance of silver eel (biomass) indicates an order of several thousand tonnes, but those estimates are extremely uncertain, due to the low impact of the fishery (near-zero statistics). Moreover, these do not take into account the origin of those silver eels, from other countries. An integrated assessment for the whole Baltic will be required to ground-truth these estimates. This would also bring the eel assessments in line with the policy to regionalise stock assessments for other (commercial) fish species (see https://ec.europa.eu/oceansand-fisheries/fisheries/rules/multiannual-plans_en). It is recommended to develop an integrated assessment for the Baltic eel stock, and to coordinate protective measures with other range states.

Keywords

Sweden; eel; stock; status; tri-annual; assessment; protection; recovery

Published in

Aqua reports
2021, number: 2021:12
eISBN: 978-91-576-9874-2
Publisher: Department of Aquatic Resources, Swedish University of Agricultural Sciences