Maryland Sea Grant: Blue Crab Stock Enhancement Lessons

In an effort to enhance its fisheries, Japan releases more hatchery-produced swimming crabs than any other nation: 28-42 million juveniles per year. The Seto Inland Sea Ranching Program was instituted in 1963 as a national initiative to enhance fisheries through mass production and release of young-of-the-year juveniles into coastal waters (Nose1985). The goals underlying this program were to:

To implement these goals, an array of research and hatchery stations now exists in Japan, consisting of 16 National Fishery Research Stations, 53 prefectural (analogous to states in the USA) Fishery Research Centers, and numerous hatcheries operated by local fishing cooperatives. Generally, the National Stations focus almost exclusively on the goals associated with propagation (goals 1-3), and these stations have developed techniques to produce juveniles of more than 80 marine species, including such difficult species to propagate as squid, eel, and bluefin tuna. The goals of mass production of young-of-the-year juveniles, as well as evaluating and maximizing fishery returns of released crabs and other species, typically have been left to the prefectural centers. Early focal species of national and prefectural stations were red sea bream (Pagrus major), Japanese flounder (Paralichtys olivaceus), tiger puffer (Takifugu rubripes), Japanese prawn (Panaeus japonicus), and portunid crab (P. trituberculatus). These species continue to dominate hatchery releases by prefectures and fishing cooperatives throughout Japan (Fushimi 1998).

Figure 3.World harvests of blue crabs C. sapidus and gazami P. trituberculatus (FAO 2000).

Beyond research and development of new and improved methods for artificial propagation, National Stations serve as important production centers themselves. The Tamano Station on the Seto Inland Sea is the single largest production center for juvenile Portunus trituberculatus, producing 10 million 1st crab instars (C-1, 4.5-5.0 mm CW) each year, which are then grown up in secondary rearing to 4th-5th crab instar (C-4 - C-5, ca. 17 mm CW) and supplied to Prefectures for release throughout western Honshu. Timing of reproduction of P. trituberculatus is controlled in the hatchery so that juveniles are released during early summer in advance of most naturally produced juveniles (Figure 4). This process in turn allows released juveniles a longer growing season and accelerates their entry into the fishery in their first fall. The Tamano Station continues to refine and communicate practical methods for rearing P. trituberculatus, and recently produced a comprehensive manual on propagation and hatchery methods (Hamasaki 2000).

Prefectural Farming Fisheries Centers are the principal producers of P. trituberculatus juveniles. Eighteen prefectural centers in Honshu, Shikoku and Kyushu (Figures 1, 5) produce ~60 million C-1 juvenile crabs each year (Fushimi 1998). As with the National Stations, it is important to note that the prefectural centers are multi-species facilities, typically producing other species such as Japanese prawn, Japanese flounder and sea bream in concert with P. trituberculatus. Prefectural centers vary in size, and staff (typically 8-12), but common features we observed included a seashore location; large pumping, filtration, and aeration systems; automated tank cleaning systems; raceways for crab brood stock; large rearing systems for C-1 crabs (typically between 100 and 200 m3); and similar sized tanks for rearing live feed (e.g., phytoplankton and rotifers) in support of crab rearing. Most tanks are constructed of poured concrete and housed in large warehouses. Due to the seashore location of these facilities, capital costs are high. Economic data available for the Kagoshima Prefectural Center (a typical center focused on production of red sea bream) put its assessed value in 1990 at $9.5 million, and annual operating costs were $470,000 (Ungson et al. 1993).

Figure 4. Sea Ranching Concept for Japanese gazami P. trituberculatus. Intermediate culture refers to "secondary rearing" in text.Typically, C-4 - C-5 crabs are released following secondary rearing. Note that hatchery-released crabs are expected to enter the fishery and contribute to the reproductive stock during their first fall of life. Diagram from Aileen et al. (2000).

Prefectures are also responsible for secondary rearing of juveniles from C-1 (~5 mm CW) to C-4 (~17 mm CW) crab instar stages. Prior to 1991, C-1 instars were released directly; however, subsequent research showed that early instars were highly vulnerable to predation losses because they had not yet settled to benthic habitats (Karakawa 1997). In shallow, tidal, sandy beach experimental areas, which represent natural nursery habitat for P. trituberculatus, Ariyama (2000) conducted a series of intensive release-recapture trials. He reported that releases of 464,000 C-1 juveniles in 1981 resulted in no settled crabs one week after release. In 1982, only 2% survival was estimated one month after 97,600 C-3 - C-4 crabs were released. In contrast, for 84,000 C-3 and 246,000 C-4 juveniles released after careful secondary rearing in 1989 and 1990, estimated survival rates were greater than 30% one month after release. Predation trials have shown that dragonets (Repomucenus beniteguri: Pisces [Perciformes, Callionymidae]) and other crabs are effective predators on C-1 - C-3 juvenile crabs that have not yet settled to the bottom (Ariyama 2000). By the C-4 stage, most juveniles have settled out of the water column which, in addition to their larger size (~17 mm CW), contributes to a sharp drop in predation losses (Karakawa 1997). Because extremely high predation losses are expected for C-1 releases, several Japanese scientists believe that prior to 1991, released P. trituberculatus juveniles did not significantly contribute to coastal fisheries (Fukada, pers. comm., Ariyama, pers. comm). Since that time, only benthic crabs (C4-C5) have been released. For convenience, we refer to these as C-4 juveniles throughout this report.

Secondary rearing from C-1 to C-4 is typically accomplished in large tanks supplemented with complex artificial structure to reduce cannibalism (Figure 6). Secondary rearing methods vary but typically require large-volume rearing systems. Prefectures are also primarily responsible for evaluation of whether released juveniles contribute to local and regional fisheries. The methods that are used to determine post-release survival rates and contribution rates to the fishery are diverse and, in general, poorly developed (see Measurement of Hatchery Contribution).

Figure 6. Refuge provided to reduce cannibalism during secondary rearing of juvenile crabs P. trituberculatus, stages C-1 to C-3 - C-4 (Osaka Prefectural Farming Fisheries Center). Right panel shows use of structure by juvenile crabs ~5-10 mm CW. This rearing period typically lasts 2-3 weeks. Left photo from Ariyama (2001). Right photo from Japan Sea Farming Association (JASFA).

Japanese Sea Ranching Program