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



249 



Smith, 1968; Gulland 1971; Edwards and Bowman, 

 1979; Grosslein et al., 1980; Rowling, 1990), al- 

 though none of these studies have dealt with a spe- 

 cies as long-lived or slow-growing as orange roughy. 

 The length frequency distribution of spawning or- 

 ange roughy consists largely of 25^10 cm fish. These 

 sizes are probably fully vulnerable to trawl gear 

 with 100 mm mesh size (legal minimum). Hence, 

 there could be relatively constant fishing mortality 

 across all size groups. There have been no indica- 

 tions of large numbers of new recruits in the length 

 frequency data. This may suggest low levels of re- 

 cruitment, or at least no entry of any strong year 

 class since 1984 that would reduce mean size. 



Interpretation of changes in size structure is also 

 limited by the lack of age data for adult orange 

 roughy. Available ageing data (Mace et al., 1990; 

 MAF Fisheries 3 ) for orange roughy suggest a wide 

 range of ages for a given length, and it is possible 

 that age structure may change more rapidly than 

 size structure. Smith et al. ( 1991) reported a reduc- 

 tion in genetic diversity of orange roughy from sev- 

 eral New Zealand areas, including the Challenger 

 Plateau, and suggested this was due to higher mor- 

 tality of older fish that may remain longer on the 

 spawning grounds than that of younger fish. 



Size at maturity showed a significant decline in 

 males, but not in females. It is possible that one sex 

 could be more vulnerable to fishing, but there are 

 no indications of unbalanced sex ratios in commer- 

 cial catches (authors' unpubl. data). Macroscopic 

 examination of gonads for identification of gonad 

 stage is more reliable in females, where criteria 

 based on colour and size of oocytes are clearer than 

 the presence of milt in male gonads. However, the 

 data may not be representative of true size at ma- 



MAF Fisheries, unpubl. data, 1993. 



turity in the total population. It is possible that the 

 size at maturity measured here is lower than the 

 true population value because fish that migrate to 

 the spawning grounds are primarily those that are 

 mature, and the relative proportions of immature and 

 mature small fish are not accurately represented. 



The apparent lack of change in size at maturity 

 is not surprising in view of the stability of the total 

 population length frequency. An increase in growth 

 rate, with a corresponding reduction in the age at 

 maturity, of individuals in an exploited population 

 is well documented (e.g. Pitt, 1975; Spangler et al., 

 1977; Borisov, 1978; Hempel, 1978; Leaman, 1991). 

 However, Pitt (plaice, Hippoglossoides platessoides, 

 on the Grand Banks) and Leaman (Sebastes spp. in 

 the north Pacific Ocean) noted that whereas age at 

 first maturity decreased with exploitation, size at 

 maturity remained the same. Orange roughy are 

 estimated to be about 24 years of age at maturity 

 (Mace et al., 1990), and hence such functional 

 changes in growth rate or maturity may not be ob- 

 vious after 10 years of fishing, despite a major re- 

 duction in population size. 



Orange roughy on the Challenger Plateau have 

 consistently spawned in the same general area at 

 the same time of year from 1984 to 1990. The go- 

 nad-stage pattern observed in trawl surveys has 

 been similar each year. However, there is evidence 

 of some regional variability in the proportion of fish 

 spawning. The Central Flat and Pinnacles have con- 

 sistently sustained levels of spawning fish over 90%. 

 On the Westpac Bank this proportion has been 

 lower, and the percentage spawning has progres- 

 sively increased from 1987 (54%) to 1990 (81%). 

 Reasons for this are not clear. Sample sizes from the 

 Westpac Bank are smaller than those from the other 

 areas but still come from at least six trawls, which 

 should give a representative sample. In 1987 no 



