Poster Abstract
15. CATCH FLUCTUATION OF KURUMA PRAWNS IN JAPAN IN RELATION TO STOCK ENHANCEMENT PROGRAMS AND CLIMATE CHANGE
Katsuyuki Hamasaki and Shuichi Kitada
Department of Marine Biosciences,Tokyo University of Marine Science and Technology, Konan, Minato, Tokyo 108-8477, Japan
hamak@kaiyodai.ac.jp
Kuruma prawns, Penaeus japonicus, are widely distributed in the Indo-West Pacific. In Japan, they occur throughout the country, except in Hokkaido, and are one of the most important crustaceans contributing to fisheries and aquaculture. The stock enhancement program through production and release of juveniles into the natural habitat for kuruma prawns started in the Seto Inland Sea (SIS) in 1964 and expanded throughout the country. Local, fisher-based statistics became available for production of kuruma prawns in 1965 (Fig. 1). Between 1965 and 1970, national annual landings decreased from 2,915 to 1,263 t. Catches recovered to a record of 3,741 t in 1985, but then declined steadily to a historical minimum of 726 t in 2008. Thus, kuruma prawn catches have fluctuated greatly. In this presentation, we analyze the catch fluctuation of kuruma prawns in Japan in relation to stock enhancement programs and climate change.
Release statistics are available from 1977. The annual number of juveniles released throughout Japan ranged from ~240 to 300 million until the mid-1990s, but then decreased steadily to ~105 million in 2008 (Fig. 1). Conversely, the body length of released juveniles increased from ~20 mm in the 1970s to ~35 mm in recent years. It appears that the annual production of kuruma prawns largely decreased according to a decline of juvenile releases in recent years.
Fig. 1. Changes in the annual catch and number of juvenile kuruma prawns released in the whole of Japan and the seven regional seas.
The reproductive season of kuruma prawns extends from spring to autumn. The juveniles, 79 mm body length (BL), settle on tidal flats after a planktonic larval stage of one month. The juveniles remain in the intertidal zone for ~210 months before migrating to deeper waters at a size of ~100 mm BL. Males and females reach maturity at 100 mm and 125 mm BL, respectively, about one year after settlement. The life span is considered to be 23 years. The prawns recruit to the fishing grounds after migrating from the tidal flats to deeper waters and are caught using several types of fishing gear, such as gill nets, set nets and small beam trawls, mainly throughout the following year (1+ age). Therefore, to elucidate the effect of juvenile releases on kuruma prawn production, we plotted the annual catch in the year ti against the mean number of juveniles released in the year ti1 and ti in regional seas excluding the North Pacific where kuruma prawn catches have been very small (Fig. 2). These relationships illustrate that the annual production tended to increase with an increasing magnitude of releases; however, catch data were highly variable.
Fig. 2. Relationship between the numbers of juveniles released and the catch of kuruma prawns.
Although catches of kuruma prawns have been greatest in regional seas with relatively extensive tidal flats, such as the Central Pacific (CP), East China Sea (ECS) and SIS, they have fluctuated similarly in each regional sea (Fig. 1), suggesting that common environmental factors have affected the abundance of kuruma prawn stocks in Japan. Two major warm ocean currents flow around the Japanese Archipelago, i.e., the Kuroshio (KU) from west southern Kyushu to middle Honshu, and the Tsushima warm current (TWC), which is derived from the KU, from west southern Kyushu to the coast around the Sea of Japan. It is known that these ocean currents have affected fishery resources through changing ocean climate. Analysis of the relationships between anomalies of KU and TWC intensity indices and kuruma prawn catches showed that annual landings of kuruma prawns tended to decrease when the KU moved northward, and when the intensity of the TWC was strong. This indicates that ocean current intensity indices should be included as explanatory variables that can affect catch fluctuations of kuruma prawns.
We applied the multiple linear regression model (LM) and linear mixed-effects model (LMM) to assess the relationships between annual catches of kuruma prawns (response variable), and the mean number of juveniles released in regional seas and the mean index of TWC intensity (explanatory variables) in the years ti1 and ti, which had higher correlation coefficients with catch data than the KU. The intensity index for TWC is an anomaly of the area >= 10 °C at 100 m depth in the Sea of Japan (data from Japan Metrological Agency). In some regional seas, catches declined greatly after 1999, so that the data were separated before and after 1999 (periods 1 and 2, respectively), and the periods were set for random intercepts in the LMM. In the linear models, the intercept can be considered as basic production of kuruma prawns and the coefficient for number of juveniles released is equivalent to the yield per released juvenile. The LM and LMM revealed that the annual landings of kuruma prawns were positively and negatively correlated with juvenile releases and TWC intensity, respectively. Furthermore, basic production of kuruma prawns decreased after 1999 in the CP, South Pacific, ECS and SIS.
Our analysis highlighted that juvenile releases could augment the kuruma prawn catches when stocks fluctuated with ocean climate change. Regime shift and decreased magnitude of releases could be responsible for recent catch decline of kuruma prawns in Japanese waters.
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