Life & Plight

The increasing scarcity of blue crabs is due to the dramatically declining regional crab harvests that by some estimates have declined by 65% over the past fifteen years. This decline should not, perhaps, be totally unexpected. Commercial fish harvests, such as that of the rockfish, have been declining in the Bay for some time. But what is more telling is the near eradication of the Chesapeake Bay oyster, one of the blue crab’s most basic food sources (fig. 11).

Figure 11 – Annual blue crab & oyster harvest in the Chesapeake Bay. Author illustration. Data courtesy

As a result of this decline, Maryland is witnessing the demise of an industry, a tradition, and a marine culture. Professional watermen have been forced to abandon their profession along with the seasonal employees who pick up work at the processing facilities that once lined the Eastern Shore. A way of life unique to Maryland is disappearing.

There are four primary causes behind the degradation of the Chesapeake Bay and the consequential habitat loss accompanying this degradation. They are listed below in order increasing significance:


GHOST POTS—There is consistent debate over whether old crab pots abandoned on the floor of the Bay are killing the blue crab. The realistic answer is most likely “No.”

FOREIGN SPECIES—The introduction of foreign species has severely impaired the Chesapeake Bay ecosystem. The failed plan to rejuvenate the flagging Bay oyster population with an Asian oyster ultimately resulted in the extermination of the Chesapeake Bay oyster.

OVERHARVESTING—Maryland watermen have consistently overharvested the Bay. In recent years this led directly to the extinction of the Chesapeake Bay oyster, and soon it may lead to the extinction of the Chesapeake Bay blue crab. Legislation has sought to restrict crab catches in terms of size and sex, but many believe these restrictions are too little too late.

INDUSTRIAL POLLUTANTS—Maryland has worked to reduce industrial pollution into the Chesapeake, and has made some real gains, but the fact of the matter is that the Chesapeake Bay can no longer support any industrial pollution given the exponential growth of residential and agricultural pollutants over the past decade.

RESIDENTIAL POLLUTANTS—As the Eastern shore has changed demographically from a farming community to a waterfront resort community, residential construction has increased exponentially. Because much of this land is unincorporated and has a high water table, developers have been installing septic fields for each new housing development. With high water tables and saturated ground, the wastewater from these new developments sluices directly into the Chesapeake—causing enormous damage to Bay plant and animal species.

AGRICULTURAL POLLUTANTS—EPA officials claim that agriculture is the largest single source of pollutants and sediment in the Chesapeake Bay, accounting for over 40 percent of the nitrogen and phosphorous and over 70 percent of the sediment. State officials say that animal manure produces more phosphorus and nearly the same amount of nitrogen pollution as all human wastewater from treatment plants in the state combined. And although the dairy and hog industry in states adjacent to the bay produce more pounds of manure per year, poultry waste has more than twice the concentration of pollutants per pound. Thus it is fair to say that chicken farming, one of Maryland’s biggest and most lucrative of agricultural industries, is also one of the Bay’s worst enemies.

These last 3 causes of degradation—industrial, residential, and agricultural pollution create vast dead zones in the Bay each summer. These areas are vast tracts of water that have been entirely depleted of their dissolved oxygen content by algae (fig. 12, overleaf). Although the science behind this phenomenon has been understood for some time, it has been exceptionally difficult to find the political will to do anything about it, and dead zones remain one of the most menacing threats to the Bay’s present and future health.

Figure 12 – How the dead zone forms. Courtesy of The Times Picayune, 2007, Dan Swenson.

Since crabs require a relatively low dissolved oxygen content of 3 milligrams/litre, they have a better chance of survival in oxygen depleted water than do some larger fishes. That said, with ¼ of the Bay dead, even the crab cannot escape the effects of asphyxiating waters (figure 13).

Figure 13 - Crabs scramble to escape oxygen depleted waters on the western shore. Image courtesy of the Chesapeake Bay Foundation’s Report on Bad Water and the Decline of Blue Crabs in the Chesapeake Bay, December 2008.


Figure 14 – Dead zone distribution alongside dissolved oxygen requirements. Author illustration.

Because blue crabs lead a migratory lifecycle, they are doubly susceptible to the lethal threat posed by the dead zones. Blue crab eggs require a minimum of 20 parts per thousand (ppt) salt content to survive, but blue crab larvae require closer to 30 ppt. Sea water is 36 ppt. Juvenile blue crabs can survive in waters with as little as 3 ppt salt content, and prefer defensible environments (not the open sea) for camouflage and access to food. As a result, the blue crab migrates up the bay to estuarine environments that afford ample opportunities for food and protection. The slow progress up the bay (blue crabs move an average of 705 feet per day) is doubly perilous due to the extent of the dead zones facing the crabs (fig. 15).

