MORTALITY ESTIMATES FOR THE NEW ZEALAND 

 ROCK LOBSTER, J/\5L/5 EDWARDSII 



John H. Annala' 



ABSTRACT 



The instantaneous total mortality rate and instantaneous fishing mortality rate were estimated for an 

 exploited population ofmale New Zealand rock lobster. Jasusedivardsn , Instantaneous total mortality 

 rate estimates were made from the seasonal size-frequency distribution of landed rock lobsters using 

 three different methods and ranged from 0.64 to 1.07. Estimates of both mortality rates were also made 

 from the rate of return of tagged rock lobstersover an entire year and by adjusting the rate for an 8- or 

 9-month fishing season. These estimates of the instantaneous total mortality rate ranged from 1.92 to 

 3.13 and were considered too high to be representative of the entire exploited population. Instan- 

 taneous fishing mortality rate estimates from the tag returns ranged from 1.17 to 1.85, with the lower 

 rates based on an 8- or 9-month fishing season. Using the results from both types of analyses, and the 

 observed lifespan of rock lobsters m the fishery, the best estimates of the instantaneous total mortality 

 rate are between 1 .00 and 1 .50 and of the instantaneous fishing mortality rate between 0.90 and 1 .40. 

 assuming the instantaneous natural mortality rate equals 0.10. 



Knowledge of the total mortality rate, and its 

 components of fishing and natural mortality, is 

 essential for an adequate understanding of the 

 population dynamics of an exploited population. 

 Mortality rates are generally estimated from 1) 

 the age composition of the population, with the age 

 composition of the catch serving as the population 

 sample; 2) the results of mark and release experi- 

 ments; or 3) some relationship between catch and 

 effort. 



The purpose of this investigation is to derive and 

 compare estimates of mortality rates for an 

 exploited population of the New Zealand rock lob- 

 ster, Jas;/s edwardsii . Rock lobsters do not contain 

 any structural parts retaining annual marks, so 

 estimates of mortality rates cannot be made from 

 the age composition of the catch. However, the 

 total mortality rate can be estimated by analysis 

 of the size-frequency distribution of th& catch, and 

 three different methods are employed. The results 

 of these analyses are compared with estimates of 

 the total mortality rate derived from a tag- 

 recapture study conducted over the same fishing 

 season and in the same area from which the size- 

 frequency distributions were drawn. The results of 

 the marking experiment are also used to estimate 

 the fishing mortality rate, which is then compared 

 with the estimates of the total mortality rate using 



'Fisheries Research Division, P O. Box 19062, Wellington, 

 New Zealand. 



Manuscript accepted December 1978 

 FISHERY BULLETIN: VOL. 77. NO. 2, 1979. 



a previously derived estimate of the natural mor- 

 tality rate. 



METHODS AND RESULTS 



Analyses of Size-Frequency Distributions 



The three methods used to estimate the total 

 mortality rate from the size-frequency distribu- 

 tion were: 1) the approximate method of separat- 

 ing a polymodal size-frequency distribution into 

 its component distributions described by Bhat- 

 tacharya ( 1967); 2 ) Method 2 of Van Sickle ( 1977), 

 where growth and size-frequency data were used 

 to estimate mortality on a size specific basis; and 3 ) 

 the partitioning of a size-frequency distribution by 

 the average annual growth increment into com- 

 ponents approximating age classes (average an- 

 nual growth increment method) described by 

 Hancock (1965). 



Mortality rates were estimated from the size- 

 frequency distribution of male rock lobsters 

 landed from the Gisborne local area during the 

 1976-77 fishing season. Females constitute only a 

 small proportion of the landings from this area, so 

 their mortality rates were not estimated. Gisborne 

 is a major fishing port located on the east coast of 

 the North Island (Figure 1). The Gisborne local 

 area is defined as encompassing the rock lobster 

 fishing grounds extending from Young Nicks 

 Head in the south to Gable End Foreland in the 



471 



