INTRODUCTION: EUROPEAN SHORE CRAB.

I had been doing some reading about the invasive Green crab or European shore crab, Carcinus maenas that we often encounter in the Cape Town harbour. Apart from the literature searching and all the reading, I decided to do my own investigation right outside the effluent of the Two Oceans Aquarium, outside the seal pool. I used a make-shift piece of nylon netting attached to a piece of string weighted down on one end with a normal-sized fishing sinker. Inside the net I placed four whole defrosted pilchards as bait, and sat back to see what happened. The net sank to the bottom quickly and I began my stopwatch. At each minute passed, I pulled up the net and deposited the crabs into a bucket. I stopped “fishing” after exactly 10 minutes and took the crabs inside the aquarium to be counted. I caught 16 European shore crabs in 10 minutes! That is 1.6 crabs per minute! (See photo).

Now that we have put this crab’s potential into perspective, lets look at a bit of its history and characteristics.

This crab is listed as among the 100 most invasive species in the world. Its carapace length can be up to 6cm long and it has 5 distinct “teeth” along the rim of the carapace by the eyes (Wikipedia.org). The colour of the European shore crab varies from different greens to browns and an orangy-red. It is thought that the various colours are associated with different environmental factors (Wikipedia.org). It can be found down to 60m and on all types of shores from muddy to rocky, but prefers shallower areas (Department of Fisheries, Australia). It can also tolerate a wide range of temperatures (0 to 33 Degrees Celsius) and salinities (4 to 54ppt) (Washington Department of Fish and Wildlife).

According to the Washington Department of Fish and Wildlife, the European shore crab has the ability to learn, to improve “prey-handling skills” while foraging for food. This has resulted in an invasive species, which is faster, more dexterous and can take better advantage of the same resources than our native species, easily out-competing them.

In the crab’s native environment it feeds on mussels of the genus Mytilus, Cockels and Dogwhelks. It is also the intermediate host for the acanthocephalan Profilicollis botulus, which kills eider ducks in Scotland (Washington Department of Fish and Wildlife).

The European shore crab is actually native to the North African coast and the Baltic Sea, as well as Norway and Iceland, but was introduced to the East coast of North America in Massachusetts in 1817 (Wikipedia.org). In Australia it was first reported in Port Phillip Bay in Victoria in the early 1900s (Department of Fisheries, Australia), and invaded New South Wales in 1971 and then parts of Southern Australia in 1976. It reached Tasmania in 1993 (Wikipedia.org).

The European shore crab was first encountered in South Africa in Cape Town harbour in 1983, only 23 years ago! It is now also found in Saldanha Bay and Camps bay (Wikipedia.org). The rate of invasion in South Africa so far has been just over 100km in 23years, or about 4.38km per year.

Suggested control measures

In Massachusetts a bounty was established in 1995 for catching the European shore crab to protect the local ecosystem. According to Wikipedia.org, 10 tonnes were caught.

Potential biological control trials in the United States using the parasitic barnacle Sacculina carcini are being investigated, however the indiscriminate nature of the infestation of the parasitic barnacle suggests that it could impact the local crab populations as well, and this would have to be seriously weighed up before being suggested as a course of action (Wikipedia.org).

One alternative is to exploit the resource of the European shore crab as a source of food. In Ireland, baited creels are used alongside the oyster farms to prevent the damage to the oysters by the feeding crabs, and the crabs are collected once caught and sold for processing into food products. The market is relatively small and uncompetitive since the value of the European shore crab is only about €500 per tonne (Wikipedia.org).

The crabs from the Cape Town harbour were often used as a source of food for the resident Black musselcrackers and Red steenbras in the Two Oceans Aquarium’s Open Ocean exhibit (Predator display), and as a food source for the octopus. The use of this crab as a source of food was ceased after a lab sample indicated that these crabs accumulated heavy metals within their tissues and could be a cause of heavy metal poisoning in our fishes in the long-term. It is therefore suggested that the European shore crab cannot be used for human consumption in South Africa, however, it could possibly be controlled using baited traps or by means of frequent dive teams and physical removal operations sanctioned by Marine and Coastal Management. Of major concern is the recent declaration of Moullie Point as a “Closed Area” under Section 77 of the Marine Living Resources Act (Act 18 of 1988), which states that no Rock Lobsters may be caught in the region 12 nautical miles seawards from the high-water mark. Using baited traps for these crabs may constitute an associated catch of these protected lobsters, and would therefore have to be strictly policed.

David Vaughan
Senior aquarist, Quarantine
Two Oceans Aquarium
Cape Town, South Africa
dvaughan@aquarium.co.za

Picture credits:

All pictures are the property of David Vaughan, Two Oceans Aquarium and may not be copied or used without permission.

References:

Wikipedia contributors. Carcinus maenas [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jul 13, 07:10 UTC [cited 2006 Aug 13]. Available from: http://en.wikipedia.org/w/index.php?title=Carcinus_maenas&oldid=63560398.

Washington Department of Fish and Wildlife: Aquatic Nuisance Species [Internet]. [cited 2006 Aug 12, 11:38]. Available from:
http://wdfw.wa.gov/fish/ans/greencrab.htm.

Australian Department of Fisheries: European shore crab [Internet]. [cited 2006 Aug 12, 12:55]. Available from:
http://www.fish.wa.gov.au/docs/pub/IMPShoreCrab/index.php?0506