Glass eels and viruses - a lesson learnt from stocking the eastern German Baltic Sea coast

Concerns about Anguillid herpesvirus 1 (AngHV-1) in European eels, especially due to stocking measures, is increasingly coming into focus and raises questions regarding disease monitoring and prevention. In the past, stocking of AngHV-1-positive eels into waters assumed AngHV-1-free has led to a rapid increase of infected eels in the wild. For this study, a glass eel stocking experiment was conducted in the eastern German Baltic coast from 2014 - 2016. Retrospective analysis of stocked glass eels shows, that virus prevalence varied from 5.3 - 37.4 % during stocking years and appeared to be influenced by a prolonged holding period in the catching region. On average, 16 % glass eels were AngHV-1 positive. Given that stocked eels were alizarin red s marked, it was possible to monitor the local eel stock between 2018 and 2019 for AngHV-1 infection and additionally two commonly coinfections, namely European Virus Eel (EVE) and Europe eel virus X (EVEX), of marked and not marked eels. In recaptured eels, mortality was significantly higher in the glass eel cohort which was found to be heavily AngHV-1 infected (stocked in 2014) compared to the less infected cohorts. Furthermore, virus-positive stocked eels turned out to be disadvantaged compared to non-infected conspecifics in terms of total length and fat content. However, co-infections with EVE and EVEX could not be detected in this study. In conclusion, eel conservation and maintaining an economically successful eel fishery is probably best achieved by stocking eels that have been shown to be free of AngHV-1. This article is protected by copyright. All rights reserved.

Identification of virus infections of European eels intended for stocking measures

The spread of viral diseases in eels is suggested to severely affect the European eel (Anguilla anguilla) panmictic population. The European Commission has initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to return the European eel stock to more sustainable levels within that measures eel restocking. However, scientific evidence evaluating the efficacy of stocking remains scarce. In addition, knowledge about the impact and contribution of eel stocking on the distribution of infectious diseases is insufficient. In this study, we aimed to investigate virus infections in batches of eels intended for restocking. We analysed samples of glass eels from certified fisheries and farmed European eels from different aquaculture farms. All analysed eels were purchased within a North Rhine Westphalian conservation program. Via a combination of cell culture and qPCR-based techniques, we detected infections of glass eels with the rhabdovirus Eel Virus European X and anguillid herpesvirus 1 infections in farmed eels (10-15 cm).

A complete check-up of European eel after eight years of restocking in an upland river: Trends in growth, lipid content, sex ratio and health status

By combining field research and careful laboratory analysis of samples over the course of an eight-year study, we met the challenge of assessing the life history traits and health status of eels restocked in freshwater ecosystems. We found that restocked eels exhibited good growth performance; moreover, the stocks were female-dominated, showed a good Fulton's condition factor (K) and lipid stores and had high survival probability estimated using the best model of Jolly-Seber stock assessment method for open populations. A necropsy revealed the absence of internal lesions. A swim bladder examination revealed the absence of the parasite Anguillicola crassus. Polymerase chain reaction (PCR) analyses revealed an increase of Anguillid herpesvirus-1 (AngHV-1) prevalence throughout the study. Most positive subjects expressed viral loads compatible with a latent infection and correlated positively with K. All restocked eels were contaminated by at least one of the organic pollutant congeners studied, but the pollution loads corresponded to the lowest range of pollutant concentrations reported in the available literature for European eels and did not exceed the maximum residue and contaminant limits in food and feed of several national and international regulations. Pollutant loads were negatively correlated with K, lipid content and eel density for polychlorinated biphenyls PCB 138, 153 and 180 and K for pesticides p.p'-DDE, p.p'-DDD, p.p'-DDT and PBDE47. This study highlights the potential role played by upland aquatic ecosystems in enhancing riverine silver eel production from the perspective of species conservation. To be successful, restocking must be accompanied by improved ecosystem quality and migration routes for eels in inland freshwaters. We also provide some recommendations for future research to improve the management of restocking programmes.

Detection of Herpesvirus anguillae (AngHV-1) in European eel Anguilla anguilla (L.) originating from northern Poland-assessment of suitability of selected diagnostic methods

The Community Action Plan requests EU member states to implement measures that ensure the recovery of the severely depleted European eel stocks. One of the main threats is posed by Anguillid herpesvirus 1 (AngHV-1) leading to increased mortality in both wild and farmed eels. Following recommendations of the OIE to minimize the risk of obtaining false-negative results, the main aim of the study was to optimize diagnostic methods for AngHV-1 detection using conventional PCR, nested PCR and in situ hybridization assay. While 53.3% of the individual organ samples were tested positive for AngHV-1 by PCR, the additional virus analysis via nested PCR revealed that the actual prevalence was 93.3%. In the cell cultivation passages, a cytopathic effect was hardly found in the first two rounds. In the third passage onto cell cultures, a lytic CPE was detected. The identification and confirmation of the viruses obtained from cell cultures as well as directly from the organ tissues were proceeded by PCR, nested PCR and sequencing of the PCR products. While no positive signal was detectable in the first round by PCR using samples from the third cell culture passages, the nested PCR provided weak but visible positive signals.