Catch Fluctuation of Kuruma Prawn, Penaeus japonicus in Japan Relative to Ocean Climate Variability and a Stock Enhancement Program

The kuruma prawn, Penaeus japonicus is widely distributed in the Indo-West Pacific. The Japanese stock enhancement program, which produces and releases juvenile kuruma prawns into their natural habitat, started in 1964 and has expanded throughout Japan. The annual number of juveniles released ranged from approximately 240 to 300 million until the mid-1990s but then decreased steadily to approximately 105 million in 2008. National annual landings of kuruma prawns recovered to a record of 3,741 t in 1985 from 1,263 t in 1970 but then declined steadily to the historical minimum of 726 t in 2008. Thus, kuruma prawn catches have decreased dramatically despite the release of juveniles. The aim of this study is to analyze the catch fluctuation of kuruma prawns relative to ocean climate variability and the stock enhancement program. The effects of ocean climate on kuruma prawn stock sizes were evaluated by generalised additive models (GAMs). In the GAMs, catch-per-unit effort and catch data transformed to reduce the dependence of the amplitude of the catch fluctuation on the level of the catch were used as response variables, and three indices for ocean current conditions were used as explanatory variables: the Oyashio (cold current), the Kuroshio (warm current), and the Tsushima Warm Current. The GAM analysis suggested that kuruma prawn stock sizes declined during the period with a strong warm current intensity, and 42 mark-recapture surveys indicated an estimated average yield per release of 0.9 g. Our analysis highlights the fluctuation of kuruma prawn stocks with ocean climate variability and indicates that the stock enhancement program could have an impact on kuruma prawn catches depending on the magnitude of the releases. Ocean climate change, decreased fishing effort, and reduced hatchery releases could be responsible for the recent decline in kuruma prawn catches in Japanese waters.