Reply to: Tuvikene et al. The inadequacies of estimating silver eel escapement by studying yellow eels caught in the coastal sea far away from the river mouth. A comment on Rohtla et al. (2021)

Abstract

European eel (Anguilla anguilla) is one of the few critically endangered (Pike et al. 2020) species that is still legally exploited by humans, despite the dramatic collapse of the stock (ICES 2023a). The long-standing scientific advice is to minimize all anthropogenic effects and, during the last three years, to stop eel fishing in all habitats (ICES 2023b). To halt or even reverse the current decline in eel numbers, a recovery plan containing multiple regulatory measures and actions was adopted in 2007 among EU member states and beyond (European Council 2007). Restocking eels from Western-European donor areas to, for example, Eastern-European recipient areas are one such action. Eel restocking is regarded as a conservation measure in the EU Council Regulation, but only when it contributes to the achievement of the 40% target level of silver eel escapement (European Council 2007). The true conservation value of eel restocking has not been confirmed, nor has the decline in eel abundance stopped or reversed (ICES 2023a). Multiple studies suggest that restocking eels may not contribute to population sustainability (e.g. Westin 2003, Sjöberg et al. 2009, 2017, Marohn et al. 2013, Stacey et al. 2015, Durif et al. 2022, Froehlicher et al. 2023). As a result of all of this, the debate over the conservation value of eel restocking continues.

In Estonia, eels are restocked to several far inland lakes in the Narva River Basin District (NRBD) to artificially sustain the local sub-population. This system, situated at the extreme eastern border of the Baltic distribution area of European eel, is under strong commercial and recreational fishing pressure (Armulik and Sirp 2021), includes a navigationally difficult out-migration route (Tambets et al. 2021a, b) and high Anguillicola crassus infection rates (Kangur et al. 2010), and ends with a hydro-power plant inflicting considerable eel mortality (Bernotas et al. 2019). The eel restocking programme began in 1956 as a compensation action after the construction of Ivangorod Hydropower Plant on the Narva River, which blocked the upstream migration of wild eels. Paradoxically, while the immigration of wild eels to the Narva River has ceased and the eel stock has collapsed, the stocking programme continues. In our study (Rohtla et al. 2021), we presented some evidence that the eel conservation goals for the NRBD are not being met and suggested that the estimate of silver eel escapement that is based on a demographic model for yellow eel in these far-inland lakes (Bernotas et al. 2020) is likely greatly overestimated due to large uncertainties in input data. The interpretations and recommendations of our study have been challenged by Tuvikene and colleagues.

We thank Tuvikene and colleagues for the progressive thinking and suggestions made to improve the silver eel escapement estimation in NRBD. However, we made the same suggestions in our article (e.g. ideally, silver eel escapement estimation should be conducted at the river mouth area in or near the sea). We stressed that there is a potential (unavoidable) sampling bias in our study, and that this needs to be addressed in future studies. We also specifically noted that further studies are needed to confirm our results and conclusions. Combined with the eel commercial landing data, our aims regarding restocking eels to NRBD were to question the validity of current silver eel escapement estimation, but also to spur discussion and encourage further research on the matter. Nonetheless, as we discuss here, a combination of different studies and data still strongly suggests that restocking eel to NRBD is not a conservation measure.

Tuvikene and colleagues had three major concerns about the data that we used to support our conclusion regarding the overestimation of the current silver eel escapement from NRBD: use of yellow eels from the Estonian coast, use of silver eels from Finnish coastal areas, and use of Estonian commercial fishing data. Regarding the first two, we have already admitted in our original study that these are potential limitations of our work. We would have gladly used silver eels sampled from the Narva River or Narva Bay. Unfortunately, there is no targeted eel fishing in these areas (nor in any coastal sea region in Estonia), despite the fact that eel is highly valued as a catch in Estonia. That being said, some eels from NRBD do descend to the Baltic Sea as yellow eels and some silver eels do revert back to the yellow eel stage, as we also saw in our data. This means that they do not avoid coastal waters, which is also true for silver eels (Pettai 1984), and are, therefore, accessible to fishers and scientific monitoring, because they were caught with eel fyke nets designed to catch yellow eels. According to the mark-recapture study by Järvalt et al. (2010), eels originating from NRBD are indeed being caught in the Estonian coastal sea, although only 0.8% of the marked eels (n = 861) were recaptured and reported in the Baltic Sea during the seven-year long study period. The study by Järvalt et al. (2010), however, does not indicate that the eels do not migrate north before they turn west.

