Sargassum muticum at the west and south coast of Ireland: an invasive species on the move?

A project run by the Irish   Seaweed Centre during the summer of 2003. This project is funded by the   Heritage Council

EXECUTIVE SUMMARY

The alien invasive seaweed species Sargassum muticum was recorded from Kilmore Quay for the first time in Ireland by the Irish Seaweed Centre in 2001. After this record, it has been found at Rath strand Kenmare River (Co. Kerry), Cashel Bay (Co. Galway) and Drum Cliff Bay (Co. Sligo) in the following years. It is important to note that these specimens were attached, indicating the presence of established populations. As shown in other studies of the invasion of Sargassum muticum in the U.K., Denmark, and Portugal, this species may have an impact on tourism, biodiversity and the aquaculture industry.

In Ireland a study was carried out from April 2003 to November 2003 to map the geographical distribution and spread of the invasive brown macroalgae Sargassum muticum.  The overall objective of the study is to determine the present status, distribution and invasive capability of Sargassum muticum at the west and south coast of Ireland.The areas where Sargassum muticum were reordered for the first time in the past were surveyed in this study including neighbouring areas. Furthermore, bays and inlets in close proximity of the original survey area were also examined. The extent of the population and spreading ratio was measured if Sargassum muticum was encountered.

The following areas were surveyed: Sligo Bay and Drumcliff Bay in Co. Sligo; Mannin Bay, Clifden Bay, Bertraghboy Bay, Kilkieran Bay in Co. Galway; Portmagee Channel, Kenmare River, Bantry Bay and Dunmannus Bay in Cos Kerry and Cork; Hook Peninsula, Ballyteige Bay and Kilmore Quay area. Detailed maps with distribution, spreading and extend of the beds are produced from the survey records. This study showed that Sargassum muticum most probably arrived in the mid nineties and is spreading along the coast of Ireland. 

The results indicate that spreading is facilitated by Yachting and perhaps via Oyster transport. Spreading rates are difficult to calculate due to the nature of the introduction and late discovery in Ireland, nevertheless, with some uncertainty a rate of spreading of 2-3 km year ^-1 has been calculated for two bays. Furthermore, this study showed that S.muticum has been found growing in Seagrass beds and in rock pools, which might have serious consequences for the biodiversity in rock pools and protective status of seagrass habitats. 

It is recommended that surveying of the spread of S. muticum should continue in order to observe the effects on local flora and fauna and possible effects on aquaculture, the Seaweed Industry and Tourism. Moreover, a detailed study of the effects of S. muticum on rock pool communities and seagrass beds should be initiated.

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