Clark and Tracey: Population changes of Hoplostethus atlanticus on the Challenger Plateau 



237 



/175°W 



* EEZ boundary 



Figure 1 



Map of New Zealand and offshore waters of the Exclusive Economic Zone 

 (EEZ), showing the location of the Challenger Plateau and other major 

 fishing areas for orange roughy (Hoplostethus atlanticus). 



mation on orange roughy. It has 

 only recently been realized that 

 orange roughy is very slow-growing 

 and long-lived. Mace et al. (1990) 

 recorded a growth rate of about 

 three cm per year for the first four 

 years of life (validated ages), and 

 an estimated age at maturity of 24 

 years and maximum age over 50 

 years. They estimated natural mor- 

 tality to be low (less than 0-1 yr -1 ) 

 and concluded that sustainable 

 yields of orange roughy would be 

 relatively low and show slow recov- 

 ery from over-fishing. Recent esti- 

 mates of the maximum age of or- 

 ange roughy from Australian wa- 

 ters approach 150 years (Fenton et 

 al., 1991). 



Quotas for orange roughy har- 

 vest from New Zealand have been 

 reduced in recent years on the ba- 

 sis of information which suggests 

 much lower productivity than origi- 

 nally assumed. However, the Chal- 

 lenger Plateau population had al- 

 ready declined markedly and pro- 

 vides some insight into the effects 

 of heavy fishing pressure on orange 

 roughy population dynamics. 

 There is an extensive literature on general re- 

 sponses offish populations to exploitation, covering 

 lake ecosystems (e.g. Regier and Loftus, 1972; 

 Spangler et al., 1977), coral reef fisheries (e.g. Russ 

 and Alcala, 1989), and relatively shallow-water 

 marine environments (e.g. Hempel, 1978; Pauly, 

 1979; Grosslein et al. 1980). There have been few 

 studies on deep-water or long-lived species such as 

 orange roughy. The closest is probably Pacific ocean 

 perch (Sebastes alutus) which is found at depths to 

 600 m in the North Pacific Ocean and has a maxi- 

 mum age of 90 years (e.g. Gunderson, 1977; Lea- 

 man, 1991). 



There are a number of general population re- 

 sponses to exploitation, which include 



1 Decline in abundance of fished species. 



2 Contraction of distribution or areas of high density. 



3 Change in age structure or size structure, or both, 

 with fewer old, large fish and the population 

 dominated by new recruits. 



4 Increase in growth rate of individuals, with a 

 decrease in age for a given length. 



5 Lower age at maturity or size at maturity, or 

 both. 



