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

Report Highlights

Findings

For the past 30 years, the Japanese Sea Ranching Program has fostered the development of technologies to cultivate large numbers of the swimming crab, Portunis trituberculatus. State of the art facilities throughout Japan now produce approximately 60 million C-1 instars (~5.0 mm carapace width) annually. Prior to 1990, these C-1 crabs were released in great numbers for stock enhancement purposes. Japanese scientists now believe that these efforts were largely ineffective. Since 1991 hatcheries have modified their approach to include secondary rearing from the C-1 to C-4 stage (~17 mm carapace width) prior to release. Over the last decade stocking efforts focused specifically on C-4 juveniles have resulted in releases of 28-42 million crabs annually.
Assessments of three stock enhancement case studies representing a range of size, physical characteristics and fisheries - Hamana Lake, Osaka Bay, and Okayama Prefecture - suggest that releases of C-4 juveniles in small scale systems (Hamana Lake, Osaka Bay) may be sustaining low catch levels and subsidizing what are presently very reduced levels of reproductive stocks. Evidence for enhancement to historical levels has not been found, although habitat degradation may be contributing to an overall reduction in carrying capacity in these systems. In the large system (Okayama Prefecture), the pattern of landings over the past 30 years suggests decadal oscillations (most likely climatic) in crab stocks; hatchery releases appear to have had little impact on crab stocks and subsequent fishery yield.
The long-term investment by the Japanese - at the federal, state and local levels - has built significant capability and has led to a far greater understanding of growth and reproduction in P. trituberculatus. Until very recently, however, there has not been a concerted effort to assess the impacts of stock enhancement programs on fisheries. Coupling production and enhancement efforts with assessment is an essential prerequisite to understanding fully the impact of investments in restoration efforts of this type.

Implications for Chesapeake Bay

With regard to the feasibility of enhancing crab populations in Chesapeake Bay, the team diverged because of differing views on the applicability of the Japanese data and differing assumptions about the nature of crab population(s) in Chesapeake Bay. Central to this divergence were differing views of the implications of scale, the importance of localized populations of blue crab and the significance of spatial and temporal dispersal patterns.
One view holds that focused efforts designed to enhance local, reproductive populations is feasible. In particular, augmenting juvenile crabs in subestuaries with hatchery-reared individuals would supplement local sub-populations in a way that overcomes dispersal and the high-mortality rates associated with larval development and should therefore positively impact fishery yield by enhancing the recruitment to the broodstock.
The other view holds that because the scale of Chesapeake Bay crab stock is immense compared with the Japanese ecosystems and because the Chesapeake Bay blue crab population replaces itself as a single unit, hatchery-based efforts cannot enhance populations in the face of natural productivity. The most effective tools for sustaining the crab population are scientifically-based fisheries management and programs of habitat restoration.
Despite this divergence, there was consensus among the authors that there are compelling reasons to develop local rearing facilities to produce releasable juvenile blue crabs (~100,000-1,000,000/year) for research purposes. Such facilities would greatly improve our understanding of the biology of this organism and provide opportunities to develop better tools to manage the fishery in Chesapeake Bay. In addition, the team agreed that habitat restoration and strong fishery management are essential if sustainability is to be achieved in the Chesapeake blue crab fishery.