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First Report of Didemnum vexillum in Strangford Lough

First Report of Didemnum vexillum in Strangford Lough

Didemnum vexillum, the non-native carpet sea squirt, was found in Strangford Lough for the first time on 5th September 2012 during a Rapid Marina Survey. 

Didemnum vexillum
is of particular concern as an non-native species because:

  • It can smother native species on the seabed as has happened across hundreds of square km of Georges Bank USA

  • It can spread on gravel substratum, stable sands and seagrass beds as well as artificial structures

  • It can smother aquaculture areas and in New Zealand has led to a 20% loss of mussel biomass on hanging culture lines

  • It is well suited to surviving and reproducing in Northern Ireland waters as it is native to the temperate waters of Japan

  • It can reproduce by both sexual reproduction (larval dispersal) and asexual reproduction (breaking off of hanging tendrils)

Eradication programmes for Didemnum vexillum have occurred in marinas in New Zealand and recently in Holyhead, Wales. These programmes mainly entail the use of divers to wrap structures in black plastic after which calcium hypochlorite is pumped in to kill the Didemnum vexillum. Wales are now in their second year of a three year programme which has cost £200k in the first round with an estimated £350-£450k for the next round (Holt, 2012) (although the pontoon structures in Holyhead are more extensive than the known extent to date in Strangford Lough).

People are asked not to try removing Didemnum vexillum manually as they may inadvertently help it to spread.  It is also important to ensure that the washings any structures being cleaned should not be allowed back into the marine environment.   The presence of Didemnum vexillum may not be obvious and even a very small amount may reproduce. 


Holt, R. (2012) Eradicating the non-native carpet sea squirt Didemnum vexillum from Holyhead harbour, North Wales. Online.

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