NOSB paper

This paper was written as part of the 2004 Alaska Ocean Sciences Bowl high school competition. The conclusions in this report are solely those of the student authors.

Threat of European Green Crab Invasion in Kachemak Bay


Monica Peters
Kirsten Baltz
Joleena Baugh
Benay Breyfogle


Team Mola Mola
Homer High School
600 E. Fairview Ave
Homer, AK 99603

Homer team photo


Invasive aquatic species have been impacting foreign coasts for hundreds of years. Some are damaging to local ecosystems by preying on native species and by out-competing them for food. The ballast water of ships is accredited with providing transportation for the spreading of many invasive species. The European green crab has spread up the west coast of North America through ballast water and is now as far north as British Columbia. This species of crab is considered a threat to Kachemak Bay. The green crab will be capable of surviving in Kachemak Bay if they do arrive in the ballast water of ships from Washington because the conditions are suitable for them. The green crab would be likely to prey on the native clams, oysters, mussels, marine worms, and other crabs, including Dungeness the same size or smaller, as they have in other ecosystems that they have invaded. To prevent the invasion of the European green crab into Kachemak Bay, the ballast water of all incoming ships might be deoxygenized to kill the green crab larvae in it. The invasion of the green crab is a significant threat that must be dealt with if it is to be avoided.


Travel by sea has played a very significant role in human history and is still making an impact now. Expansion of the Egyptian, Greek and Roman Empires was greatly assisted by sea exploration. Voyages across the Atlantic Ocean from Europe led to the discovery of North and South America. Ever since these discoveries, ships have been carrying people and merchandise across the ocean. This presented new dangers to ports at both ends of the journey. Years before, stowaway rats infected with the Black Death spread the plague to several European port cities. These rats are perhaps the most destructive and deadly creatures to have used ships as transportation. The use of ballast to keep large cargo ships balanced as they traveled across seas and oceans became an invitation for aquatic animals to become hitchhikers. The European green crab (Carcinus maenas) is likely to have traveled to U.S. shores during the 19th century, a stowaway in a cargo ship, arriving somewhere between New Jersey and Cape Cod. Although it is not certain how this highly destructive green crab spread to the west coast, one of the most likely possibilities is that it caught a ride in the sea ballast water of a ship during its larval stage, as this is the mode taken by most marine invaders. The European green crab quickly spread up the West coast, harming local ecosystems by taking up resources. Alaska is now at risk of being intruded by this voracious predator.

The European Green Crab

The European green crab is native to the Atlantic coast of Europe, including Norway and the British Isles, and northern Africa. During the 19th century, the green crab traveled across the Atlantic Ocean to the upper east coast of America. A population of green crabs was found further north in Nova Scotia during the 1960's. The first documented green crab on the west coast was in 1989 in San Francisco Bay, California. Since then, the green crab has spread northward up the west coast, and was first spotted in Oregon in 1997, Washington in 1998 and British Columbia in 1999. In addition to both coasts of North America, the European green crab has spread to South Africa, Australia and Japan.

The European green crab is occasionally not green, regardless of its name. The carapace of the crab is usually mottled green or brown with small yellow spots. The color of the crab's underside can vary between green, brown, yellow, white and red. The color may change during the molting process ( The green crab has a few distinctions that make it recognizable from other crabs. Unlike Pacific Northwest crabs, the European green crab has five spines on either side of the eyes on the front end of the carapace. There are also three small knobs in-between the eyes that are used for identification because they are unique to the green crab (Figure 1). The green crab does not grow to be much bigger than 4 inches (~100 mm) and an average crab is around 2.5 inches (~63 mm) (

Fig. 1, green crab diagram

Figure 1. This figure identifies the five spines and the three small lobes between the eyes. These are the unique features of the European green crab.

The life span of the green crab is similar to that of other crabs. It usually lives less than 80 days in its larval stage as plankton. The ocean currents can disperse the larvae up and down the coast. Juvenile crabs are swept into the upper intertidal zone by ocean tides and currents and settle there. If the conditions are good, the crab will survive and be able to reproduce, extending the green crab population further along the coast (Smith and Copping, 2001).

The European green crabs have proven to be very competitive predators in the habitats they have invaded. Since the arrival of the green crab in Bodega Bay, California in 1993, there have been noticeable reductions in the populations of the native clams and a species of shore crab. On the east coast, the green crab is credited with the significant decline of the soft shell clam fisheries in the 1950s. Green crabs are known to prey on clams, oysters, marine worms, mussels, and small crustaceans, including other crabs. Laboratory studies have shown green crabs will readily feed on Dungeness crabs the same size or smaller than themselves (

Risk to the Kachemak Bay

The European green crab has been introduced to some coasts with very different characteristics and has survived in many of them. This species of crab can tolerate any salinity between 4 and 54 ppt (parts per thousand) and temperatures between 0° and 33° Celsius (WDFW 2002). In the spring, summer, and fall, the salinity of Kachemak Bay is usually between 29 and 32 ppt. Water temperatures in Kachemak Bay are rarely below the green crab's tolerance range, so it is capable of surviving these conditions of the bay.

The European green crab is a very competitive and often successful predator. Because of the damage that the green crab has done to other coasts that they have invaded, biologists believe that this species of crab poses a significant threat to local ecosystems and fisheries. After the introduction of the green crab to the coasts of Maine, the number of clams harvested went from 14.7 million in 1938 to only 2.3 million in 1959. On the west coast, the European green crab has already cost about $44 million in damages (Licking 1999). In Kachemak Bay, the European green crab would be capable of eating juvenile clams and oysters, among others. The green crab would even eat some of the smaller native crabs, including Dungeness crabs. Green crabs are also capable of out-competing other crabs. It is suspected that the European green crab will greatly reduce the populations of, and possibly even kill off, native species that are vital to the local coastal ecosystems by preying on them and out-competing them for food and resources.

