Eel Unit

The Eel Unit is a unit that conducts research on the stock assessment and management of Japanese eel and related eels as part of the Fisheries Agency’s Fisheries Resource Survey and Evaluation Promotion Commissioned Project (Commissioned Project) and the International Fisheries Resource Fluctuation Mechanism Analysis Project (Subsidized Project). Professor Hiroshi Hakoyama of the Institute of Freshwater Biology is the head of the Eel Unit, and 39 experimental and research institutions are working together to advance project research.

The Japanese eel was listed as an endangered species by the Ministry of the Environment in 2013 and an endangered species by the IUCN in 2014, raising concerns about its extinction. The recent decline in Japanese eel catches is an international issue, and scientific research contributions are being sought for various management issues, such as the IUCN Red List Review (Review in 2019, Asia Pre-Assessment in 2017, Japan Pre-Assessment in 2016), CITES considerations, domestic management measures being promoted by the Fisheries Agency, and the establishment of an international resource management system (Informal Consultation on International Cooperation for Conservation and Management of Japanese Eel Stock and Other Relevant Eel Species, and the Eel Scientific Meeting and Eel Task Team held under the Informal Consultation).

Against this background, the Eel Unit assesses and understands the current status of Japan’s eel resources through the collection of catch information, surveys, and mathematical and statistical analysis, and considers appropriate management measures, surveys, mathematical modeling, and advanced statistical analysis to ensure the recovery and sustainable use of the resource. It also actively publishes and uses its findings to contribute to the government.

Specifically, we conduct research on the following four tasks:

Task 1. Resource assessment and management: resource trend analysis, pond input volume recommendation, and extinction risk assessment

It is necessary to ensure the sustainable use of eel resources by recovering resources while continuing eel fishing and eel farming. The recent decline in Japanese eel catches is an international issue, and contributions from fishery resources research are being sought for various management issues, such as the review of the IUCN Red List, the consideration of regulations at the CITES Animals Committee, domestic management measures being promoted by the Fisheries Agency, and the establishment of an international resource management system. The most important role in addressing these issues is research and development on eel resource trends, extinction risk assessment, and pond input management, with mathematical models at the center of the framework, and Nagano University is conducting research. Regarding the understanding of eel resource trends, the main sources of information are the monitoring of glass eel fishing conditions and sea surface eel fishing conditions, and glass eel surveys, and glass eel surveys are conducted as qualitative information outside the fishing season. In particular, with regard to the glass eel fishing conditions, a conference is held to share and publish data from glass eel harvesting prefectures. In addition, with regard to mathematical and statistical models, we will develop reliable resource dynamics models and improve the statistical analysis methods. We will develop and analyze management methods using data on catch volume, population genetics (Task 2), and satellite tag migration surveys (Task 3) (funded project), and will also conduct resource assessments such as extinction risk assessment and resource trend analysis.

Task 2. Population genetic analysis, including estimation of effective population size

Effective population size will be estimated from the genome as an indicator of resource trends independent of fishing. Effective population size can be interpreted as the number of parental individuals that contribute to maintaining the genetic diversity of the intraspecific population and is therefore an indicator of the genetic diversity of the resource. It is often required as an important metric, especially for endangered species. CITES also requires an index of resource trends other than fishing, and effective population size is considered to meet this requirement. Glass eel and yellow eel samples will be collected from Chiba, Shizuoka, Wakayama, Miyazaki, and Kagoshima prefectures, and Nagano University and the Fisheries Resources Research Institute will collaborate to perform sequence analysis using next-generation sequencing (NGS). In addition, historical glass eel samples from Associate Professor Nozomioka of Kyushu University will be used to conduct a time-series analysis of past effective population size and serve as a material to assess the health of the current population. A paper by Igarashi et al. (2018) presented a counterargument to the idea that Japanese eels consist of a single population (panmixia). If the results of Igarashi et al. are true, we will need to reconsider the basis for estimating the effective population size and resource management units for this species, so we will revisit the single population hypothesis based on resequence analysis.

Task 3. Improvement of pop-up electronic tags using the ARGOS satellite for Japanese eel and field research

Linking the nursery and spawning grounds of Japanese eel and identifying the origins (habitats and abortive migration areas) of parent eels that contribute to reproduction is important for understanding the resource structure. In addition, tracking spawning migrations that do not reach the spawning grounds is one of the basic knowledge of eel resources required by CITES, IUCN, FAO, and international informal eel conferences. Tracking to spawning grounds using satellite tags has been successful for large American eels and European eels, and is a promising method internationally (Beguer-Pon et al., 2015. Nature Comm., Righton et al., 2016. Sci. Adv.). However, satellite tagging has not yet been established for smaller-weight Japanese eels to monitor their long-distance migrations between nursery and spawning grounds, and current challenges include improving satellite tagging survey methods and collecting and accumulating knowledge through surveys. With this in mind, the Eel Unit will conduct research to improve satellite tagging and its use in surveys. In the migration survey, eel satellite tags will be used to survey multiple habitats and obtain data on ocean movements (Nagano University, Kumamoto Prefecture, Miyazaki Prefecture, Kyushu University). In addition, the small and less cumbersome ARGOS pop-up satellite tag for Japanese eels will be improved (Shinshu University, Nagano University). In addition, experiments on attaching dummy tags to actual living organisms (short-term and long-term attachment experiments) will be conducted in large tanks at Nagano University, and the results of attachment resistance will be fed back into tag improvement. Shinshu University will also conduct numerical calculations and experiments in running water tanks to improve the shape of the tags to reduce fluid resistance. This year, the goal is to improve the tags to make them even more reliable, smaller, and less resistant, with the ultimate goal of conducting satellite tag surveys that can withstand travel up to 2,500 km to spawning grounds.

Task 4. International cooperation in the Eel Task Team

The research activities will be carried out in two Task Teams. The tasks of each Task Team are as follows: Task Team 1: To understand and predict trends in Japanese eel stocks in Northeast Asia, to establish close relationships among scientists in the region, and to collect and organize long-term time series data on Japanese eel (Team Leader Leanne Faulks); Task Team 2: To exchange information on tracking technologies to follow the migration routes of Japanese eel and other related eels from rivers to spawning grounds in Northeast Asia and other regions, and to analyze and evaluate tracking data (Team Leader Hiroshi Hakoyama). The Eel Unit will work with members of foreign task teams to discuss and collaborate on the compilation of international catch data, the distribution and current abundance of eels through eDNA surveys, and satellite tag tracking surveys in East Asia.