The role of fishing gear in the spreading of allochthonous species: the case of Caulerpa taxifolia in the Ligurian Sea

Qualitative and quantitative changes in fish communities are described when a sea grass (Cymodocea nodosa) is replaced by a green alga (Caulerpa taxif olia) at Imperia (western Ligurian Sea). In general, the number of species, number of individuals and weight increase when the soft bottom is colonize d by the alga, but the catch of valuable fish, and consequently the fisherm en's income, decrease. The spreading of the alga is facilitated by fishing activity, in particular by bottom trawlers and trammel nets. Fishermen are themselves strongly affected by the spreading, not only because of the decr ease in valuable fish, but also because the massive presence of the alga in terferes with the use of the gear. (C) 2000 international Council for the E xploration of the Sea.