Feature

Trawling the Depths

Deep-sea fishing off New Zealand may have long-lasting effects.

Keith Probert

Fisheries science is concerned primarily with the biology of particular target species so that their catches by the fishing industry can be maximised. The species exploited by fisheries do not, however, live in isolation. Many, for instance, are important predators or prey species. If there is large-scale removal of a key species from a marine food web, how do the remaining species in the community adjust to this loss? Many fisheries are relatively unselective so that large amounts of unwanted by-catch species are also taken. In the case of trawling, the heavy gear physically disturbs the seabed and the bycatch may include many seabed animals.

Marine biologists have in recent years become increasingly interested in the wider environmental effects of commercial fishing, including changes to habitats and effects on non-target species. Understandably this interest has focused mainly on continental shelf and shallow-water areas where major fishing grounds have traditionally been located. Increasingly, however, deeper water species are being sought. This situation has occurred because many inshore stocks have become seriously depleted, fishing technology has improved greatly and, with a new international regime for the oceans, many maritime nations have claimed extensive economic zones.

New Zealand began exploiting its deep-water fisheries in the late 1970s. The country's Exclusive Economic Zone is enormous, some 15 times larger than the land area, and includes huge deep-water plateaux and rises that were found to support dense populations of orange roughy, hoki, oreos and other important deep-water species.

Environmental
Implications

The rise of deep-water fisheries in New Zealand and elsewhere has not, however, been accompanied by research into the wider environmental implications of these operations. Particular concerns have been raised with regard to bottom trawling where intensive fishing can significantly alter the seabed environment and the animal communities it supports.

Environmental effects of trawling on the seabed were recently reviewed by Brian Jones, then at the Fisheries Research Centre in Wellington. He pointed out the urgent need to carry out trawling impact studies in deeper water since this is where environmental impacts could be severe and where recovery may take decades.

Environmental conditions and life-history characteristics of deep-sea benthic (that is, seabed) species are very different to those at shelf depths. At temperatures of only a few degrees Celsius and with limited input of food reaching these depths from the productive surface waters, species of the deep sea are generally slow-growing and of low reproductive potential.

Following the development of new deep-water grounds in New Zealand, trawler operators noticed a decline in the bycatch of benthic invertebrates in the trawls. Initially, large amounts of deep-water hard corals, soft corals, and sponges were being taken, but the catch of these species has declined markedly.

Apart from anecdotal observations, however, little is known about the effects of intensive trawling on New Zealand's deep-water benthic fauna and what implications these operations may have for the sustainable management of deep-water fisheries. At least from studies of shallow-water environments, there is evidence that alterations to the seafloor may affect the fisheries themselves.

Bycatch Survey

A first evaluation of the potential impact of trawling on the deep-water benthos has been carried out as part of a joint study by the National Institute of Water and Atmospheric Research (NIWA) in Wellington and the Department of Marine Science at the University of Otago.

During the course of an orange roughy survey of the Chatham Rise being undertaken by fisheries scientists on the Research Vessel Tangaroa, the opportunity was available to collect and record the bycatch of invertebrate benthos taken by trawling. The Chatham Rise is a broad submarine ridge extending eastwards for more than 1,000 km off central New Zealand to east of the Chatham Islands. The Rise is generally flat-topped at water depths of 200-400 metres, but depths increase to more than 2,000 metres to the north and south.

During the bycatch survey, 73 trawl tows were examined from water depths of 660 metres to more than 1,500 metres taken along the northern and eastern slopes of the Rise. Trawls were from two types of sea floor. Two-thirds were from flat muddy areas typical of the seabed at these depths. The other third, however, were from "pinnacles" or "hills", which are small submerged seamounts that are dotted around the Chatham Rise. Most would probably be of comparable size to Wellington's Mount Victoria. When orange roughy fishing began on the Chatham Rise it was confined mainly to flat areas but, more recently, sophisticated fishing techniques have been developed to enable the summits of pinnacles to be trawled. This is a tricky procedure but worth perfecting since fish collect at these features, in most cases probably because of concentrations of prey.

