A research team led by the Chuo University Research and Development Organization * has conducted research on natural eels and cultured eels released in Japan. It was revealed that there is a possibility of adverse effects on the survival and growth of eels.

 In Japan, the catch of eels in rivers and lakes is 69 tons, the release of eels is 201 million individuals (about 30 tons) (2018), and the release of eels is the largest resource conservation measure in Japan.However, there is no paper that examines how many eels survive when released (effect of release).Therefore, the research team conducted research on the relationship between natural eels and cultured eels (released eels) through three methods: behavioral observation, mixed breeding, and labeled release.

 In behavioral observation, natural and farmed eels (same size) are placed in a small aquarium one by one, and the behavior is analyzed based on the biting behavior and the occupancy rate of the pipe (hidden place).Natural eels dominated.In mixed breeding, natural and farmed eels (of the same size) were bred in the same concrete aquarium for about two years.Cultured eels had lower survival rates and growth rates than single-reared eels.Surveys on the release of labels were conducted in four rivers in Japan.In rivers with a high density of natural eels, the growth of released eels was slow, and the population of released eels decreased by 1% in two years.

 From the above results, it was shown for the first time that natural eels may reduce the fitness of cultured eels through intraspecific competition, that is, the release effect may be reduced.The research team attributed this to the breeding process in the farm, but should also be aware of the possibility of pathogen spread.He said that it is necessary to reexamine the current release of eels, citing necessary items as research on improving the release effect.

* In addition, Aomori Prefectural Industrial Technology Center Inland Water Surface Research Institute, Kagoshima Prefectural Fisheries Technology Development Center, Kobe University, Fisheries Research and Education Organization, Fukui Prefectural Agriculture, Forestry and Fisheries Department Fisheries Division, Chuo University Faculty of Law participated.

Paper information:
Experiment 1 / Experiment 2[Journal of Fish Biology] Agonistic behavior of wild eels and depressed survival and growth of farmed eels in mixed rearing experiments
Experiment 3[Journal of Fish Biology] Slower growth of farmed eels stocked into rivers with higher wild eel density

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