The decline of the Chesapeake Bay blue crab fishery over the past decade has catalyzed numerous debates throughout the region. Callinectes sapidus represents the most important Bay fishery, and in recent years the blue crab stock has hovered near the historical low-point recorded in 1968. Faced with this condition, in 1999 the governors and legislatures of Maryland and Virginia initiated a broad-based effort to establish new criteria for the Baywide management of this resource. Over the past two years, the Bi-State Blue Crab Advisory Committee (BBCAC) forged a unique partnership that linked scientists, managers and watermen in an effort that has led to the implementation of new harvest regulations designed to set thresholds to protect spawning stocks in the Bay. These precautionary thresholds and related regulations are strongly endorsed by the three authors.
As the bi-state process developed, it became clear that there remained a number of key unknowns about this very complex organism and its fishery. Maryland Sea Grant, the NOAA Chesapeake Bay Office, the Maryland Department of Natural Resources, and research entities have mobilized resources and rapid progress is being made towards improved science and management of blue crabs. In parallel to these research efforts, questions emerged regarding the potential to use hatcheries as a means of enhancing blue crab populations in Chesapeake Bay. Funding from the Maryland legislature and private industry was provided in 2000 to initiate a research program focused on rearing blue crabs in closed systems and on developing a better understanding of reproductive biology, growth and mass cultivation, as well as to provide a foundation for potential large-scale hatchery production of juvenile crabs. Discussions regarding the role and feasibility of hatcheries as a viable element in the management of Chesapeake Bay blue crabs very rapidly reached the stage of a heated and at times publicized debate within the academic community.
One of the rationales for undertaking an effort to develop hatchery technology for C. sapidus was that hatchery rearing and enhancement had been successful for a similar species - the swimming crab, Portunus trituberculatus - in the Seto Inland Sea of Japan. While some information was available in the scientific literature, there was very little data upon which to judge the success or failure of the Japanese effort and perhaps more importantly to assess its implications for blue crab management in Chesapeake Bay. With this in mind, Maryland Sea Grant provided funding for three researchers to travel to Japan in the summer of 2001 to assess the crab hatchery program. While all three members of the team were engaged in some aspect of blue crab research, all had very different scientific backgrounds. Their contrasting expertise provided an opportunity to view the Japanese experience through very different lenses. The team was charged with the following tasks:
The research team returned with a tremendous amount of information. There was clear agreement on many issues and a shared interpretation of most of the Japanese data. That said, divergent interpretations and opinions were also apparent. As much as is possible, this report summarizes the team's findings, points of agreement and differing views.
Findings and Conclusions
Context, Extent and Success of Crab Rearing in Japan
According to the research team, fisheries enhancement efforts in Japan take place within the context of a highly modified coastal environment with severe degradation of important habitat for fisheries reproduction and recruitment. Consequently, controlled rearing of commercially important species has a long history and plays an important role in the coastal economy. Government investment in the infrastructure required to support this effort is extensive and occurs at both the national and prefectural (state) level. Japan's sea ranching effort is a capital-intensive operation that itself is integrated into a system designed to cultivate and release a wide range of marine species. The world leader in the development of artificial propagation methods for marine fishes and crustaceans, Japan's mass-production methods for Portunus trituberculatus extend back 30 years. A close linkage exists between the propagation and releases of P. trituberculatus and the fishing cooperatives that ultimately harvest them. The cooperatives have direct input into decisions regarding production levels and may provide support or actually operate small local crab hatcheries. In view of the service role that enhancement programs provide fishing cooperatives, the aim of the Japanese release programs is towards fishery enhancement ("put and grow") rather than enhancement of stock productivity.
The Japanese have implemented rearing facilities capable of producing approximately 60 million C-1 instars (~5.0 mm carapace width) per year. Production facilities range from a large-scale, nationally supported facility (Tamano Station) capable of producing as many as 10 million C-1 instars/year to smaller scale, prefecture-based operations that produce on average about 3 million C-1 instars/year. It was clear that the investment in hatchery technology and long-term support for research has facilitated the development of an extensive knowledge base - much of which has been translated directly into practical applications for rearing and grow out. The science of crab cultivation and the associated knowledge of the reproductive biology of P. trituberculatus is very advanced in Japan.
Implementation and Impacts
The Japanese swimming crab stock enhancement program has evolved over its 30-year history. The early success of rearing technologies led to a concerted effort to release millions of C-1 instars into coastal ecosystems. While this strategy lasted for nearly 20 years, there was very little systematic collection of key data on the fate of released crabs. The discovery that survivorship of the C-1 instars was extremely low (<1%) led to a major modification of protocols and associated technologies. By the early 1990s, secondary rearing to the C-4 (~17 mm) stage prior to release was implemented by hatchery operators throughout Japan. Secondary rearing facilities achieved 12-61% survival of C-4 instars from C-1 instars. During the period 1991-1997, 28-42 million C-4 were released annually throughout Japan. This shift in strategies has been accompanied by a recognition of the need for a more concerted effort to evaluate the impacts of the enhancement program on fisheries.
While data were not complete nor definitive for any of the local programs visited by the research team, three different case studies provide insights into the efficacy of hatchery enhancement in Japan.
