Clark and Tracey: Population changes of Hoplostethus atlanticus on the Challenger Plateau 24 1 



In the following text, three colloquial area names 

 have been used. These are given below with specific 

 strata numbers (see Fig. 2): 



Central Flat 

 Pinnacles 

 Westpac Bank 



strata 1, 4 

 stratum 10 

 strata 9, 11 



Results 



Distribution 



Trawl surveys The distribution of orange roughy in 

 the survey area changed substantially between 

 years (Fig. 3). In 1984 high catch rates were ob- 

 served across much of the Central Flat area. (No 

 trawls were made on the Pinnacles although heavy 

 marks were observed on the echosounder; the sur- 

 vey did not cover the Westpac Bank area.) In 1987 

 fish were still widely distributed in the Central Flat; 



there were two main schools and further concentra- 

 tions around the Pinnacles and the Westpac Bank. 

 In 1988 there was a marked contraction in the area 

 of high catch rates; a single small aggregation was 

 observed on the Central Flat, and by 1989 there 

 were no aggregations in the Central Flat region. High 

 catch rates still occurred on the Pinnacles and Westpac 

 Bank in 1989, and these actually increased in 1990, 

 after the TAC and fishing effort were greatly reduced. 



Commercial fishery The commercial fishery has 

 been centered mainly inside the EEZ, targeting ag- 

 gregations of orange roughy on the Central Flat and 

 Pinnacles. Distribution of effort (number of tows) 

 and catch between these two areas has changed over 

 time (Table 3). In the period 1982-87, over 80% of 

 the catch from the two areas was taken from the 

 Central Flat with over 75% of the number of tows. 

 In 1988 there was a marked increase in the propor- 

 tion of catch and effort on the Pinnacles, and a corre- 

 sponding reduction on the Cen- 

 tral Flat. This shift continued 

 in 1989 and 1990, during which 

 the Pinnacles accounted for 

 65-70% of the catch. These 

 changes reflect the change in 

 distribution observed in the re- 

 search trawl surveys. 



Relative abundance 



Trawl surveys Biomass indi- 

 ces (estimates of relative bio- 

 mass) from trawl surveys in 

 1987, 1988, and 1989 are 

 given in Table 4. The indices 

 indicate a marked decline in 

 biomass over the period. The 

 distribution of biomass among 

 strata changed over the years 

 1987-90 (Table 5). In 1987 

 and 1988 over 60% of the bio- 

 mass was in the Central Flat 

 area, but only 30% in 1989 

 and 1990. Over this period, 

 there was an increase in the 

 proportion on the Pinnacles, 

 especially between 1989 and 

 1990. Biomass levels in the 

 surrounding areas have fluc- 

 tuated but were particularly 

 high in 1989. The proportion 

 of biomass on the Westpac 

 Bank has remained compara- 

 tively constant. 



