CHANGES IN A POPULATION OF ORANGE ROUGHY, HOPLOSTETHUS-ATLANTICUS, WITH COMMERCIAL EXPLOITATION ON THE CHALLENGER PLATEAU, NEW-ZEALAND

The commercial fishery for orange roughy on the Challenger Plateau developed in 1981, increased markedly throughout the mid-1980s, and then declined rapidly by 1990. Data from research trawl surveys and commercial fishing returns over the period are examined, and changes in the population are described. The distribution of orange roughy changed over the period examined; there was a contraction of the areas of high density and apparent fishing-out of aggregations on relatively flat bottom. Aggregations are now largely confined to pinnacles. Biomass of orange roughy, measured by bottom trawl survey indices and commercial catch per unit of effort, declined substantially and is currently estimated to be about 20% of virgin levels. Most other incidental species in the trawl surveys have also declined in abundance, and there are no indications of 'species replacement.' Data on size, reproductive stage, size at maturity, and feeding have also been examined. Size structure of the population has not changed over time. Time of spawning (July) and the pattern of gonad development have been consistent over the years. Diet composition has also remained similar; dominant prey groups are natant decapod crustaceans and small fish. It is suggested that biological changes have not been apparent because orange roughy are a long-lived, slow-growing species, with low productivity. There could be a long response time to fishing pressure, yet orange roughy populations can be quickly reduced to low levels by commercial fishing.