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NIWA: New Year survey marks 20th anniversary of Chatham Rise

On New Year’s Day, NIWA’s research vessel Tangaroa departs for its first voyage, since its recent $20 million dollar upgrade, making its twentieth consecutive trip to the Chatham Rise to study the abundance of important fish species.

The Ministry of Fisheries funded survey has been undertaken by NIWA scientists onboard Tangaroa every year since 1992. The main aim of the survey is to estimate the abundance of hoki and other commercially important species (such as hake and ling), but during the 20 consecutive surveys NIWA scientists have also been able to study other aspects of deepwater biodiversity on the Chatham Rise, including fish distribution, abundance, and ecology.

The surveys are conducted by trawling at depths of 200–800 metres. Trawling locations are randomly selected using a specialised computer programme. The trawl is towed for 3 nautical miles (5.5 km) at a speed of 3.5 knots (6.5 km/h) at each location.

Since the surveys began, scientists have recorded a total of 558 species or species groups and analysed more than one million individual fish, squid, crustaceans, and benthic fauna to help establish biomass trends and spatial and depth distributions. They enter a catch record of all species into a trawl database and bring any species unidentified at sea back to NIWA for identification.

The surveys have also provided the Ministry of Fisheries with a lot of important information about hoki stocks. Hoki is New Zealand’s largest fishery, with a current total allowable catch (TAC) of about 120,000 tonnes. In 2009–10 the commercial catch of hoki from the Chatham Rise was 39,000 tonnes.

Over the last 20 years the proportion of hoki in the trawl survey catch declined from nearly 60 percent in 1993 to 21 percent in 2004, but has increased again to make up 30-40 percent of the total biomass in the past six years.

The other two target species (hake and ling) typically make up 3-4%, and less than 2%, respectively, of the total survey biomass.

NIWA fisheries principal scientist Dr Richard O’Driscoll says the surveys are very important for New Zealand fisheries.

“For most of the species we analyse, the trawl survey is the only fisheries-independent estimate of abundance on the Chatham Rise. The Chatham Rise is also the major nursery area for New Zealand hoki. Juvenile hoki from both eastern and western hoki stocks mix on the Chatham Rise, so this survey provides an opportunity to get an idea of how many small hoki are out there, before they recruit to their respective areas and are caught by the commercial fishery. This allows the Ministry of Fisheries to set an appropriate catch limit which is responsive to the abundance of small fish coming into the fishery.

“The survey provides essential input into the stock assessment for hoki, but also fulfils an important ecosystem monitoring role by providing additional information to improve our knowledge of species distribution and biodiversity,” says Dr O’Driscoll.

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