Science Centres: Fisheries

New research on orange roughy stocks by NIWA scientists Matt Dunn and Ian Doonan is beginning to answer some longstanding puzzles.

Juvenile orange roughy. (Photo: Malcolm Clark, NIWA)

The scientists located orange roughy nursery grounds for the first time by analysing data from more than 13 000 trawls. They found that juveniles occupy a narrow depth range close to the spawning grounds. At an average depth of 850 m, the juveniles are shallower than the adults, which average 1150 m.

Locating the nursery grounds enabled the scientists to directly estimate the number of juveniles available to recruit into the fishery – something that hasn’t been attempted before. Preliminary results indicate that current recruitment to the Chatham Rise stocks is likely to be much lower than estimated by stock assessment models, which assume a constant recruitment rate. There is also suggestion of a ‘recruitment hole’ dating from the late 1980s on the Chatham Rise, when the spawning aggregations were rapidly fished down. Further analysis is needed to test these hypotheses.

Analysis of the spatial structure of orange roughy populations indicates that the larger fish are confined to seamounts, where most fishing activity occurs. This uneven size distribution needs to be taken into account in stock assessments.

The research was funded by the Ministry of Fisheries.