688 



Fishery Bulletin 89(4). 1991 



Discussion 



Size structure 



Evidence from length-frequen- 

 cies indicates that in some re- 

 gions the fishery may still be sus- 

 tained by the accumulated stock. 

 Although reliable catch-per-unit- 

 effort figures are not available 

 for the commercial fishery, it is 

 probable that the west coast of 

 Stewart I. represents such a re- 

 gion. The weather and sea condi- 

 tions render this region inacces- 

 sible for much of the year, but it 

 is a favored place to fish when 

 conditions permit because of the 

 large sizes and abundance of 

 paua. In contrast, sites on the 

 east coast of Stewart I. are con- 

 siderably more sheltered, gener- 

 ally accessible to fishing, and 

 closer to a port. The observed 

 population structure in the east- 

 ern region, with most paua below 

 MLS, is consistent with a popula- 

 tion in equilibrium with the 

 fishery. 



There are alternative explana- 

 tions for the differences in size-frequencies. One 

 possibility is a difference in growth rates among 

 regions: L M could be smaller in the eastern region of 

 Stewart I. than the western region. Another possibil- 

 ity is that the population structure seen at the west 

 coast of Stewart I. may have resulted from a short 

 period of good recruitment followed by poor recruit- 



Table 5 



Estimates of total mortality rate Z based on length-frequencies, using the method of 

 Fournier and Breen (1983). N is the assumed number of age classes; NFULL is the 

 number of the first age-class included in the mortality estimate; "SDevs" are the stan- 

 dard deviations of lengths in the first and last age-classes; T is the value of the objective 

 function minimized by the procedure. Asterisks indicate constrained values. 



Region 



N NFULL SDevs 



K 



Northern Marlborough Sounds 14 7 4.1* 4.2* 144.4 0.28* 92.8 0.49 



D'Urville I. 12 6 4.2* 4.2* 138.0 0.37* 162.0 0.51 



ment. A third possibility is that natural mortality is 

 much higher in the eastern region of Stewart than in 

 the other regions. Similarly, population size structures 

 seen in the Marlborough Sounds could be explained by 

 high natural mortality, recruitment variation, or 

 stunted growth in some regions. Only repeated 



Table 7 



Northern Marlborough Sounds region: YPR (top) and egg pro- 

 duction (bottom) for three different levels of assumed M, for 

 three different minimum legal sizes. YPR is in grams whole 

 weight; egg production is expressed as a percentage of the 

 eggs produced by a virgin population. 



MLS 



M 



115 



125 



130 



0.05 

 0.10 

 0.15 



0.05 

 0.10 

 0.15 



219.0 

 175.9 

 139.0 



7.4 

 14.0 

 22.6 



196.1 

 151.1 

 126.3 



33.4 

 31.1 

 31.1 



230.6 

 176.2 

 132.5 



9.9 

 18.1 

 28.2 



199.0 

 151.1 

 122.2 



35.0 

 33.3 

 34.6 



236.5 

 171.9 

 122.9 



12.8 

 12.5 

 33.9 



205.4 

 147.0 

 114.4 



34.1 

 36.6 

 39.1 



