Ensuring a sustainable Ross Sea toothfish fishery
NIWA has been carrying out research on toothfish biology and the exploratory longline toothfish fishery in the Ross Sea for several years, initially as a PGSF project, but more recently under contract to the Ministry of Fisheries. We have provided advice and carried out analyses on a wide range of topics, such as catch sampling methods, genetics, age and growth, monitoring abundance, and bycatch and seabird mitigation measures.
The greater Ross Sea area and the CCAMLR boundaries used for managing the fishery.
Ice conditions make for good photographs but make fishing in the Ross Sea difficult and dangerous.
It’s physically demanding work measuring and weighing up to 100 toothfish from each longline set.
As a significant contribution to the international management efforts for toothfish, NIWA staff contribute to the Fish Stock Assessment Working Group meetings held annually in Hobart for the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR). This group reviews research work from the previous year, and develops stock assessments to estimate sustainable yields for the main icefish and toothfish fisheries. In 2004, Stuart Hanchet will take over as convenor for 2–3 years.
Toothfish distribution
There are two main species of toothfish: the Patagonian toothfish (Dissostichus eleginoides), which occurs mainly in Subantarctic waters, and the Antarctic toothfish (D. mawsoni), which is found only in Antarctic waters. The Antarctic toothfish is caught from the Polar Convergence (at about 60° S) south to the Antarctic continent: the Patagonian toothfish is mainly caught north of 65° S. Between these latitudes both species are equally abundant. The Ross Sea region is unusual because the species show considerable overlap in their distribution there and over 90% of the longlines set in the fishery in this area have both species on the same line. Both species are found to depths of over 2000 m.
The Ross Sea toothfish fishery
The Ross Sea fishery is the southernmost fishery in the world, and the extreme cold and ice conditions can make fishing difficult and dangerous. For most of the year the Ross Sea itself is covered by ice. However, during January and February areas of open water (called polynas) develop, allowing access to the continental shelf and slope. New Zealand longliners have taken advantage of this to develop an exploratory fishery. Vessels start working in the deep south; as the season progresses they move north to stay ahead of the freezing sea ice, and by May are restricted to the northernmost fishing grounds. The catch has steadily increased from about 40 t in 1998 to over 1350 t in 2002. Because it is a high latitude fishery, Antarctic toothfish has formed over 95% of the catch. Up to three New Zealand vessels have been involved in the fishery in any year, but in the 2003 season there may be up to six.
Biology
Although considerable research has been carried out elsewhere on Patagonian toothfish, little was previously known about the biology of Antarctic toothfish. In collaborative studies with fishing industry and overseas scientists, NIWA has investigated age, growth, maturity, and feeding of Antarctic toothfish.
With the help of other scientists we have developed a validated methodology for ageing Antarctic toothfish – the first for any toothfish species. US scientists at McMurdo Sound in the Ross Sea have tagged Antarctic toothfish for over 20 years, and before release injected the fish with oxy-tetracycline. This chemical is deposited in the hard parts of the fish and acts as a marker of when the fish was released. Recent recapture and analysis of six fish (including one that had been at liberty for 7 years) confirmed that rings in the otolith are formed annually. Other US scientists carrying out research trawl surveys in the South Atlantic provided otoliths from small (13–51 cm long) Antarctic toothfish, which enabled validation of the younger ages. Toothfish have relatively fast growth rates that are comparable to those of hoki and ling. They reach about 60 cm after 5 years, about 100 cm after 10 years, and about 150 cm after 20 years. The maximum age recorded is 39 years, but there are few fish older than about 30 years caught in the fishery. It is thought that fish mature at about age 10.
Little is known about the exact timing and location of spawning, as the area is covered by ice during the winter when spawning is thought to occur. Some ripe and running ripe fish have been found in the north of the area in May, and it is likely that spawning continues from then until late winter. Larvae and juvenile fish are pelagic (until at least 15 cm) after which they start to spend more time on or near the bottom. We have found that most of the diet of adult toothfish is other fish, rattails and icefish predominating. Other unusual stomach contents included three penguins, numerous skate egg cases, and a large quantity of rocks. Stomach samples are being collected during the 2003 season to extend our knowledge of feeding.
Stock structure
Molecular methods have been used to determine the genetic relationships among populations of the Patagonian toothfish in the Southern Ocean. Microsatellite DNA markers in samples from the Atlantic, Indian, and Pacific Ocean sectors of the Southern Ocean showed significant heterogeneity in the total data and a high level of genetic subdivision. The microsatellite DNA data indicate that there is restricted gene flow through the Southern Ocean, and that the different fishing grounds may support independent stocks.
Other research
There are many management measures in place to ensure that the effect of the fishery on the environment is minimal. These include catch limits for toothfish and bycatch species, seabird and fish bycatch mitigation measures, and no discarding of any waste or offal. NIWA has been involved in monitoring and characterising various aspects of the fishery, including the toothfish catch and the fish and invertebrate bycatch. We have identified the main bycatch species as Whitson’s grenadier and the starry skate, and looked at aspects of their biology, including age, growth, length-weight relationship, and maturity. We have also investigated ways to monitor their abundance and to minimise the bycatch.
Elsewhere in the Southern Ocean there has been a high incidental mortality of seabirds during longline operations. NIWA scientists have been involved in two projects on seabird bycatch issues. Seabird distributions within the Ross Sea region were reviewed, and new information from scientific observers analysed. We have also been involved in analysing data collected during longline sink rate experiments in conjunction with the fishing industry and the Ministry of Fisheries. To date there has been a zero bycatch of seabirds in the Ross Sea fishery.
Management of toothfish fisheries
The two toothfish species support valuable international longline and trawl fisheries. Finfish fisheries in Antarctic waters are largely managed under CCAMLR jurisdiction. The CCAMLR Convention area covers the area north to the Antarctic Convergence and varies from 60° S in the Pacific to 45° S in the western Indian Ocean. Patagonian toothfish are taken both inside and outside the Convention area, and catches to the north of the Convention area are taken both within national EEZs and in the high seas. There is considerable concern at the level of unregulated and illegal fishing of this species, and in some areas the populations have been depleted to a small fraction of their original size. In contrast, Antarctic toothfish is restricted to waters managed solely by CCAMLR, and the only fishery for this species is in the Ross Sea. At this stage the fishery is very small compared to the estimates of yield, and this combined with the short season, ice conditions, and low illegal catch means that the Ross Sea fishery should be sustainable for many years to come.