Seamount benthic macrofauna off southern Tasmania: community structure and impacts of trawling

Abstract

The benthic macrofauna of a group of small seamounts south of Tasmania was surveyed with a dredge and camera to assess the impact of trawling for orange roughy (Hoplostethus atlanticus; Trachichthyidae) and the efficacy of a proposed marine reserve. The seamounts were generally 300 to 600 m high and the peaks ranged from 660 to 1700 m depth. The fauna was diverse: 262 species of invertebrates and 37 species of fishes were enumerated, compared with 598 species of invertebrates previously reported from seamounts worldwide. On seamounts that peaked at depths <1400 m and that had not been heavily fished, the invertebrate fauna was dense, diverse and dominated by suspension feeders, including a matrix-forming colonial hard coral (Solenosmilia variabilis) and a variety of hard and soft (gorgonian and antipatharian) corals, hydroids, sponges and suspension-feeding ophiuroids and sea stars. Of the invertebrate species, 24 to 43% were new to science, and between 16 and 33% appeared to be restricted to the seamount environment. Trawl operations effectively removed the reef aggregate from the most heavily fished seamounts. The benthic biomass of samples from unfished seamounts was 106% greater than from heavily fished seamounts and the number of species per sample was 46% greater. Living S. variabilis was not found on seamounts peaking at depths >1400 m. These seamounts were dominated by sea urchins and had lower biomass and fewer species per sample. However, few species were restricted to either the shallowest or deepest depths sampled. The fauna unique to the region's seamounts appears to be adequately represented within a recently established 'Marine Protected Area' that encloses 12 seamounts that peak at depths >1150 m.KEY WORDS: Seamount · Benthos · Impacts of trawling · Community structure 213: 111-125, 2001 rockhopper gear -large rubber bobbins and metal discs along the footrope -and precise electronic positioning systems both for the vessel and to monitor net performance. Seamounts are one such environment to become subject to intensive trawl fishing in recent decades. Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerSeamounts provide a unique deep-sea environment due to the topographically-enhanced currents in their vicinity (Roden 1986). In the water column, substantial aggregations of deep-bodied fishes, such as the pelagic armourhead (Pseudopentaceros wheeleri), Sebastes spp., orange roughy (Hoplostethus atlanticus) and oreosomatids are commonly found around seamounts (Boehlert & Sasaki 1988, Koslow 1996, 1997. These aggregations are supported in the otherwise food-poor deep sea by the enhanced flux of prey organisms past the seamounts and the interception and trapping of vertical migrators by the uplifted topography (Tseitlin 1985, Genin et al. 1988, Koslow 1997. Discovery of these aggregations led to seamounts being increasingly targeted by trawlers throughout the world's oceans: i.e. the massive but short-lived fishery for pelagic armourhead in the N...

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