Around the world, large deep-water sponge fields are hotspots of biodiversity. Their three-dimensional structure provides highly varied habitats for a wide range of species. But deep-water sponges are being swept up as by-catch, crushed or scarred by bottom trawlers or clogged by the clouds of sediment churned up by their heavy fishing gear.
Sponges of the deeper regions of the ocean margin, like many other deep-water species tend to be slow-growing, making them particularly vulnerable to changes to their environment and therefore, particularly at risk from the devastation wreaked by deep sea bottom trawlers. Some of the oldest multi-cellular animals, sponges have been living in the world’s oceans for over 600 million years. Glass sponges many species of which were thought to have died out with the dinosaurs, today continue to create, towering sponge reefs the height of office blocks off the coast of British Columbia. Scientists fear that the spectre of bottom trawling may now drive such fragile deep-water species to extinction, even before they can be discovered and properly researched. Scientists are only just beginning to unlock the mysteries and wonders of the deep. Only a handful of the oceans’ estimated 100,000 or more seamounts have been sampled. (1) The Canadian glass sponge reefs referred to above, were only discovered in 1987 by researchers curious to find out what was responsible for puzzling shapes showing up on sonar survey maps. The reefs grow by the attachment of new sponges on top of the remains of their predecessors – a process that has been going on for at least 9000 years. Individual sponges can reach over a metre in height and are thought to be over one hundred years old. In some places the reefs reach 19 metres high and discontinuously cover 425-700 km2. The Canadian Department of Fisheries and Oceans set up trawl fishery closures to protect these glass sponge reefs and some have suggested that they should be included on the UNESCO list of World Heritage sites. The North Atlantic is also home to large sponge fields that, like seamounts and cold-water coral reefs, are proving to be hotspots of biodiversity, sheltering a rich variety of deep-water creatures. Jan Helge Fosså of the Institute of Marine Research in Bergen and Ole S. Tendal from the Zoological Museum, University of Copenhagen, suggest that there are normally twice as many species on sponge fields as on the surrounding seabed. In a study of sponge fields in Faroese waters, more than 250 species of invertebrates were found to be associated with the sponges. “We have also regularly observed young redfish, sometimes seeking shelter inside the cavities of large sponges.” (2) Fosså and Tendal, however, also observed that a number of North Atlantic sponge species are well known to fishers, often being pulled up as by-catch. Although most bottom trawlers tend to avoid dragging trawl nets over sponge fields because of the danger of the fish they catch becoming contaminated by silicious needles (spicules), some may accidentally trawl through sponge fields or even deliberately use trawl gear to “sweep up” the sponges so they can trawl over the area afterwards. This is in spite of the fact that fishermen also claim that fish catches decrease after sponge grounds are damaged by trawling. Sponges may be destroyed by bottom trawlers in three ways: as a result of being dragged up in nets; damaged as the nets are scraped across the sea bed; or have their canal system clogged by clouds of sediment churned up by the fishing gear. Sponges thrown back overboard cannot survive because their interior canal system has been drained of water and filled with air while on the surface. Investigations in Australia showed that 89% of the sponges hit by trawl gear were damaged and in laboratory experiments 60% or more died within 3 months after damage. Although there are no detailed records about the impact from fisheries on sponge grounds in the Northeast Atlantic, Fosså and Tendal conclude that it is likely that trawling is damaging some areas. “Because of their slow growth it is likely to be a long time before sponge-dominated areas recover even after partial damage; repeated disturbance may lead to permanent removal of the species in an area.” They recommend further research into the ecological importance of sponge fields and point to the need for further mapping of these areas.
Notes: (1) A great deal of deep-seas biodiversity is concentrated around seamounts – —underwater islands that rise 1,000 meters or higher from the seabed. (2) Discovering deep-water sponges, ICES article by Jan Helge Fosså, Institute of Marine Research in Bergen and Ole S. Tendal, Zoological Museum, University of Copenhagen.