Unnatural Methods of European Green Crab Dispersal

The quickest method for the European green crab to spread to new areas is to travel in the ballast water of ships. This is how it is believed that the green crab crossed the Atlantic Ocean and arrived on the east coast of the United States. Since then, the green crab has been hitchhiking in ballast water as it spread all along the east coast and eventually across to and all along the west coast as well. Ballast water hitchhiking can be particularly risky to Kachemak Bay, which regularly receives regular large ship traffic, including oil tankers and wood-chip ships.

Ballast water is stored in the hold of a ship to produce stability and keeps the ship at a certain depth in the sea to reduce the possibility of capsizing while en route. Many different planktonic species are drawn up into the tanks as well as the water. Upon arrival at the port of destination, the ballast water is released from the tanks into the foreign waters. All the aquatic species that were traveling in the ballast water are released into that coastal area. These introduced species can survive in the new environment if its conditions are suitable for them. A few of the animals that survived the trip become invasive species.

Ballast Water Regulations

Ballast water regulations are used to enforce the provisions of the Non-Indigenous Aquatic Nuisance Prevention and Control Act of 1990 ( These guidelines for ballast water treatment apply to all vessels in the United States. Since water is acquired at one port and eventually discharged at another, there is a great chance that invasive species will survive in the water during the journey, and then be introduced to that new environment (L. David Smith,, December 4, 2002). The extensive regulations for ballast water created by the United States Coast Guard are designed to prevent the transport of invasive species in a ship's ballast water.

Since there is no ballast water management technique that is capable of effectively removing all organisms from ballast tanks, precautions must be taken to avoid contamination of waters. Ballast tanks are to be cleaned often and on a regular schedule. The Coast Guard also states that water from ballast tanks should be discharged only by minimal amounts when in coastal and internal waters ( Personnel on each vessel are required to have been trained in the management and treatment of ballast water. A vessel-specific management plan is also required. The removing of organisms from the hull, tanks, and piping regularly helps prevent invasive species transport. These substances should be disposed of following regulations at the federal, state, and local levels ( To be sure that the regulations are being followed, the Coast Guard asks that every vessel not bound for the Great Lakes, Hudson River, or north of the George Washington Bridge report the required ballast water records at the first port of call by mail, electronically, or fax. Keeping accurate records while following laws and regulations is a large part to preventing the spread of invasive species from ecosystem to ecosystem.


Taking preventive measures against the invasion of the European green crab in Kachemak Bay has become very important since the organism was discovered in Washington and British Columbia. This can be done by regulating where ballast water is dumped or by killing everything alive in ballast water while it is in the tanks. Ballast water exchange, which is exchanging ballast water in the open ocean where the aquatic species are not as likely to be able to survive in coastal environments, is a good idea. However, it is not always practical because of bad weather, sea surface conditions, and the fact that many of the ships traveling between Washington and Alaska never leave coastal waters.

Treatment of ballast water can be done in several different ways. Toxic chemicals can be introduced into the ballast water, but this may be more damaging to the environment when the water is released. An electrical current could also be passed through the ballast water to electrocute all the living organisms. Filtration and the introduction of biocides into ballast water are other possibilities. These methods are all fairly costly, though. Another way of killing the aquatic hitchhikers in ballast water is to deoxygenate the water. These are some possibilities for preventing the arrival of invasive species by ballast water.


The European green crab poses a great threat to Kachemak Bay. It is a very capable predator that is able to do significant damage to the native aquatic species, particularly those that live primarily in the intertidal zone. The green crab spreads to new areas in the ballast water of ships. Prevention of the invasion of the European green crab and other foreign aquatic species into Kachemak Bay will require that all ships with ballast tanks that enter the bay first kill off their stowaways.

Deoxygenation would be one practical and cost-effective way of doing this. Though deoxygenation is sometimes thought of as too expensive a method of controlling invasive species in ballast water, it really is not prohibitively expensive because it also stops the corrosion of ballast tanks, and therefore takes care of two problems at once. Marine ecologists at the Monterey Bay Aquarium Research Institute ( have done laboratory experiments that have proven that deoxygenated ballast water very effectively kills off European green crab larvae, which is the stage in which they travel in ballast water. They were usually dead within only two or three days while cargo ships crossing oceans typically take a couple weeks to arrive at their destination (

References Cited

Ecological Society of America. http://esa.sdsc.edi/invas3.htm. 10/23/2003.

Houghton Mifflin Company, online dictionary. 12/6/2003.

Licking, Ellen. 5/24/1999. Environment: ECOLOGY: THEY'RE HERE, AND THEY'RE TAKING OVER. Business Week.

MBARI News, ballast water deoxygenation study. 12/2/2003.

National Research Council, ballast water deoxygenation research. 10/24/2003.

Prince William Soumd Regional Citizen's Advisory Council. 10/26/03.

Sea Grant, ballast water treatment options. 10/24/2003.

Smith, Scott and Andrea Copping. Washington Sea Grant Program. 12/2/2003.

Tangley, Laura. 10/26/1998. Unwelcome Sea Voyagers. U.S. News and World Report.

United States Coast Guard, United States ballast water regulations. 10/24/2003.

Washington Department of Fish and Wildlife. 12/2/2003.

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