Maryland Sea Grant: Blue Crab Workshop - Presentations & Discussions

Summary of Presentations & Discussions

Concern over Blue Crab Declines in 2000

Douglas Lipton, Maryland Sea Grant Marine Economic Specialist, provided a chronology of the recent political and industry actions that have focused attention on the blue crab industry. In particular, he discussed the petition filed by domestic crabmeat producers to place a quota or tariff on imported crabmeat from Asia. This petition led to discussions among stakeholder groups to improve the situation regarding crab and crabmeat production in Maryland. On July 11, the International Trade Commission voted against the domestic industry by a 4-2 vote, thus, denying them federal assistance to compete with imported products. Meanwhile, the General Assembly appropriated $100,000 for crab research at the University of Maryland Center of Marine Biotechnology in 2000.

Research Issues

Blue Crab Ecology in the Chesapeake Bay

Victor Kennedy, Horn Point Laboratory, University of Maryland Center for Environmental Science (UMCES), presented an overview of the research that has led to the current understanding of the basic biology of Callinectes sapidus in Chesapeake Bay. Kennedy detailed the ecology and life history as well as the role of blue crab as both predator and prey. He emphasized the complex interaction between the highly motile blue crab and the physical as well as chemical dynamics of the estuarine system. Both estuarine circulation and signals from key habitats (e.g., seagrasses) are central elements in the blue crab's life history. The regulation of these interactions is largely unknown and constitute an important area of study that would almost certainly benefit long-term management efforts. Migration, spawning and molting are all thought to be tied to specific environmental cues that, to date, are not well understood. Similarly, it is known that a variety of small-scale interactions among populations within a given locality relate to food availability, habitat diversity and population level. Kennedy noted that the effect of predation on the crab population is a complex process and that cannibalism is important throughout the organism's life-history. In particular, our understanding of predation on crab larvae, post-larval stages and juveniles is poorly known. Crabs are thought to possess very keen olfactory senses that could be critical in the turbid, low light environments they occupy. The link to specific habitat types at different stages of the life-cycle is thought to be very important, as is the concept of physical refuges – particularly for soft shelled, post-molt individuals. Crabs are also susceptible to diseases of various types, both in natural populations as well as more intensive crab shedding operations and may be impacted by anthropogenic influences as well.

Reproductive Biology

Anson (Tuck) Hines, Smithsonian Estuarine Research Center (SERC), discussed various aspects of the reproductive biology of blue crab and its implications for resource management strategies. Females mate once during their terminal molt while males mate multiple times. The quantity of sperm delivered by males per mating is dependent upon the interaction of several factors, including the size of the individual and the interval between mating. Hines noted that complete recharge of seminal fluid requires about 10 days and hence repeated matings during that interval yield less sperm per event. Data from the Rhode River suggests that about 50% of males are sperm-depleted at any given time. Pressure from the fishery also interacts with the organism's reproductive biology. Harvests are weighted preferentially towards large males and therefore lead to the development of a population dominated by small individuals. Thirty years of monitoring data by George Abbe, (Academy of Natural Sciences Estuarine Research Center, personal communication) on the Patuxent River has revealed that although the size of females has remained steady, there has been a distinct decrease in the size (carapace width) of males in his samples.

Taken together, natural mating strategies and selective harvest pressure yielding a preponderance of small males, suggest that over time a greater percentage of male crabs could be sperm-limited, therefore, delivering less sperm per mating. The implications of this depletion may be quite profound, but is, as of yet, unquantified. While Hines estimated that 95% of females in the Bay are mated, these matings may be based upon less than optimal sperm delivery and may yield insufficiently fertilized broods. Females mate only once, hence all fertilizations are tied to the quantity of sperm delivered during that event. They may contribute from 1-3 broods per season, each containing between 1-6 million eggs. On average, it is thought that females contribute 3-6 broods during their lifetime. The quality and fertility of eggs in a given brood is poorly understood, as is the age structure of the female population and its fecundity. In addition, it is also not known if sperm quality declines over time so that later broods have lower fertilization success.

