20th Chatham Rise fish survey a success

NIWA's research vessel Tangaroa has successfully completed its first voyage since its recent $20 million dollar upgrade, surveying for hoki and other commercially important species on the Chatham Rise.

NIWA has run the Ministry of Fisheries-funded Chatham Rise survey twenty years in a row, 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 surveys NIWA scientists also study other aspects of deepwater biodiversity on the Chatham Rise, including fish and invertebrate distribution, abundance, and ecology.

This year 114 successful survey trawls were run on the 27-day voyage, including 88 tows to a depth of 200-800 metres and 23 tows in deep water areas from 800-1,300 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.

More than 280 species or species groups were identified from a total catch of nearly 130 tonnes of fish, squid, crustaceans, and benthic fauna. This catch included 48.3 tonnes of hoki, 28.2 tonnes of silver warehou, 4.6 tonnes of orange roughy, 3.4 tonnes of javelinfish, 3.3 tonnes of spiny dogfish, 3.2 tonnes of ling, and 0.7 tonnes of hake. Information from the catch helps NIWA scientists to establish biomass trends and spatial and depth distributions to assist the Ministry of Fisheries set catch limits for commercial species.

New species are also often identified during the survey. If a species cannot be identified at sea, samples are taken back to NIWA's laboratory in Wellington for formal identification.

Many interesting species were caught on this year's trip including a rarely seen frill shark.

In total, 65,536 fish were measured during the trip. If laid end-to-end these would stretch 29.5 kilometres. More than 21,000 fish were individually weighed and 3,467 sets of otoliths (ear bones) were collected to provide information about the age of the fish. Two hundred and seventy stomachs were collected from the deepwater species to understand more about their diet and feeding habits and 252 tissue samples were take for stable isotope analysis.

NIWA fisheries principal scientist Dr Richard O'Driscoll says the voyage was very successful, in terms of trawls completed and fish surveyed.

"For most of the species we study, the trawl survey is the only fisheries-independent estimate of abundance on the Chatham Rise, providing essential stock assessment information for key commercial species such as hoki, hake, and ling. By having a consistent 20 year time-series and analysing and measuring all the associated species we are also getting a clear picture of the trends in the structure, health and biodiversity of the overall ecosystem and species composition," says Dr O'Driscoll.

"Preliminary biomass estimates suggest that there are good numbers of young (1 and 2-year old) hoki out there which will feed into the fishery over the next couple of years. However, the numbers of older hoki appear to have decreased."

These results will be considered by the Ministry of Fisheries at their Fishery Assessment Working Group meetings in March, and used to guide the setting of catch limits for 2010/11.

RV Tangaroa is now on another two week voyage to the Chatham Rise, this time on a pilot study on surface ocean aerosol production.