Date of introduction and origin
Sargassum muticum was first found attached in
Bembridge, Isle of Wight, in 1971 where it had arrived from France
(Farnham, Fletcher & Irvine 1973). Critchley, Farnham
& Morrell (1983) indicated that it was probably first found in
the English Channel in the late 1960s and Farnham (1980) indicated
by at least 1966. This species naturally occurs in Japanese and
Method of introduction
It was an associated unintentional introduction with
commercial introductions of oysters from the Canadian state of
British Columbia or Japan to France. Spread from northern France is
presumed to have occurred by natural means. Spores may be
transported in ballast water, on ships' hulls and by rafting or
floating of entire plants or detached fragments (Critchley et
al. 1990). Marginal dispersal (up to 30 miles) is most likely
to occur by the latter method (Farnham et al. 1981).
Reasons for success
This species has a rapid growth rate (Hales & Fletcher
1989). It is highly fecund (Norton & Deysher 1989), producing
fertile receptacles which are cast off during the summer months.
These float and can survive for up to 3 months (Farnham et
al. 1981). The receptacles are androgynous with
self-fertilisation; viable germlings are released.
Rate of spread and methods involved
It spread rapidly along the English south coast at about 30
km/year and along the north-west American coast at an average rate
of about 60 km/year, mostly by drifting, fertile adults (Farnham
et al. 1981).
It is found in the Isles of Scilly, entire Channel coast
(Hiscock & Moore 1986; Devon Wildlife Trust 1993) and east
coast north to Suffolk, however the Norfolk population appears to
be no longer extant (Critchley, Farnham & Morrell 1983; W.F.
Farnham pers. comm.). It was recorded from south Wales as drift
specimens in 1983 (N.C. Eno pers. obs.) and Lundy in 1993 (Andrew
Gibson pers. comm.), and as attached specimens from Strangford
Lough, Northern Ireland, in 1995 (Boaden 1995) and Constantine Bay,
north Cornwall, in 1991 (K. Hiscock pers. comm.). Plants 2-3 years
old were found in Crackington Cove, north Cornwall, in 1992 (W.F.
Farnham pers. comm.). Elsewhere in Europe it is known from the
Mediterranean and along the North Sea and Atlantic coasts of
Portugal, Spain, France, Belgium, The Netherlands, Denmark,
southern Norway and Sweden (Critchley, Farnham & Morrell 1983;
Rueness 1989; W.F. Farnham pers. comm.).
Factors likely to influence spread and distribution
Higher temperatures are favourable and will encourage its
spread further south. Lower temperatures are unfavourable and will
limit its spread north. Ideal conditions for growth are 25·C and
34‰ salinity, although this species will grow at temperatures from
10 to 30· and salinities from 6.8 to 34‰.
Effects on the environment
It causes the physical displacement of native species through
over-growing and shading underlying species (Critchley, Farnham
& Morrell 1986). There is documented replacement of
Laminaria saccharina and Zostera marina at
Grandcamp on the French Atlantic coast (Givernaud, Cosson &
Givernaud-Mouradi 1991). In Britain, there is observed growth of
Sargassum on eel-grass beds in the Isles of Scilly (Raines
et al. 1992) and in deep pools and channels Halidrys
siliquosa can be displaced by Sargassum muticum as
the dominant species (George, Tittley & Wood in prep.). Withers
et al. (1975) reported a rich epiphytic community
associated with Sargassum collected from the east Solent,
suggesting that native epiphytic species are not particularly
Effects on commercial interests
This species is a pest and fouling organism which is reported
to interfere with recreational use of waterways, particularly when
it becomes detached from hold fasts and floats off forming large
masses (Farnham 1980). It blocks propellers and intakes (Critchley,
Farnham & Morrell 1986). It is also a fouling organism on
oyster beds and a nuisance to commercial fishermen, fouling their
nets (Critchley, Farnham & Morrell 1981).
Control methods used and effectiveness
Removing Sargassum by hand is extremely
time-consuming and needs to be repeated, probably indefinitely
(Farnham 1980). Removal by trawling, cutting and suction have also
been tried. Chemical methods using herbicide have been tried but
failed due to lack of selectivity and the large doses needed. Small
germlings can be consumed by molluscs and amphipods but this has no
restrictive effect on S. muticum. Whatever method is used
the alga always quickly regrows and effective methods for its
permanent removal have not been found, although cutting and suction
is the preferred method applied (Farnham et al. 1981;
Critchley, Farnham & Morell 1986).
It is of possible commercial value to the alginate
In its native habitat off the coast of Japan S.
muticum is much smaller than in Britain (Rueness 1989). The
eradication of this species in British waters has been attempted
but has failed.
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muticum (Yendo) Fensholt in Strangford Lough, Northern
Ireland. Irish Naturalists' Journal, 25:
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brown alga, Sargassum muticum, 1973-1981. Journal of
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Acknowledgements (contributions from questionnaire)
Prof. T. A. Norton, Port Erin Marine Laboratory.