Identification of cymothoid isopods probably responsible for mass mortality of hatchery-reared Pacific bluefin tuna Thunnus orientalis juveniles after transfer to sea cages.

Hatchery-produced Pacific bluefin tuna (PBT) Thunnus orientalis juveniles experienced mass mortality after being transferred to sea cages in a cove on Kakeromajima Island, Kagoshima, Japan, in July 2022. Manca and juveniles of cymothoid isopods were highly prevalent in the branchial cavity of the few surviving PBT juveniles. We identified two species of cymothoid parasites based on their mitochondrial DNA sequences. Most parasites had cytochrome c oxidase subunit 1 (COI) and 16S rDNA sequences similar to those of Norileca indica, while some were identified as Ceratothoa carinata. Adult cymothoids were found in the bigeye scad Selar crumenophthalmus in the same area and were also morphologically and molecularly identified as N. indica. Therefore, it is likely that the mancae and juveniles of the cymothoids accidentally infested the PBT juveniles after they were released from adult parasites in wild natural hosts, such as S. crumenophthalmus, living around the PBT farming cages. This is the first record of N. indica in Japan and the northernmost record of this species. To our knowledge, this is the first report of mass mortality in PBT juveniles caused by parasitic isopods. Our study is the first step in developing measures to avoid the damage caused by the parasites. [ABSTRACT FROM AUTHOR]