Regarding the use of commercial fishing data, but also doubting the motivation and fishing skills of Estonian commercial fishers, Tuvikene and colleagues conflate eel availability with high eel numbers. They do this both for fresh water and brackish water. Of course, eels are able to escape NRBD and they are available at and near the Estonian coast. We do not doubt that. However, the central question here is how abundant they are at different locations in NRBD and coastal sea. Tuvikene and colleagues claim that the commercial fishers operating in the River Emajõgi, Lake Peipsi, Narva Reservoir, and River Narva do not fish for eel or use large-meshed traps and only catch eel as incidental by-catch. However, the same fishers have fished for eel successfully in the past when the abundance of eel was higher due to higher numbers of eel that were restocked to NRBD (Väino Vaino, personal observations). The market price for eel is the highest among all fishes in Estonia and eel would, therefore, make a nice profit for the fishers. Why would fishers outside the lakes that are restocked not fish for eel if catches from a single trap can reach tons as Tuvikene and colleagues claim? We argue that commercial eel fishing is just not economically sound anymore outside the lakes that are restocked, because there are too few eel.

It is known that recreational fishers fish for eel with hook and line in the River Emajõgi in May and June during the migration period. Eels migrate out of Lake Võrtsjärv also in late summer and autumn, but no significant recreational fishing for eel occurs then. Spring and early summer migrating eels are visually often silvery, but still continue feeding. However, this magnitude of eel recreational fishing does not carry over to the River Narva, where the same eels should arrive in a few weeks or later. Although hook and line fishing is also popular in the River Narva, and some eels are indeed caught (Järvalt et al. 2010), this is not comparable with the recreational eel fishing in the River Emajõgi. More generally, a recent survey estimated that up to 29 tons of eel were caught by recreational fishers in 2022 (www.kliimaministeerium.ee). Although the confidence intervals of this survey were wide due to low sample size, it is likely that a large majority of recreational eel landings come from the restocked lakes and the River Emajõgi.

The same argument applies for coastal waters of Estonia. Bottengaarns, special pound-nets designed for catching migrating silver eel, were still used until about 30 years ago in northern Estonia to catch migrating silver eels, as was done 100 years ago (Pettai 1984) and during the Soviet occupation (Sorokin and Härm 1953). None of this occurs today. Even in Narva Bay, which is a small area compared to the surface area of the entire Gulf of Finland and where (arguably) 65–86 tons of silver eel migrate annually, there is no commercial eel fishery. In fact, zero eel catches were reported from the ca 20 km coastal strip closest to the Narva River in the years 2018–2022, while the total eel catch for the entire Estonian side of the Gulf of Finland was 303 kg for the same period. We argue that the commercial fishers of Estonia, and especially of Narva Bay, would surely notice the annual 65–86 tons of silver eel passing by “at the place and time of their migration, e.g. near the mouth of the Narva River in October and November” if there were indeed that many eels. Although eel fishing is now banned in the Estonian coastal sea during the main migration period (various periods from September to March), this has only been in effect for a few years.

Tuvikene and colleagues also claim that our conclusions and interpretations, if used as a scientific basis for eel conservation and management, could lead to erroneous decision-making and, thus, harm the status of eels. However, it is not explained how underestimation of silver eel escapement can harm stock status. We can only see this happening when one considers restocking as an eel conservation measure because when 40% escaping silver eel biomass is not reached then restocking is not justified anymore, which leaves the “surplus glass eels” dying in the donor areas (a popular argument among the keen supporters of restocking), and this could harm the stock if this surplus is not restocked. However, the conservation value of eel restocking is not scientifically confirmed (see references above), nor is the claim that there is surplus glass eel mortality in some donor areas (Froehlicher et al. 2023). Rather, true problems arise when silver eel escapement is greatly overestimated, which further justifies the restocking and perhaps even encourages restocking larger numbers of eel. A recent study by Höhne et al. (2023) demonstrated that silver eel escapement estimates from a yellow-eel based demographic model overestimated the true escapement six-fold. And this was in a system where navigation to the sea is relatively easy because eel can follow the river flow discontinuously until they reach the sea. In Estonia, a similar demographic model is used (Bernotas et al. 2020) in a system where navigation to the sea is complex as also demonstrated by the maladaptive behaviour of the tagged eel (Tambets et al.  2021a, b).

To paraphrase Tuvikene and colleagues: Is it adequate to estimate the escapement of silver eel based on yellow eel data originating far inland as is the current practice in Estonia? We can also ask: Is it reasonable to continue restocking eel to far inland waters with high anthropogenic mortality, complex outmigration routes, and high A. crassus infection rate, while the species is in a fragile state? While acknowledging some potential and unavoidable sampling biases in our study, we stand by our general conclusions that silver eel escapement has most likely been greatly overestimated in NRBD and that restocking eels to NRBD is not a conservation measure.

Conflict of Interest

The authors have no conflicts of interest to declare.

Data availability

This article has no additional data.

References