Pegasus Canyon hoki survey
Science Centres: Fisheries
FV Independent 1
Pegasus Canyon. Depth contours are 250, 500, 750, and 1000 m.
Echogram of hoki schools in Pegasus Canyon on 7 September 2002.
Typical hoki catches on Independent 1
The hoki fishery, New Zealand’s largest fishery, is also notable as the first ‘white fish’ fishery in the world to be certified by the Marine Stewardship Council. The Hoki Fishery Management Company, NIWA, and individual fishing companies have been proactive in developing collaborative research projects, working together towards sustainable management of hoki. A recent example was an acoustic survey off the east coast of the South Island in September 2002.
Hoki spawn in winter, forming dense schools which may contain millions of fish. The main spawning areas are on the west coast of the South Island and in Cook Strait. NIWA has carried out regular acoustic surveys to measure abundance in these areas since 1988. The survey results are a key input into hoki stock assessment.
In January 2002, one of the major hoki fishing companies, Independent Fisheries Ltd, interested in knowing how much hoki spawned in Pegasus Canyon on the east coast of the South Island, approached NIWA. Spawning hoki were found in Pegasus Canyon during an exploratory survey in 1987, but there has been no research in this area since. However, in the last few years there has been increasing interest in fishing in Pegasus Canyon, with skippers reporting that hoki schools are increasing.
Independent Fisheries and NIWA prepared a joint proposal, and the Hoki Fishery Management Company, an organisation representing all hoki fishing groups, agreed to fund an acoustic survey in September 2002. The Pegasus Canyon survey immediately followed an acoustic survey of Cook Strait in July–August 2002 to allow a comparison of abundance levels.
The survey was carried out from Independent 1, a modern 45.6 mfactory/freezer stern trawler equipped with trawl monitoring equipment which allows it to accurately target 10–20 t bags of hoki. Hoki are then filleted, packaged, and frozen on board. The crew can process up to 3 t of fish per hour, which means trawls are usually 3–8 hours apart. This processing time provided opportunities for other activities. The acoustic survey was fitted into these times, allowing us to survey between trawls without compromising fishing operations.
Pegasus Canyon is about 50 km long and up to 15 km wide, ranging in depth from 200 to over 1000 m. The survey consisted of a number of subsurveys or ‘snapshots’. Within each snapshot there were 8–11 randomly spaced lines (‘transects’) across the canyon. Running transects at 8–10 knots, it was possible to carry out each snapshot in two or three ‘processing windows’ or 24–36 hours. This enabled seven snapshots during the 10-day survey.
Acoustic data were collected by the vessel’s commercial sounder, calibrated by NIWA before the survey. Raw acoustic data were stored at sea on compact disk and later analysed using NIWA’s ESP2 software. Biological data on size and spawning condition were collected from hoki caught during commercial trawls.
Dense marks from hoki schools were present in Pegasus Canyon throughout the survey, and biological sampling confirmed that the fish were spawning. The size of the spawning aggregations was a surprise to many researchers, and indicated that Pegasus Canyon may be a significant secondary spawning area for the eastern hoki stock which also spawns in Cook Strait.
This survey was the first of its kind conducted in the New Zealand hoki fishery, proving that under certain conditions research can be carried out while fishing commercially. However, we don’t see surveys from commercial vessels replacing research vessels. There is still a need for directed research using specialised equipment. Nevertheless, commercial vessels provide opportunities for research that would otherwise never be done. If the vessel can ‘pay for itself’ by continuing to fish commercially during the survey, the cost is reduced. There is also a large pool of commercial vessels available in New Zealand, with potential to carry out surveys over wide areas at the same time.