Effectiveness of stock enhancement of a rare species, spotted halibut Verasper variegatus, in Fukushima, Japan

The effectiveness of stock enhancement trials of a rare flatfish Verasper variegatus was evaluated through assessment of fishery statistics, geographical distributions of catches, and intensive market surveys in Fukushima Prefecture, Japan. In all, 426,704 cultured individuals within 23 age-0 cohorts and 4 older cohorts (7.5-10.8 cm and 20.0-51.0 cm total length, respectively) were released at six coastal sites off Fukushima between 1993 and 2007. Commercial landings before the release trials decreased from 5.3 metric tons (MT) in 1987 to 1.0 MT in 1993. After the release trials, commercial landings increased rapidly to 2.7 +/- 0.6 MT during 1996-2010 (maximum of 3.8 MT in 2008). Fish of 25-70 cm total length (mainly 1-3 years old) were caught equally by gillnet and trawl fisheries at different depth ranges (mainly <50 m and 100-150 m), corresponding respectively to those of feeding and spawning grounds. The numerical contribution of released fish increased rapidly, exceeding 70% during the mid-2000s; up to 91% in 2004. Multiple regression analysis revealed two factors (number released and release date from first January) that significantly and negatively affected the recapture rates of age-0 cohorts (0-24.8%, mean 6.0%). Our results demonstrate that local stocks of a rare species can be augmented through the release of cultured juveniles. In addition, the strong direct impacts of hatchery releases (rapid increase of landings and contribution rate) suggest that stocking effectiveness of a rare species were less influenced by the fluctuating natural populations than common species are. Results also suggest that the long-term and comprehensive assessments of release trials from release, to fishery catches, and to market landings are necessary to detect potential factors affecting recapture rates and to clarify the ecological characteristics of a rare species. The release strategy can be refined based on the feedback process of these results. (C) 2012 Elsevier B.V. All rights reserved.