Abstract. — The commercial 

 fishery for orange roughy on the 

 Challenger Plateau developed in 

 1981, increased markedly through- 

 out the mid-1980s, and then de- 

 clined rapidly by 1990. Data from 

 research trawl surveys and com- 

 mercial 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 aggrega- 

 tions on relatively fiat bottom. Ag- 

 gregations are now largely con- 

 fined to pinnacles. Biomass of or- 

 ange roughy, measured by bottom 

 trawl survey indices and commer- 

 cial catch per unit of effort, de- 

 clined substantially and is cur- 

 rently estimated to be about 20% 

 of virgin levels. Most other inci- 

 dental 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 develop- 

 ment 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 pres- 

 sure, yet orange roughy popula- 

 tions can be quickly reduced to low 

 levels by commercial fishing. 



Changes in a population of 



orange roughy, 



Hoplostethus atlanticus, 



with commercial exploitation on the 



Challenger Plateau, New Zealand 



Malcolm R. Clark 

 Dianne M. Tracey 



MAF Fisheries Greta Point. PO. Box 297 



295 Evans Bay Parade, Wellington, New Zealand 



Manuscript accepted 4 November L993 

 Fishery Bulletin 92:236-253 (1994) 



Orange roughy (Hoplostethus 

 atlanticus Collett) has a worldwide 

 distribution on the continental 

 slope at depths of 700 to 1,500 m. 

 However, it is fished commercially 

 only off New Zealand, Australia, 

 and in the northeastern Atlantic 

 Ocean. The New Zealand fishery is 

 the most established, having 

 started in 1978; the others date 

 from 1988 and 1991, respectively. 

 Orange roughy is one of the most 

 valuable commercial species in 

 New Zealand waters, with annual 

 landings of 40-50,000 metric tons 

 (t) and export earnings of NZ $100- 

 150 million (Robertson, 1991). 



The New Zealand fishery for or- 

 ange roughy occurs in a number of 

 areas (Fig. 1), including the Chal- 

 lenger Plateau, a broad submarine 

 plateau off the west coast of New 

 Zealand. The commercial fishery on 

 the Plateau developed in late 1981 

 and rapidly expanded into one of 

 the most important orange roughy 

 fisheries in New Zealand waters, 

 with annual catches up to approxi- 

 mately 16,000 t (Table 1). The fish- 

 ery operates primarily during win- 

 ter (June-August), when the fish 

 form large spawning aggregations at 

 depths of 850-900 m (Clark, 1991a). 



The fishery has been managed by 

 a Total Allowable Catch (TAC) sys- 

 tem since 1982. Tracey et al. (1990) 



and Clark (1991a) discussed details 

 of this management regime. Ini- 

 tially, catches were limited to 7,000 

 t by the TAC for all areas of the 

 New Zealand Exclusive Economic 

 Zone (EEZ), outside the established 

 fishing grounds on the east coast. 

 A TAC of 4,950 t was set for 1983- 

 84 and 1984-85 (October-Septem- 

 ber fishing year) on the Challenger 

 Plateau and west coast of the 

 South Island. This was raised to 

 6,190 t specifically for the Chal- 

 lenger fishery in 1985-86 based on 

 biomass estimates from a trawl 

 survey in winter 1984. The quota 

 was raised to 10,000 t in 1986-87, 

 and further to 12,000 t in 1987-88, 

 in order to assess the effects of 

 heavier fishing on the population 

 dynamics of orange roughy ("adap- 

 tive management"). In the follow- 

 ing fishing year only 8,200 t of 

 quota were allocated, the rest with- 

 held because of signs that the or- 

 ange roughy population was declin- 

 ing rapidly. During 1989-90 the 

 TAC was reduced to 2,500 t after 

 new stock assessments showed the 

 population was overexploited and 

 had declined to low levels (Clark 

 and Francis, 1990). The TAC was 

 further reduced for 1990-91 to pro- 

 mote rebuilding of the population. 

 These changes occurred against 

 a background of increasing infor- 



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