690 



Fishery Bulletin 89(4), 1991 



Our experience underscores the need to validate age 

 estimates from independent data (Beamish and 

 McFarlane 1983). ForH. iris, the use of growth rings 

 leads to the erroneous conclusion in at least two popula- 

 tions that they are more lightly fished than indicated 

 by length-frequency analysis based on mark-recapture 

 data. 



Mortality 



For the reasons discussed above, we consider that the 

 mortality rates based on ring counts are spurious for 

 H. iris, at least for the regions examined. Mortality 

 rates derived from length-frequency distributions are 

 heavily dependent on the assumptions of the method. 

 One critical assumption is that recruitment to the 

 population is roughly constant. If recent recruitment 

 has varied with any trend, analysis of a single length- 

 frequency sample cannot provide a realistic estimate 

 of mortality (Shepherd and Breen 1991). We have no 

 data on annual variation of recruitment at our sampling 

 sites. Interannual variation can be high in some species 

 of abalone (Tegner et al. 1989), but it is systematic 

 change in recent recruitment that causes estimates to 

 be dubious. However, it is encouraging that our 

 estimates of mortality are reasonably robust to varia- 

 tion in the input parameters (Table 5). 



Other important assumptions are that natural mor- 

 tality is age-independent (over the age range examined) 

 and also constant for all areas. For abalones general- 

 ly, the first assumption is not unreasonable for mature 

 animals (Shepherd and Breen 1991) and, for H. iris, 

 it is consistent with the conclusions of Sainsbury (1982). 

 Our estimates of current fishing mortality rates are 

 sensitive to the assumption that M = 0.10 (Table 6). This 

 is the best estimate of natural mortality derived by 

 Sainsbury (1982). 



Both reference points are sensitive to several as- 

 sumptions, similar to those discussed above: random 

 recruitment variation, constant age-independent M, 

 constant growth rates, and the assumed value of M 

 (reviewed by Hancock 1979, Vetter 1988, Breen 1991). 



The impact of varying the minimum legal size ap- 

 pears-small in comparison with the sensitivity of YPR 

 and egg production analysis to M (Table 7). Neither 

 YPR nor egg production at equilibrium would be af- 

 fected much by changing the MLS, at least over the 

 range examined. Maximizing YPR by adjusting the 

 MLS would require a much more precise knowledge 

 of M. YPR is robust to changes in F, but decreasing 

 F to the reference level F .i would increase equilib- 

 rium egg production; this would be true whatever com- 

 bination of M and MLS is examined. This result is 

 similar to that reported by Sluczanowski (1984, 1986) 

 for Southern Australian abalones. He found that egg 

 production could be increased markedly with little 

 sacrifice in YPR in the fishery. 



Because of the ease with which paua can be found 

 in most sites, there is a temptation to assume that 

 stocks are healthy and in no imminent danger of over- 

 fishing. However, based on our estimates of fishing 

 mortality rates, the recent fishery may be simply 

 removing an abundant accumulated stock at a rate 

 greater than annual production in two study regions. 



Our best estimates of fishing mortality rates are 

 higher than the standard reference levels, indicating 

 that fishing pressure should be reduced in these 

 regions. Under an ITQ management regime, this can 

 be addressed by reducing quotas in a QMA. Translating 

 reductions in fishing mortality to reductions in TACCs 

 for Quota Management Areas can be done by evalu- 

 ating the Baranov catch equations (Ricker 1975). 

 Another approach is to estimate maximum constant 

 yields from historical catch data (Schiel 1989). 



Modeling and implications for management Acknowledgments 



Of the two reference points we use for evaluating fish- 

 ing mortality rate in the H. iris fishery, F A is smaller 

 than F 25 % for both regions. This result is insensitive 

 to the assumed M and indicates that F .i management 

 is more conservative than F 2 5% management. 



The estimated current fishing mortality rates for 

 both regions are greater than both reference levels of 

 F; this result is also insensitive to the assumed value 

 of M. If the underlying assumptions hold, therefore, 

 the current fishery is more intense than can be sus- 

 tained in the long term in these regions. Egg produc- 

 tion at the estimated present level of fishing mortality 

 is about 18% (Table 7). This is lower than prudent 

 management would probably require. 



We thank Dr. Talbot Murray for kindly providing ac- 

 cess to his unpublished data. We also thank the follow- 

 ing: S. Mercer, R. Maulder, K. Michael, and R. Welsh 

 for assistance in the field; S. Mercer for reading the 

 paua rings; T. Kendrick, 0. Anderson, S. Mercer, and 

 K. Michael for help in analyzing the data; the crew of 

 the RV Kaharoa for field support; Drs. Linda Jones, 

 Paul McShane, Scoresby Shepherd, and Mia Tegner for 

 comments on the manuscript. 