Population Dynamics

Thomas Miller, Chesapeake Biological Laboratory, UMCES, posed fundamental questions about population dynamics that are critical for developing management strategies:

What have been the patterns of abundance in the past and is there evidence for population regulation?
What is the population structure and what are patterns of abundance likely to be in the future?
Is the population sustainable and how can we ensure population sustainability?
What patterns of exploitation are sustainable and what needs to be protected, e.g., life history stages, areas?

A major element in developing population models to answer these questions is a more accurate knowledge of blue crab growth in the field–in particular the rate of natural mortality. An integrated framework for management, Miller points out, must enable resource managers to identify (1) levels of crab abundance at which exploitation can occur, (2) combinations of crab abundance and exploitation that are not acceptable for maintaining sustainable populations, and (3) combinations of crab abundance and exploitation that promote biologically optimal "use" of blue crabs. Miller argued that despite the lack of full knowledge about blue crab growth and ecology, population dynamics is sufficiently advanced to do the following:

Project future population status over the short term
Help guide resource management
Quantify the impacts of alternative management actions
Identify conservation measures compatible with sustainable exploitation

Miller's efforts are closely linked to the activities of BBCAC and aim at developing targets for blue crab fishing mortality and population abundance to support a sustainable fishery. The targets are based on models of the expected yield for every crab entering the population, but researchers acknowledge that there is an apparent uncoupling of spawning stock size and subsequent recruitment in this blue crab stock. BBCAC recommendations to fishery managers on harvest levels for ensuring sustainable populations are currently being discussed and managers will have to decide the mix of fishing activities (e.g., peeler and soft versus hard crabs) that they want to achieve.

Habitat

Court Stevenson, Horn Point Laboratory, UMCES, discussed the relationship between blue crab and Chesapeake Bay habitat. He noted that crabs are able to exploit a variety of habitats over the course of their life history, an observation supported by T. Hines who noted that crabs require an array or mosaic of habitat types to thrive. The actual make-up of these mosaics may vary and crabs appear to be adaptable to change. Stevenson suggested that research is needed to assess what types of alternate substrates can fill the role once provided by SAV and to what extent the decline of grasses impacts population levels. Marshes and small creek systems may play an important role for large adult crabs, while woody debris may be essential for juveniles. Anthropogenic influences have the potential to impact populations as well. New shoreline structures such as rip rap, jetties and piers may provide suitable physical substrate and cover while removal of woody debris for esthetic or navigational purposes has the potential to have a negative impact. Another important habitat issue is that of contaminants. Little is known regarding the impact that a variety of chemical contaminants present in the ecosystem have on the health of the blue crab population. Hence, anthropogenic alterations result in complex impacts that need to be assessed carefully within the context of crab biology and population dynamics.

Stevenson's presentation emphasized that habitat issues and responses in this area must consider multiple factors. He suggests that there is a net "habitat burden" to a population. If key habitats are lost, this burden may become sufficiently large to force declines or negatively impact the recovery of the population. It is useful to think in terms of what habitat features are essential to crab stocks at given life cycle stages and where potential bottlenecks in population growth can occur if these areas are degraded or eliminated. The concept of refuges, therefore, needs to be very carefully considered within the context of the biology of the organism.

Stock Enhancement

Discussions during the workshop focused attention on the concept of blue crab stock enhancement–in particular the possibility of developing systems to efficiently produce mass quantities of larvae and/or juvenile crabs for release into Chesapeake Bay. Fisheries scientists and others voiced strong concerns that large scale stocking efforts would be ineffective as a means to enhance this fishery as it presently exists in a meaningful and economically viable way and noted that analogous efforts overseas had not fared well. While production of larvae and juveniles was viewed as potentially feasible, participants felt there were significant barriers to restocking. These included issues of predation, natural mortality and habitat, as well as the impact of natural variability in blue crab populations driven by multiple factors. In addition, the technology to assess whether or not stocking was having an impact on natural populations does not exist at present. Importantly, others felt that the issue of stock enhancement warranted further examination and should not be dismissed at this stage. Examples of success with non-crustacean species were noted, as were efforts to cultivate other crab species in the Pacific. There was little consensus among the groups on this point. We note, however, that there was interest within both groups for the development of intensive closed systems to produce and rear blue crabs under highly controlled conditions for research purposes. Given many of the research areas outlined previously, such systems were seen as an important tool for advances in many areas and indeed could be catalytic for the development of new collaborations within the Maryland scientific community.