Hamana Lake. Hamana Lake, a small coastal lagoon with a single inlet to the sea, experienced a dramatic (10-fold) decline in swimming crab harvests in the mid 1980s; the decline was thought to be due in part to changes in the physical dynamics of the system that limited entry of larvae and juveniles from outside the lagoon. In addition, resort and residential development pressure, coastal modifications, and low flushing rates within the system may have resulted in an increased duration of hypoxic events. Efforts to stock the system with C-4 instars began shortly after the population crash and at present about 1.4 million juvenile crabs are released into the system each year. A mark-recapture study in 1998 and a more long-term cohort analysis conducted between 1992-1998 have provided information regarding the contribution of stocked crabs to local fisheries. The recapture rates in the fishery of released C-4 instars was estimated to range from 0.3-2.2% and suggests that the stocking efforts are responsible for an 18% contribution to the fishery as it stands now in Hamana Lake. Based upon analysis of long-term trends in the harvest data, the stocking effort does not appear to be enhancing the fishery as annual harvests remain near or below the levels found when the fishery experienced its large decline. Evidence may suggest that the effort has helped to sustain the level of the fishery following its collapse in this ecosystem.
Osaka Bay. Osaka Bay is a heavily impacted, mid-sized ecosystem with profoundly modified shorelines and severe degradation of natural habitat. The crab fishery has seen large historical declines - most recently in the late 1970s - that are likely associated with habitat loss, and periods of degraded water quality. Enhancement efforts using C-3 - C-4 instars were implemented, and since the early 1990s approximately 0.7 million/year juvenile crabs have been released. Data for long-term trends in crab harvests in Osaka Bay suggest that the releases have not led to an enhancement in fisheries to historical levels prior to collapse in the fishery. Recently, extensive studies have provided details into the dispersal, growth and survivorship of released crabs. Juvenile crabs released into shallows showed limited dispersal over their first month at large, grew rapidly, and attained harvestable size within their first year of life. Employing these data, a fisheries model for Osaka Bay predicted a recapture rate of 31.3% for released crabs in 1990, with a concurrent contribution to 1990 landings of about 59%. This rate is sensitive to fishery model assumptions, but suggests that hatchery releases could be subsidizing to a substantial degree the severely depressed Osaka Bay fishery.
Okayama Prefecture. Okayama Prefecture's segment of the Seto Inland Sea is the largest and most open of the systems examined and also supports the largest crab fishery (~2-10 fold greater). Early efforts focused upon the release of C-1 instars and there was an apparent increase in landings. However, Japanese scientists reported that this correlation was unexplained and attention since 1991 has been on the release of larger C-4 instars reared in coastal impoundments. Approximately 1 million juveniles are currently released per year. The Okayama fishery has experienced wide variations over the past 30 years and at present harvests are in a period of decline. Studies based upon cohort analysis suggest that the recapture rate for released crabs was 5.7%, with a concurrent contribution to the fishery of 9%. Recent studies suggest that most released crabs are retained within a fairly small area (5-20 km of release) over a period of two years, suggesting to some on the team that the geographical range or ambit of stocks of this species is restricted.
The three facilities studied provide a number of key insights and there was general agreement among the group on several points. These include:
Implications for Chesapeake Bay
Interpretations of how Japanese experiences and data relate to the Chesapeake Bay differed among the scientists participating in this fact-finding mission. A point of agreement was that enhancement strategies should not be directed only at increasing fishery yields, but should be directed at enhancing reproductive potential of the Chesapeake Bay stock. As noted earlier, the lenses through which each viewed the issue led to a focus on specific aspects of the enhancement effort there. Two relatively distinct opinions and interpretations emerged. Central to these divergent views was the importance of localized populations in Chesapeake Bay relative to a unified Baywide stock of blue crabs. This divergence led to very different conclusions:
One view holds that examination of the fisheries data for the three case studies suggests that P. trituberculatus has a limited dispersal and rapid growth rate, and accordingly is more amenable to efforts designed to sustain small-scale local fisheries through large localized releases, followed by intense exploitation. The case for Chesapeake Bay presents a different set of constraints based in great part upon the fact that C. sapidus has a very large seasonal and annual dispersal range and can be managed only as a single Baywide stock. Because the Chesapeake Bay's population of blue crabs is so much larger than the Japanese stocks, efforts to enhance the population - given the estimates of hatchery contributions to the fishery in Japan - would require such large numbers of juveniles that it would make the effort unfeasible on this scale.
The other view holds that definitive data on the Japanese effort is only now emerging and, as such, provides a guide for ways to proceed in Chesapeake Bay. The importance of localized crab subpopulations has been underappreciated in the Bay, as has the use of protected areas to provide sanctuary for reproductive populations. Based upon the success of rearing technology in Japan, it is feasible to use hatchery-raised juveniles to enhance specific local populations or reproductive females in specific areas and, in conjunction with strong fisheries management, thereby achieve positive impacts on the fishery.
Last modified October 07, 2002
Maryland Sea Grant Publication Number UM-SG-TS-2002-02 (September 2002)
Blue Crabs in the Chesapeake
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