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Fishery Bulletin 92(2), 1994 



Commercial fishery Mean catch per tow for all 

 New Zealand vessels in the fishery from 1983 to 

 1991 is given in Table 6. Catch rates in winter, when 

 the fish are aggregated for spawning, are generally 

 higher than in other months. Although aggregations 

 occur at other times, presumably for feeding, they 

 are not as large or as stable as in winter. Catch rates 

 in both periods declined steadily from 1983 to 1989 

 to between about 15% and 20% of original levels. 

 The trend is slightly different in the two periods; 

 winter catch rates declined more sharply to 1988, 

 whereas in the other months the largest decrease 

 was between 1983 and 1984. Catch rates increased 

 in 1990, following a reduction of the TAC, when there 

 were less vessels and fewer trawls on the grounds. 



Individual trawl catch rates for orange roughy can 

 be highly variable, consisting of 'hits' and 'misses.' 

 Therefore it is not useful to describe the variance 

 around these mean catch rates, beyond commenting 

 that there is wide variation. It should be stressed 

 that the changes in catch rates presented here may 

 give an indication of changes in stock size but should 

 be treated with caution. Difficulties in interpreta- 

 tion of such data for orange roughy are described in 

 the 'Methods' section, and the form of relationship 

 between mean catch per tow and stock abundance 

 is uncertain. 



Stock reduction results 



Abundance indices used in, and estimates of virgin 

 biomass from, the stock reduction analyses are given 

 in Table 7. Point estimates of B Q range from 95,000 t 

 to 278,000 t. The best fits of data to the model (those 

 with the lowest CV) are from the winter CPUE se- 

 ries. Results from trawl survey data have higher 

 CVs but confirm that an estimate of the order of 

 100,000 t is reasonable. 



The 1987-89 trawl survey series gave the lowest 

 virgin biomass estimate. It was not considered reli- 

 able because there were only three indices, and high 



fishing mortality rates were required to support the 

 catch history. A maximum F of 1.0 is regarded as 

 realistic for orange roughy (Francis et al., 1992). 

 This constrains the virgin biomass to a minimum 

 value of 94,000 t. 



The estimate from non-winter CPUE is compara- 

 tively high. It has a large CV and is based on rela- 

 tively low numbers of trawls (because most fishing 

 effort is in winter). Such a biomass level would also 



