Schiel and Breen: Population, ageing, and fishing mortality of Hahotis ins 



687 



Figure 4 



'Age' estimates from growth ring counts (n 335) in one region 

 (Marlborough Sounds D'Urville I.), assuming that one growth 

 ring represents one year of age. 



Marlborough Sounds and D'Urville I. regions. Growth 

 estimates from ring count data were not used because 

 of the discrepancy seen in Table 3. Estimates of F 01 

 and F 25 % resulting from YPR analyses are given in 

 Table 6 for three assumed M values. In all cases, 

 F 2 5% was greater than the estimated Fo.i obtained 

 from the same parameters. 



The best estimates of actual 

 fishing mortality rates, obtained 

 from the procedure shown in 

 Table 5, were substantially high- 

 er than F .i in both regions ex- 

 amined (Table 6). This result is 

 not sensitive to the assumed M, 

 although higher M values in- 

 crease F .i and decrease the 

 estimated current level of F. 

 Response surfaces in YPR and 

 egg production resulting from 

 variation in M and the size-at- 

 first capture are shown in Fig- 

 ures 6 and 7, respectively. These 

 show the typical form of such 

 surfaces (cf. Tegner et al. 1989). 

 When M is higher, maximum 

 YPR is obtained at a lower size 

 for a given F, or at a higher F for 

 a given size. Egg production 

 follows a simpler pattern, but is 

 greater at a given combination of 

 F and size-at-first-capture when 

 M is greater. Yield and egg pro- 

 duction resulting from different 

 values of M and MLS are shown 

 in Table 7. 



Table 3 



A comparison of von Bertalanffy growth parameters estimated from growth ring-count 

 data from several sites in the D'Urville I. and northern Marlborough Sounds regions 

 (Fig. 1) and mark-recapture data from experiments at the same sites (Talbot Murray, 

 unpubl. data). Sample sizes are unequal because more animals were subjected to ring 

 counting than marks were recovered. 



Place 



Mark-recapture data 

 n L„ K 



Ring-count data 



(SE) 



K (SE) 



D'Urville I. 79 141.6 0.379 131 131.5 (2.21) 0.197 (0.029) 



Northern Marlborough 53 144.5 0.285 180 126.4 (2.65) 0.214 (0.052) 



