Japanese Hatchery-based Stock Enhancement:
Lessons for the Chesapeake Bay Blue Crab

Summary of Case Studies

Case study systems ranged considerably in physical size, geomorphology, fishery yield and estimates of recapture rates of hatchery released crabs. Still, similarities existed in the number of released crabs each year and the methods used to produce those crabs. In all three systems and in other Prefectures in Japan, there was an important shift in the early 1990s from releases dominated by C-1 instars to releases of C-4 - C-5 instars. Research by Ariyama (2000) and Karakawa (1997a) indicated that releases of the smaller pelagic C1 crabs during the period 1973-1990 were largely ineffective in enhancement of fishery resources. Thus, analysis of hatchery contribution rates has been limited to the last decade.

Because release levels tend to be constant (set by prefectural quotas) there are only minimal ranges within which to contrast landings data. However, for the recent period of C-4 releases, there was no apparent correlation between number of crabs released and increased harvest levels in any of the case study systems. Still, lack of any apparent correlation between release and harvest levels does not imply that hatchery crabs are not contributing to fisheries. Patterns in landings are more likely driven by ecosystem-level changes. Hamana Lake and Osaka Bay have both undergone fishery crashes from 1980 levels and low harvests have persisted throughout the last decade. Landings in Okayama Prefectural coastal waters show decadal oscillations, which suggest that climate or oceanographic factors affect P. trituberculatus yields. Under the assumption that hatchery enhancement has been implemented with crabs of sufficient size only after 1990, it is apparent that hatchery releases have done little to restore these fisheries to historical levels.

On the other hand, hatchery releases into Hamana Lake and Osaka Bay may be contributing to sustaining low catch levels, and subsidizing the reduced reproductive stock of crabs. In support of this view, recent research in the smaller of the systems visited (Hamana Lake and Osaka Bay) indicated that that hatchery-released crabs were contributing to fisheries (Ariyama 2000, Okamoto unpubl ms, Kochi Fisheries Coop. Assoc.), while evidence for fishery enhancement in the larger system (Okayama Prefecture) was insufficient to make this determination.

Predicted harvest recapture rate was much higher for Osaka Bay than for Hamana Lake, Urado Bay, or Okayama Prefecture (Table 1), and because no physical evidence is yet available on harvest contribution rates, model assumptions need to be carefully considered. While exploitation rates for Osaka Bay are believed to be quite high, uncertainty remains regarding the calculation of the actual rates. The Delury method used for estimating fishing efficiency (q) was conducted during cold weather months (December-April) when crabs are more easily captured. This could have inflated estimates of q and fishing mortality rate (F). Still, given the rapid growth and relatively high survival of the released crabs in Ariyama's 1990 study, there seems a strong possibility that hatchery releases are making significant contributions to harvests in Osaka Bay. Post-release survival estimates for Osaka Bay were based upon a single study (1990) in which secondary rearing and release conditions may have been substantially improved over those that occur using routine prefectural practices. In particular, leg loss during collection of tank-reared C-4 instars is believed to contribute to early mortality of released crabs. Ariyama (2000) conducted a similar study in 1982 on post-release survival but for this study, secondary rearing was conducted in cement tanks (the more typical prefectural practice), rather than net pens. Survival during the first month after release of C-3 - C-4 juveniles was only 2%. A clear conclusion from this case was that the method of collecting and handling hatchery-produced juvenile crabs for transportation and release can have negative consequences. Highly concentrated batches of juvenile crabs can lead to significant limb loss that in turn will reduce survival of released crabs.

These results emphasize the importance of detailed assessment and analysis of factors affecting successful survival of released juveniles. Though we have highlighted the research of several prefectural scientists who are active in developing quantitative evidence for the assessment of hatchery enhancement, in general such research is nascent and is greatly underfunded relative to the investment in hatchery production. There remain many untested fundamental assumptions in the methods used to evaluate harvest recapture rates, and we should attribute high uncertainty levels to these rates (Table 1). Particularly valuable in the future evaluation of hatchery contribution will be recent developments in tagging crabs with micro-wire tags and more conventional external tags (Okamoto 1999; Ariyama et al. 2001). Also, more experimental approaches, such as those utilized by Dr. Okamoto in Hamana Lake, will substantially advance evaluations of how hatchery released crabs contribute to fisheries, and how they interact with wild P. trituberculatus and potentially affect the ecosystems into which they are released. As another example of nascent research, the mtDNA study of Scylla tranquebarica (see sidebar, Applications of Population Genetics to Assess Survivorship of Hatchery-Reared Crabs) also indicated that hatchery releases could make local contributions to populations of a retentive crab species. Genetic markers should be very useful in the future evaluation of scientific approaches in P. trituberculatus as well. These examples of scientific approaches in evaluating the effectiveness of hatchery-based enhancement (see Japanese Sea Ranching Program, Goal 6), while promising, are still embryonic and in need of substantially increased support by government entities, should they be successful. We re-emphasize a central point here: within the Japanese Sea Ranching Program, there has been little scientific evaluation of effectiveness of mass releases of swimming crabs and other species despite its 30-year existence.