By Philip Mladenov
Seamounts are distinctive and dramatic features of ocean basins. They are typically extinct volcanoes that rise abruptly above the surrounding deep-ocean floor but do not reach the surface of the ocean. The Global Ocean contains some 100,000 or so seamounts that rise at least 1,000 metres above the ocean floor.
Seamounts represent a very special kind of biological hotspot in the deep ocean. A prolific group of suspension feeding animals dominate the summit and flanks of seamounts, creating dense thicket-like communities comprising cold water stony corals, sea fans, black corals, and sponges that create habitat for a host of other animals including dense aggregations of fish. Many of the corals associated with seamounts show extreme longevity. For example, a type of black coral, Leiopathes, sampled from a seamount in the Pacific Ocean, was shown using radiocarbon dating to be about 4,200 years of age, making it one of the world’s longest living animals.
Seamounts support a great diversity of fish species; the latest census reveals close to 800 species have been recorded living around seamounts. In the 1960s deep sea trawling vessels looking for new stocks of fish began to trawl seamounts and discovered large aggregations of commercially important species. This triggered the creation of new deep-ocean fisheries focused on seamounts. Heavily built bottom trawls are towed from the summit down the flanks of seamounts to capture the fish. Commercial fish species that are targeted include orange roughy, oreos, alfonsinos, grenadiers, and toothfish. These fish are not generally permanent residents of seamounts, but aggregate at seamounts at certain times of the year to spawn, to feed on squid and small fishes, or simply to rest. They are very slow-growing and long-lived and mature at a late age, and thus have a low reproductive potential. A good example of this is the orange roughy, which is known to live for more than a hundred years and reaches maturity at around thirty years of age, with the females producing relatively small numbers of eggs. Such a life history is typical of many deep-ocean fish species.
Seamount fisheries have often been described as mining operations rather than sustainable fisheries. They typically collapse within a few years of the start of fishing and the trawlers then move on to other unexploited seamounts to maintain the fishery. The recovery of localised fisheries will inevitably be very slow, if at all, because of the low reproductive potential of these deep-ocean fish species.
The destruction of fish stocks is not the only concern associated with seamount fishing. The trawling of seamounts causes extensive damage to the fragile coral communities, with the trawls bringing up not only fish, but large numbers of corals and other benthic animals associated with the corals. The intensity of trawling on seamounts can be very high, with many hundreds to thousands of trawls often carried out on the same seamount over time. Tens of tons of coral can be brought up in a single trawl and in one new seamount fishery it was estimated that almost one-third of the total catch consisted of coral by-catch. Comparisons of “fished” and “unfished” seamounts have clearly shown the extent of habitat damage and loss of species diversity brought about by trawl fishing, with the dense coral habitats reduced to rubble over much of the area investigated.
Not surprisingly, seamount-based fisheries have become very controversial. An increasing number of countries have begun to close some of the seamounts present in their exclusive economic zones (EEZs) to fishing. Unfortunately, most seamounts exist in areas beyond national jurisdiction which makes it very difficult to regulate fishing activities on them, although some efforts are underway to establish international treaties to better manage and protect seamount ecosystems.
It appears that the future for seamount ecosystems lies in a delicate balance between protecting some from fishing altogether while allowing fishing on others under some form of regulation. What sort of balance is achieved remains to be seen in the face of declining fish stocks globally versus an unrelenting growing world demand for fish protein. Let’s hope that legislators, fisheries managers, and marine conservationists can work together to ensure that a good proportion of these remarkable seamount ecosystems will be preserved in an intact state as part of a more general effort to sustainably manage our ocean resources.
Philip V. Mladenov is the Director of Seven Seas Consulting Ltd and Chief Executive of the Fertiliser Association of New Zealand. He has more than 35 years of professional experience in marine biological research, teaching, and exploration. He is the author of Marine Biology: A Very Short Introduction and some 80 scientific papers and a broad range of popular articles, consulting reports, and government reviews.
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Image credit: By NOAA Office of Ocean Exploration, Public domain, via Wikimedia Commons