The eel population in the Netherlands is increasing slightly but is still in poor condition, with too high a mortality rate and too low a biomass.
The results of the most recent eel assessment by Wageningen Marine Research covering the period 2014-2016 show that since the introduction of the Dutch eel management plan in 2009, mortality caused by human activities has fallen sharply but that silver eel migration remains much lower than the European objective. The current biomass of migrating silver eel, at 1,795 tonnes, remains below the target of 4,160 tonnes for the Netherlands.
Too low silver eel migration
Since the 1980s, glass eel migration and the eel population in Europe have declined dramatically. Eel are listed on the IUCN Red List as critically endangered. For years, the International Council for the Exploration of the Sea (ICES) has been advising that all human activities that reduce the production and migration of adult eel (silver eel) to the sea, such as fishing and fish migration bottlenecks, should preferably be reduced to zero. Adult eel (silver eel) migrate from inland to their reproduction area in the Sargasso Sea. To enable the recovery of the eel population, the European Union drew up the Eel Regulation in 2007, which obligates member states to draw up a national eel management plan. The European objective is that of the estimated possible biomass of silver eel, at least 40% can escape to the sea. In order to determine how much silver eel escapes with respect to the objective, Wageningen Marine Research has used models and survey and catch data to estimate the biomass of yellow eel and silver eel. In the Netherlands, the best possible biomass of silver eel (or pristine biomass) is estimated at 10,400 tonnes of silver eel in fresh water. The objective of the management plan is that in time, 40% (4,160 tonnes) of this best possible biomass of silver eel migrates to the reproduction area in the Sargasso Sea. This objective is not (yet) being met and is estimated at 1,795 tonnes in 2014-2016.
To estimate the biomass of eel in the Netherlands, an eel survey is carried out on the IJsselmeer. Good survey data is not available for a number of other large lakes (Markermeer, Grevelingenmeer, Randmeren). It is therefore assumed that the fishing mortality in these lakes is the same as that for the IJsselmeer. Together with local catch data, an estimate of the biomass in these lakes is made. In the IJsselmeer and Markermeer, a decline in biomass was measured after 2005-2007. From 2008 to 2016 the estimated biomass remained more or less stable.
In other national and regional waters (other small waters such as ditches, ponds, smaller rivers and canals), the biomass is estimated by surveys with an electroscopic net. To estimate the biomass, these surveys are scaled up to the surface of the waters. The biomass of eel in the national waters was found to have increased in 2014-2016. This increase is almost entirely due to an increase in the Lower Rivers. The largest eel biomass is in smaller waters such as ditches, ponds, smaller rivers and canals, the so-called regional waters. The total biomass of eel in the Netherlands therefore appears to have remained stable or increased slightly in recent years.
The eel evaluation also looked at eel mortality caused by human activity (anthropogenic mortality). After 2007, there is a clear decline in the anthropogenic mortality. Until 2007, this mortality was 81%. In the period 2014-2016, this mortality decreased to 49%. In particular, catches from commercial and recreational fishing have fallen sharply in recent years. Also the silver eel mortality during migration through barriers such as pumping stations, hydroelectric power stations and sluices has decreased from 20% to 18% in recent years. This barrier mortality seems a small decrease, but it requires large investments by the government (Rijkswaterstaat) and other managers (water boards). There is a lot of public attention for the removal of migration barriers for the intake of fish and to a lesser extent for the departure of fish (such as silver eel).
Eel regulation under scrutiny
Eel is a long-lived species. Measures only have an effect in the long run. Years pass before a glass eel eventually becomes a silver eel and migrates back to sea to provide a new generation. It remains uncertain whether the measures taken will lead to a good eel stock in the long term because it is not certain that all the factors causing the decline in eel stocks are known.
The European Commission has decided to evaluate the 2007 EU eel regulation and to see whether the member states have implemented the eel management plans properly. Based on the findings, it will be decided whether the Eel Regulation needs to be amended or whether the national management plans need to be implemented better.