ANNALA: MORTALITY OF ROCK LOBSTER 



The third factor, emigration out of the area, does 

 not appear to be important. None of the males 

 tagged in July 1976 had been recaptured outside 

 the Gisborne local area. The inshore fishing 

 grounds outside the Gisborne local area are all 

 heavily fished, so the movement of significant 

 numbers of tagged animals would probably be de- 

 tected, unless they were migrating to deeper, 

 unfished areas. 



The methods of Paulik 1 1963) and Ricker 1 1975) 

 used to estimate the fishing mortality rate from 

 the tag returns (Table 7) are both dependent on 

 estimates of the total mortality rate. Any bias in 

 the estimates of total mortality rate will affect the 

 estimates of fishing mortality rate. If the tagged 

 rock lobsters were more susceptible to capture 

 then the untagged individuals, then the estimates 

 of fishing mortality rate may be overestimates. 



Other factors affecting estimates of fishing mor- 

 tality rate (Type A errors, Ricker 1975) include: 1) 

 the death of tagged rock lobsters due to the pres- 

 ence of the tag, or the loss of their tags, shortly 

 after marking; and 2) incomplete reporting of tags 

 recaptured by fishermen. As mentioned previ- 

 ously, preliminary experiments indicate the ef- 

 fects of the first factor are minimal. However, pre- 

 dation by fish on the newly released animals may 

 be important and warrants further investigation. 

 It is known that some of the recaptured tags went 

 unreported by fishermen, although the numbers 

 were not large. This would result in an underesti- 

 mate of the fishing mortality rate. 



The estimates of fishing mortality rate adjusted 

 for an 8- or 9-mo fishing season (1.17-1.35) are 

 considerably lower than the unadjusted estimates 

 (1.56-1.85). If the estimate of M = 0.10 (Annala 

 1977) applies to the Gisborne fishery, then Z = 

 1.27 to 1.45 based on the estimates ofF for an 8- or 

 9-mo season. These estimates of Z are more com- 

 parable to those derived from the analyses of the 

 size-frequency distributions (0.64-1.07) than to 

 the estimates derived using the method of Robson 

 and Chapman (1961) (1.92-3.13). Moreover, these 

 lower estimates are more consistent with the es- 

 timated lifespan in the fishery than are the higher 

 estimates. Based on the average annual growth 

 increment of 4.8 mm and the seasonal size- 

 frequency distribution in Table 1 and Figure 2, 

 rock lobsters appear to survive for about 4 or 5 yr 

 in the fishery. The preliminary empirical growth 

 model developed by Saila et al. (in press) also indi- 

 cates that male rock lobsters from the Gisborne 



local area remain in the exploited phase for a 

 minimum of 4 to 5 yr. 



In summary, the estimates of total mortality 

 rate for the entire exploited population from the 

 rate of tag returns are considered too high. The 

 estimates of total mortality rate from the size- 

 frequency distribution analyses, and those based 

 on the estimates of F over an 8- or 9-mo fishing 

 season with the addition of M = 0.10, are more 

 consistent with the observed lifespan in the 

 fishery. Thus, the best estimates of Z are between 

 1.00 and 1.50, and ofF are 0.90 and 1.40, assuming 

 M = 0.10. 



ACKNOWLEDGMENTS 



I would like to thank the National Research 

 Advisory Council of New Zealand for fellowship 

 aid during part of this study. Saul Saila provided 

 many helpful suggestions on the application of 

 these methods. J. Booth, R. Francis, J. McKoy, S. 

 Saila, G. D. Waugh, and two anonymous reviewers 

 made many constructive comments on the manu- 

 script. B. Bycroft. B. Hvid, and J. McKoy provided 

 valuable assistance in the field. Finally, I would 

 like to thank the commercial fishermen for their 

 excellent cooperation in returning tagged rock 

 lobsters, and the staffs of the fish processing sheds 

 and the Ministry of Agriculture and Fisheries 

 Office in Gisborne for leceiving and holding the 

 same. 



LITERATURE CITED 



ANNALA, J. H. 



1977. Effects ofincreases in the minimum legal size on the 

 Otago rock lobster fishery. N.Z. Fish. Res. Div, Occas. 

 Publ. 13, 16 p. 

 BEVERTON, R. J. H.. AND S. J, HOLT. 



1956. A review of methods for estimating mortality rates 

 in exploited fish populations, with special reference to 

 sources of bias in catch sampling. Rapp. P.-V. Reun. 

 Cons. Perm. Int. Explor, Mer 140:67-8,3. 

 BH.ATTACHARYA. C. G. 



1967. A simple method of resolution of a distribution into 

 Gaussian components. Biometrics 23:115-135. 

 Chittleborough, R. G. 



1974. Development of a tag for the western rock lob- 

 ster. Aust.C.S.I.R.O. Fish.Oceanogr.Div.Rep.56. 19p. 



Hancock, D. a. 



1965. Yield assessment in the Norfolk fishery for crabs 

 [Cancer pagurus). Rapp. P.-V, Reun. Cons. Perm. Int. 

 Explor. Mer 156:81-94. 



Jones R, 



1976. The use of marking data in fish population 

 analysis. FAO Fish. Tech. Pap. 153. 42 p. 



479 



