FISHERY BlILLETIN VOI. 77, NO 



Gable End 

 Foreland 



30 



Young Nicks Head 



39° S 



30' 



Figure l, — Location of the study area between Young Nicks 

 Head and Gable End Foreland on the east coast of the North 

 Island, New Zealand. 



north ( Figure 1 ). The landings from this area were 

 chosen for analysis because it has been the site for 

 a series of tag-recapture studies, which were also 

 used to estimate mortality rates. The results re- 

 ported here are part of an extensive study of the 

 biology of J. edwardsii in the Gisborne area. 



Size-frequency distributions in the landings 

 were determined on a monthly basis from July 

 1976 to February 1977, with the exceptions of Au- 

 gust and January, during the 1976-77 season. The 

 fishing .season is defined as extending from 1 June 

 to 31 May of the following year. There is a natural 

 break between seasons due to a period of low 

 catchability and resulting low effort expenditure 

 during April and May. During each month the 

 landings were chosen on a nonrandom basis for 

 sampling, and the entire landing on a given day 

 for an individual boat was measured. The mea- 



surement used was the carapace length taken 

 from the base of the antennal platform to the dor- 

 sal, posterior margin of the carapace along the 

 midline. These individual samples were given 

 equal weights and combined directly to yield a 

 monthly sample. The monthly samples were then 

 weighted by the proportion of the total seasonal 

 landings landed during that month and combined 

 to give a weighted seasonal size-frequency dis- 

 tribution (Table 1, Figure 2). This weighting pro- 

 cedure was applied to average out changes in the 

 size-frequency distributions due to fluctuations in 

 catchability, recruitment, and mortality to permit 

 estimation of the average annual total mortality 

 rate. 



The approximate method of separating the com- 

 ponent distributions of a polymodal size-frequency 

 distribution described by Bhattacharya ( 1967) in- 

 volves a cubic approximation of density within a 

 size class and a quadratic approximation to the 

 logarithm of the frequency of each class. It was 

 assumed that the frequency distribution is com- 

 posed of Gaussian component distributions that 

 are adequately separated so that each component 

 has a sufficiently broad region where the effects of 



Table l. — Weighted seasonal size-frequency distribution of 

 male New Zealand rock lobsters from the Gisbome local area 

 during the 1976-77 season. Observed frequency values are 

 weighted frequencies x 10-'. 



472 



