Maryland Sea Grant: Blue Crab Stock Enhancement Lessons

Recent declines in Chesapeake blue crab landings, and evidence for declines in spawning stock and recruitment (CBSAC 2001; Sharov et al. 2001) have prompted actions for increased regulation of harvest, increased habitat management and restoration, and improved scientific understanding of factors that control blue crab production. In addition, seafood industry representatives, state and federal legislators, and some scientists have suggested that the blue crab stock in Chesapeake Bay might be enhanced artificially through controlled spawning, hatching and rearing technologies, and release of juvenile crabs into natural ecosystems. This use of hatchery-based stock enhancement for a variety of species is controversial (Hilborn 1999; Leber 1999; Hilborn and Eggers 2000; Secor et al. 2000a). Critics note that research has focused mainly on hatchery production methods without adequate assessment or analysis of factors affecting successful survival of released juveniles. Advocates of stock enhancement for Chesapeake Bay blue crabs point to the large hatchery-based ranching program undertaken in Japan as a model. Some 28-42 million juvenile swimming crabs -Portunus trituberculatus -are released in Japan each year (since 1992, Ariyama 2000). Juveniles (c. 15-20 mm carapace width, CW) are released at numerous coastal locations within the Seto Inland Sea and other areas including the islands of Kyushu, Shikoku, and along the south-eastern coast of Honshu (Figure 1). Throughout most of the nearly 30-year history (1973-present) of Japan's Sea Ranching Program, there has been successful development of hatchery production methods but little critical evaluation of the effects of hatchery releases on stock enhancement (but see Ariyama 2000, Okamoto unpubl ms).

To further evaluate the Japanese approaches of using hatchery technologies for supplementing portunid crab stocks and the potential lessons for hatchery technologies in Chesapeake Bay, we traveled to Japan during July 2001; we visited hatcheries involved in large-scale production of juvenile portunid crabs and interviewed Japanese scientists who have conducted research on aspects of hatchery-based enhancement. Because Portunid crab markets in the U.S. are increasingly international, we begin by comparing portunid crab fisheries among the United States, Japan, and other major national producers. The Japanese Sea Ranching Program is then presented in the context of fisheries enhancement and management in Japan; particularly important is the government's role in providing considerable support for the research and physical infrastructure necessary to rear millions of juvenile swimming crabs. We then discuss three cases of hatchery-based stock enhancement in Japan for systems that vary in fishery yields, size, geomorphology and methods used to evaluate hatchery contribution to the catch. Finally, we apply lessons learned from Japan's experience to evaluating the issue of artificial propagation of blue crabs and the enhancement of their populations in Chesapeake Bay.

Figure 1. Case study sites where Japanese swimming crab P.trituberculatus hatchery-based enhancement was evaluated: Okayama Prefecture portion of Seto Inland Sea, Osaka Bay, and Hamana Lake. Urado Bay, site of a mangrove crab enhancement study is also shown.

Introduction