Comparison of body color anomaly on blind side in hatchery-reared Japanese flounder (Paralichthys olivaceus) between pre-release and post-release

Abstract

Body color anomaly on the blind side (hypermelanosis) is a mark of hatchery-released fish from Japanese flounder (Paralichthys olivaceus) stock enhancement programs, but little information has been reported about changing degrees of hypermelanosis after release. We compared the frequency of hypermelanosis occurrence on the blind side (hypermelanosis rate) in juveniles before release and in recaptured fish. From a survey of 14 released fish groups identified using otolith marking with alizarin complexone, no significant difference (P > 0.01) was found except for one group. For six released groups with more than 100 recaptured fish, the frequency distribution of six anomaly grades was classified by a visual observation-based pigmented area ratio on the blind side (no pigmentation, pigmented area less than 1%, 5%, 10%, 75% and more than 75%: hypermelanosis level) among juveniles. Recaptured fish at 0 or 1 year old, and 2 or more years old were also investigated. From comparison among juveniles and recaptured fish, no significant difference (P > 0.01) was found except for one group.
These results suggest that blind side body color anomaly of Japanese flounder changed only slightly after release. We inferred that blind side body color anomaly is useful to mark hatchery-released flounder to evaluate stocking effectiveness. Additionally, results suggest that the number of recaptured flounder, as underestimated by a lack of body color anomaly on the blind side, can be corrected by dividing the number of recaptured fish with body color anomaly by the hypermelanosis rate of juveniles before their release.