During the study nearly a hundred species of benthic invertebrates were collected from the trawls, These included many brittle-stars (12 species), prawns, sea stars and gorgonian corals (11 species each), sea cucumbers (seven species) and sponges (six species). Cluster analysis, a program that groups samples of similar composition, confirmed that the trawl bycatch from the flats and hills differed significantly. Bycatch from the flat tows was dominated by sea cucumbers, sea stars and prawns; whereas the invertebrates most commonly recorded from hill tows were gorgonian and stony corals. The largest volumes of bycatch were from hill tows and comprised different types of corals, in particular the stony coral Goniocorella dumosa, the stylasterid coral Errina chathamensis, and a black coral, probably Bathyplates platycaulus.

Deep Water Corals

We think of corals primarily as tropical shallow-water organisms, and certainly that is where they are most spectacularly developed. But there are also many deep-water forms. Although these do not form true reefs, some of them do grow sufficiently luxuriantly to contribute an important structural dimension to the seabed and thereby provide opportunities for many associated species. They undoubtedly create patches of enhanced biodiversity.

For example, the deep-water stony coral Goniocorella, which has been collected from a number of localities around New Zealand, can form dense thickets that support a diverse assemblage of associated organisms, including other corals, bryozoans, polychaete worms, brittle-stars, sea stars, gastropod and bivalve molluscs, and anemones.

Similar deep-water coral faunas are known from other sites around the world. The stony coral Lophelia pertusa of the North Atlantic forms deep-water banks on prominences along the continental margin, where it hosts a highly diverse fauna. Mature patches of Lophelia can be 10-50 metres across and probably of the order of 200-400 years old. There are no age estimates for comparable New Zealand species of deep-water stony corals, but they are likely to be similarly slow growing and long lived.

Black corals belong to a different group to the stony corals and, instead of a calcareous skeleton, have an internal skeleton of a black horny material. They mainly occur in deeper water, although the best known New Zealand species, Antipathes fiordensis, lives in relatively shallow water (less than 100 metres) in the fiords, where limited light penetration appears to enable many deeper water species to occupy anomalously shallow niches. Studies by Ken Grange of NIWA show that this species has a very slow growth rate and that colonies over four metres high are probably more than 300 years old. Large black coral trees on the Chatham Rise pinnacles are also probably centuries old.

Trawling Impacts

The floor of the deep sea is almost entirely covered by muddy sediments. Exposed rock surfaces are relatively uncommon, and one of the places they occur is on pinnacles and seamounts where they can provide a major opportunity for the development of communities dominated by attached fauna, such as corals. Pinnacles in the Chatham Rise region allow the development of a benthic fauna in which large attached species form a distinctive and major component. Trawling on these small seamounts is likely to have considerable and long-lasting impacts on their benthic communities. Coral thickets appear to be especially vulnerable to trawling damage. They are physically fragile and take a very long time to recover from disturbance. In addition, they support many associated species.

It is quite possible that some species of fishes exploit the distinctive benthic environment of seamounts, or at least at certain stages of their life history. If this were to be the case, then damage to the benthos might impact on the sustainability of fish populations. Certainly, strong links between fish and benthos are known to occur in some shallow-water environments, where alteration to the seabed environment due to trawling has reduced the benthic fauna and in turn has led to a decline in the fisheries catch.

A Case for Conservation?

The composition of benthic faunas associated with seamounts in the New Zealand region has yet to be assessed in any detail. In terms of marine biodiversity of the New Zealand region, seamounts may prove very significant for certain organisms. Stylasterid corals, for instance, prefer deep shelf and slope depths of offshore submarine ridges and seamounts, and New Zealand's existing stylasterid fauna is the most diverse known.

So far there has been little interest worldwide in the conservation of deep-sea habitats, but there may be a case for examining options for the conservation of seamounts given the relative scarcity of such habitats, their biodiversity, and the potential vulnerability of components of their biota.

New Zealand's Marine Reserves Act 1971 has been used to establish 13 marine reserves, but the legislation applies only to the Territorial Sea (that is, up to 12 nautical miles offshore). Section 27 of the Territorial Sea and Exclusive Economic Zone Act 1977 does provide for measures for the protection and preservation of the marine environment of New Zealand's Exclusive Economic Zone, but has never been used to set aside areas which could be protected from deep-water trawling.

Dr Keith Probert works in the Department of Marine Science at the University of Otago.