238 



Fishery Bulletin 92(2), 1994 



6 Possible change in species composition over time 

 ('species replacement'). 



Such responses are often observed in short-lived, 

 fast-growing species (e.g. Pauly, 1979; Grosslein et 

 al., 1980). Some have also been noted with Sebastes 

 alutus (Gunderson, 1977; Leaman, 1991), but it is 

 not clear whether these changes would occur in such 

 a long-lived species as orange roughy, or over what 

 time period such changes would become apparent. 

 Orange roughy on the Challenger Plateau have been 

 exploited for only 10 years, and hence it seems un- 

 likely that marked changes in biological character- 

 istics could occur over such a relatively short time 

 period in relation to the longevity of the species. 

 Therefore, we might expect to observe changes in 

 biomass and distribution, as well as age and size struc- 

 ture of the population, but not changes in growth rate 

 or reproductive potential. 



In this paper, we summarize some of the available 

 data on distribution, abundance, and biology of or- 

 ange roughy on the Challenger Plateau, primarily 

 over the period 1984-90. This period covers the 

 early years of the developing fishery, to maximum 

 levels of exploitation, and subsequent decline of the 

 population. We describe the reduction in size and 

 distribution of the stock and investigate associated 

 changes in size structure, aspects of reproduction, 

 and feeding. 



Methods 



Research trawl surveys 



Trawl surveys have been carried out in the winter 

 (June-July) of each year from 1984 to 1990. The 

 vessel used, area covered, intensity of trawling, and 

 survey design differed between 

 years, and all are not directly 

 comparable (Table 2). Surveys 

 from 1987 to 1989 were treated as 

 fully comparable, but only se- 

 lected data have been used from 

 other surveys: distribution from 

 1984 and 1990, and biology (size, 

 reproductive, and feeding data) 

 from 1984, 1985, 1986, and 1990. 

 The general survey design was 

 two-phase stratified random (af- 

 ter Francis, 1984). The survey 

 area was divided into a number of 

 strata based on depth and certain 

 bottom features (e.g. pinnacles). 

 General stratification is shown in 



Figure 2. The depth range covered was 800 to 1,200 

 m. New, random station positions within strata were 

 selected each year, except in strata 10 and 11 on 

 pinnacles where a random tow direction was ad- 

 justed to avoid untrawlable ground, and these 

 trawls were repeated each year. A similar net de- 

 sign and gear set up was used for each survey. Tow 

 length was standardized where possible at 1.5 nau- 

 tical miles (nmi). Trawling speed was 3.0-3.5 knots. 

 Biomass indices were calculated by the area swept 

 method as described by Francis ( 1981). Biomass and 

 its standard error were calculated from the follow- 

 ing formulae: 



and 



B=^(X,a,)/cb 



S B = J^sfaf/c 2 b 2 



where B is biomass (t), X is the mean catch rate 

 (kg-km -1 ) in stratum i, a i is the area of stratum i 

 (km 2 ), b is the width swept by the gear (defined as 

 doorspread (m) by MAF Fisheries), c is the catch- 

 ability coefficient (an estimate of the proportion of 

 fish available to be caught by the net), S B is the 

 standard error of the biomass, s ( is the standard 

 error of X t 



The catchability coefficient was assigned a value 

 of 0.27, which represents the wingend spread di- 

 vided by the doorspread, because orange roughy 

 form schools which are not believed to be herded 

 substantially by doors or sweeps. 1 



Approximate 95% confidence limits (CD were 

 calculated as 



CL = B±2S B . 



1 Orange Roughy Working Group, MAF Fisheries, Greta Point, 

 P.O. Box 297, Wellington, New Zealand, pers. commun. 1991. 



