No abstract

In this review of seamount ecology, we address a number of key scientific issues concerning the structure and function of benthic communities, human impacts, and seamount management and conservation. We consider whether community composition and diversity differ between seamounts and continental slopes, how important dispersal capabilities are in seamount connectivity, what environmental factors drive species composition and diversity, whether seamounts are centers of enhanced biological productivity, and whether they have unique trophic architecture. We discuss how vulnerable seamount communities are to fishing and mining, and how we can balance exploitation of resources and conservation of habitat. Despite considerable advances in recent years, there remain many questions about seamount ecosystems that need closer integration of molecular, oceanographic, and ecological research.

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...

Complex biogenic habitats formed by corals are important components of the megabenthos of seamounts, but their fragility makes them susceptible to damage by bottom trawling. Here we examine changes to stony corals and associated megabenthic assemblages on seamounts off Tasmania (Australia) with different histories of bottom-contact trawling by analysing 64 504 video frames (25 seamounts) and 704 high-resolution images (7 seamounts). Trawling had a dramatic impact on the seamount benthos: (1) bottom cover of the matrix-forming stony coral Solenosmilia variabilis was reduced by 2 orders of magnitude; (2) loss of coral habitat translated into 3-fold declines in richness, diversity and density of other megabenthos; and (3) megabenthos assemblage structures diverged widely between trawled and untrawled seamounts. On seamounts where trawling had been reduced to < 5% a decade ago and ceased completely 5 yr ago, there was no clear signal of recovery of the megabenthos; communities remained impoverished comprising fewer species at reduced densities. Differences in community structure in the trawled (as compared to the untrawled) seamounts were attributed to resistant species that survived initial impacts, others protected in natural refugia and early colonisers. Long-term persistence of trawling impacts on deep-water corals is consistent with their biological traits (e.g. slow growth rates, fragility) that make them particularly vulnerable. Because recovery on seamounts will be slow, the benefits from fishery closures may not be immediately recognisable or measureable. Spatial closures are crucial conservation instruments, but will require long-term commitments and expectations of performance whose time frames match the biological tempo in the deep sea.

Deep-sea fisheries operate globally throughout the world's oceans, chiefly targeting stocks on the upper and mid-continental slope and offshore seamounts. Major commercial fisheries occur, or have occurred, for species such as orange roughy, oreos, cardinalfish, grenadiers and alfonsino. Few deep fisheries have, however, been sustainable, with most deep-sea stocks having undergone rapid and substantial declines. Fishing in the deep sea not only harvests target species but can also cause unintended environmental harm, mostly from operating heavy bottom trawls and, to a lesser extent, bottom longlines. Bottom trawling over hard seabed (common on seamounts) routinely removes most of the benthic fauna, resulting in declines in faunal biodiversity, cover and abundance. Functionally, these impacts translate into loss of biogenic habitat from potentially large areas. Recent studies on longline fisheries show that their impact is much less than from trawl gear, but can still be significant. Benthic taxa, especially the dominant mega-faunal components of deep-sea systems such as corals and sponges, can be highly vulnerable to fishing impacts. Some taxa have natural resilience due to their size, shape, and structure, and some can survive in natural refuges inaccessible to trawls. However, many deep-sea invertebrates are exceptionally long-lived and grow extremely slowly: these biological attributes mean that the recovery capacity of the benthos is highly limited and prolonged, predicted to take decades to centuries after fishing has ceased. The low tolerance and protracted recovery of many deep-sea benthic communities has implications for managing environmental performance of deep-sea fisheries, including that (i) expectations for recovery and restoration of impacted areas may be unrealistic in acceptable time frames, (ii) the high vulnerability of deep-sea fauna makes spatial management—that includes strong and consistent conservation closures—an important priority, and (iii) biodiversity conservation should be &gt; balanced with options for open areas that support sustainable fisheries.