248 



Fishery Bulletin 92(2), 1994 



1987 



1988 



1989 



was not observed prior to 

 1988, and so does not ex- 

 plain the consistent 

 downward trend in CPUE 

 indices. Hence, although 

 alternative hypotheses 

 cannot be discounted, 

 fishing is most likely to 

 have been the major fac- 

 tor in the observed deple- 

 tion of the stock. 



Most other 'bycatch' 

 species also declined in 

 abundance, although 

 changes were relatively 

 minor compared with 

 that of orange roughy. 

 There was no strong in- 

 dication of any 'species 

 replacement' of orange 

 roughy. The fishery on 

 the Challenger Plateau 

 targets specifically or- 

 ange roughy, and other 

 species are not caught in 

 large quantities. In the 

 trawl surveys, orange 

 roughy have generally 

 accounted for over 95% 

 of the biomass. The com- 

 mercial fishery would 

 probably take even less 

 bycatch than the surveys 

 because it focuses on the 

 aggregations which are 

 usually almost exclu- 

 sively orange roughy. In- 

 tuitively, some compen- 

 satory increase in other 

 components of the Chal- 

 lenger Plateau commu- 

 nity is to be expected 

 because of the extent of 

 orange roughy depletion, 

 but there is no informa- 

 tion on biomass, trophic 

 interactions, and produc- 

 tivity of other fish or in- 

 vertebrate species. In 



addition, even if most of the other finfish species 

 have higher fecundity and faster growth rates than 

 orange roughy, there could be a relatively long re- 

 sponse time to changes in species dominance. 



There has been no apparent change in the size 

 structure of the orange roughy population. With 



Female 



1990 



July date 



Figure 6 



Relative proportions of maturing, ripe, and spent gonads of orange roughy 

 iHoplostethus atlanticus) by day during research surveys in 1984 and 1987-90 (5 

 day running mean; maturing=dashed line, ripe=solid line, spent=dotted line; 

 maturing=stage-3 male, stage-3 female; ripe=stage-4 male, stages 4 and 5 female; 

 spent=stage-5 male, stage-6 female). 



heavy exploitation, a truncation of the length fre- 

 quency distribution and a reduction in the mean size 

 of fish in the population might have been expected, 

 as larger fish were removed and new recruits en- 

 tered the population. Such changes in size structure 

 of exploited populations are well documented (e.g. 