Figure 15 – Blue crab travel distance per day. Author illustration.

While blue crab males make the trip up the Bay only once, Blue crab females must make the trip thrice—once up for growth and mating, once down while eggs are brooding (14-17 days), and then up again to live out the remains of their three-year lifespans. While males mate many times, the female blue crab mates only once. Of the two-to-eight million eggs she can deliver for spawning, only 1% of the brood is expected to survive long enough to mate upstream and make the trip to the breeding grounds adjacent to Hoopers Island. After the eggs have hatched, the crab emerges as larvae, and evolves through seven zoeal stages over a course of thirty to fifty days. Subsequently the crab goes through one postlarval stage, that of megalopae, over a course of 6-58 days. Once the crab reaches what scientists call the juvenile state, it begins a process of feeding, molting, and northern progression that will lead to adulthood (fig. 16).

Figure 16 – Blue crab lifecycle mapped against Chesapeake Bay salinity. Author illustration.

Figure 17 – Population decline by gender. Author illustration.

Given this migratory lifecycle, it is no coincidence that the heart of the Maryland crabbing industry is located adjacent to the blue crab’s spawning grounds. This also may account for the marked decrease in the female population relative to the male population over time (figure 17). Given the threats of overharvesting, dead zones, and habitat degradation, it may come as a surprise that blue crabs are not farmed. Fish farming became widely practiced in the early 1990s. Couldn’t the same principles be employed to raise and produce crabs as a local commercial foodstuff?

The answer is that up until very recently, it was assumed that crabs could not be farmed like fish. The reason for this assumption is twofold. As a crustacean, blue crabs possess a hard exoskeleton made of chitin. This exoskeleton protects the crab from predators, but it is also imposes limitations on growth. In order to grow in size from a juvenile crab to a fully grown adult, the crab must molt—evacuate its hard exoskeleton and inflate its body with water while it waits for its exoskeleton to harden with the body in its bloated state. While the molting process does not take long (½ hour to 3 hours), the crab’s new shell will not harden for 24-36 hours, during which time the crab is known as a soft-shell and is incredibly susceptible to predators. This is not a one-time process. The blue crab molts an average of 27 times on its way to adulthood (figure 18).

Figure 18 – molting and cannibalism. Author illustration.

Most crustaceans display greater and lesser degrees of communal behavior. Lobsters, for instance, are communal creatures that protect members of the school when they molt. The blue crab exhibits the opposite behavior. The blue crab is a fierce cannibal and appears to prefer the taste of its own kind to that of any other food source. As such, crabs invariably attempt to benefit from their siblings’ brief moments of weakness after molting. Behaviorally, the blue crab is also hyper-aggressive, far more so than any other crab species. This fatal combination makes the survival rate for blue crabs very slight indeed (fig. 19).

Figure 19 – Agression Scale. Author Illustration. Information provided in 04.08.09Williams Interview.

To further complicate matters, the blue crab emits a pre-molt pheromone that entices any nearby blue crabs to mate or feed. This counterintuitive natural process is directly related to the blue crab’s mating process. At 12 to 18 months, the female blue crab is fully grown and ready to mate. When she reaches this point, she releases a pheromone in her urine which attracts males of the species. Prior to this final, “pubertal” molt, males will compete for the right to mate with her, and the selected mate will protect the female (referred to as "cradle carrying") until molting occurs. After this final molt, while the female’s shell is still soft, the pair will mate. During mating, the female stores the male's sperm in sac-like receptacles so that she can fertilize her eggs at a later time. Once the female's shell has hardened, the male will release her and the female will migrate to higher salinity waters for spawning. She will spawn two to nine months after mating and will release 2 to 8 million eggs, of which less than 1% are expected to survive to maturity. Recently the viability of rearing blue crabs to adulthood in confined spaces has been a topic of some debate. At the University of Maryland’s Biotech Institute’s Center of Marine Biotechnology (COMB) in Baltimore, scientists have been developing general outlines of the attrition rates associated with captivity. Based upon current market value, a mortality rate of upwards of even 95% could potentially be profitable. At COMB, scientists are working diligently to determine a method of synchronizing the blue crab’s molt. If an entire population of blue crabs were to molt at the same time, its believed that self-inflicted mortality rates would decline exponentially. And while it is not practiced in the US, crab farming has become a viable and lucrative industry in many other parts of the world even without synchronized molting.
[14] Author’s interview with Odi Zmora, master crab nutritionist at COMB, conducted 02.12.09.
[17] Author’s interview with Odi Zmora, master crab nutritionist at COMB, conducted 02.12.09.