Study

Slower growth of farmed eels stocked into rivers with higher wild eel density

  • Published source details Wakiya R., Itakura H., Hirae T., Igari T., Manabe M., Matsuya N., Miyata K., Sakata M.K., Minamoto T., Yada T. & Kaifu K. (2022) Slower growth of farmed eels stocked into rivers with higher wild eel density. Journal of Fish Biology, 101, 613-627.

Actions

This study is summarised as evidence for the following.

Action Category

Release wild-caught captive-reared eels to re-establish or boost native populations ('head-starting')

Action Link 		Eel Conservation in Inland Habitats
  1. Release wild-caught captive-reared eels to re-establish or boost native populations ('head-starting')

    A controlled study in 2016–2019 in four rivers in Japan (Wakiya et al. 2022) found that wild-caught captive-reared Japanese eels Anguilla japonica decreased in density, grew less and travelled more than naturally occurring wild eels during two years after release. Overall, average density of wild-caught captive-reared eels declined from 251 eels/ha (average biomass: 7,449 g/ha) at the time of release to 86 eels/ha (average biomass: 2,993 g/ha) three months after release, and 13–14 eels/ha (average biomass: 671–919 g/ha) 6–24 months after release. For wild eels, average density and biomass did not differ significantly over the same two years (overall 0–112 eels/ha, biomass data not reported). Captive-reared eels had lower average daily growth rates (0.04 g/day) and travelled further from release points (500 m upstream to 3,400 m downstream) than captured and tagged wild eels (0.13 g/day, 200 m upstream to 200 m downstream). In June 2017, a total of 1,940 captive-reared eels (wild-caught and reared at an eel farm for six months) were tagged and released at 10 sites in each of four rivers. Wild eels were captured and tagged at the same sites. Captive-reared (total 34 eels) and wild eels (total 26 eels) were recaptured by electrofishing three, six and 24 months after release. Fishing efficiency was used to estimate eel density and biomass.

    (Summarised by: Vanessa Cutts)

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