Veined whelks: Wanted dead or alive

In the summer of 1997, a fisherman working in the Bay of Quiberon, on the Brittany coast, was surprised to find a strange-looking whelk (a type of marine snail) hanging in his nets. Intrigued, he kept it and took it back to shore where he handed it in to the local marine laboratory. A number of other fishermen had also brought in these strange whelks and the scientists at the Ifremer Laboratory in La Trinite-sur-Mer now had 10 of them, and they were very uneasy.

Rapana venosa

The whelks were quickly identified as Rapana venosa, the veined whelk, a voracious consumer of shellfish, such as mussels and oysters.

The problem was that veined whelks should not have been living off the Brittany coast and in fact they should not have even been in the Atlantic Ocean. They normally live on the other side of the world in the Sea of Japan. But this was not the first time they had turned up thousands of miles from their normal habitat.

Veined whelks are thought to have first arrived in the Black Sea in the 1940s either on ships’ hulls, in ships’ ballast water or with Pacific oysters imported into the area for aquaculture. However they arrived, they were tough enough to survive low oxygen conditions and low salinities and soon thrived on the abundant local shellfish stocks.

In fact they ate so well that they are now blamed for almost wiping out many local stocks of mussels, scallops and oysters. Over the years they have also expanded their range from the Black Sea down to the Adriatic Sea and they are now slowly moving west across the Mediterranean.

They have also turned up in other sea areas. In June 1998, a year after the Brittany finds, a research trawler owned by the Virginia Institute of Marine Science, was busy working in Chesapeake Bay, on the east coast of the United States. The first haul of the day had been fairly standard with a few fish, crabs and assorted strands of algae; but they had also found an unusual-looking whelk.

It turned out to be a veined whelk and since then more than 6000 have been collected in the Bay. The biggest whelks reach a size of 17cm, a similar size to a mango, while the average size has been between 10 and 16cm.

And it seems that the whelks intend to be a permanent feature of Chesapeake Bay: in the last couple of years a number of smaller whelks at less than 10cm have been collected, which indicates that the whelks have reproduced successfully and the next generation are coming through.

Scientists think that what gives the whelks the advantage in Chesapeake Bay is that they grow so quickly that they are soon too big and thick-shelled for the local predators, such as turtles, to fit in their mouths. It seems that because the native whelks do not grow as broad, or as quickly, they end up being chosen for food over their Asian cousins.

And once the veined whelks have reached the size where they are too big for predators to tackle them, they reign as unchallenged predators for the rest of their lives, which can be more than ten years: and that means eating a lot of shellfish.

The huge appetite of the marauding whelks has united both scientists and local fishing communities in their concern over the effect on Chesapeake Bay’s local stocks of mussels, oysters and in particular hard clams (Mercenaria mercenaria), which support a fishery worth $3 million a year.

Like humans, veined whelks are particularly fond of hard clams and adult whelks are capable of munching their way through up to 2.7 grams of clam tissue a day. They are also quite partial to oysters and can eat up to two oysters a day.

And when a female whelk has eaten enough to grow to sexual maturity she turns to reproduction. She lays her fertilized eggs in mats. Each mat may contain 50-500 egg cases and in each egg case there can be 200 – 1000 eggs. When the eggs hatch, after two to three weeks, thousands of whelk larvae are released to float round in pelagic - or surface - waters for about three weeks before moving down to settle on the seabed.

This drifting larval phase is thought to be one of the reasons why the veined whelks of the Black Sea have been able to disperse out into the Adriatic and Mediterranean Seas so successfully. It is also thought that pelagic larvae could have been taken onboard ships traveling from the area with ballast water.

Then when ship’s captains have ordered the release of their ballast water in areas such as Chesapeake Bay, they have unwittingly delivered a potential timebomb for local shellfish stocks.

But the veined whelks are not getting it all their own way. In Chesapeake Bay the scientists and fishers are fighting back. The Virginia Institute of Marine Science has put a bounty on veined whelks and local fishers bring them in dead ($2) or alive ($5).

In Chesapeake Bay, veined whelks have a bounty on their heads

The Institute has also distributed descriptions of the egg cases around the Bay’s fishing communities along with information on how to destroy them. There are also proposals to try and stimulate a targeted fishery for the whelk, as has developed in the Black Sea, to keep the numbers down. As part of this initiative they even have produced a selection of “tasty” veined whelk recipes for locals to try.

Back on the other side of the Atlantic, on the Brittany coast, the veined whelk does not seem to have established itself as quickly. After intense searches since the first finds in 1997, the number reported has been less than thirty; although, ominously, some veined whelk egg cases have also been found.

In answer to the question as to how the whelks came to be living on the Brittany coast, French scientists think that they originally arrived in Brittany as ballast to weigh down bags of live Manilla clams (Tapes philippinarium), which were transported from the Adriatic for mariculture.

Having worked out where the whelks were coming from the scientists are now trying to make sure that they do not gain a foothold on the French coast, by starting a renewed eradication programme using nets and dredges, complemented by an intensive public education campaign.

The success of this programme will ultimately depend on whether they have been quick enough to catch the pioneer whelk population. It will also depend on how suitable the conditions are for the veined whelk; because if it flourishes it can be virtually impossible to eradicate as the residents of Chesapeake Bay have found out.

By Neil Fletcher

For more information on the veined whelk please consult the ICES Report Alien. Species Alert: Rapana venosa (veined whelk).
http://www.ices.dk/pubs/crr/crr264/crr264.pdf

For information on the battle against the veined whelk in Chesapeake Bay please see:
http://www.vims.edu/mollusc/research/rapaw/rapup.htm

Go to ICES Code of Practice on the Introduction and Transfer of Marine Organisms.
http://www.ices.dk/reports/general/2003/Codemarineintroductions2003.pdf

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