WNALA: MORTALITY OF ROCK LOBSTER 



estimates of the annual instantaneous total 

 mortality rate were 0.64 and 1.03 for the smaller 

 and larger groupings, respectively. 



Estimates of the annual instantaneous total 

 mortality rate from the monthly samples (see 

 Table 5) using the 4 mm grouping ranged from 

 0.46 to 1.00, with a weighted mean of 0.69 and QSVf 

 confidence limits of 0.50 and 0.88. The monthly 

 estimates using the 5 mm gi-ouping ranged from 

 0.90 to 1 . 18, with a weighted mean of 0.99 and 95'7f 

 confidence limits of 0.89 and 1.09. 



Hancock (1965) estimated the total mortality 

 rate of Cancer pcigurus in the Norfolk (England) 

 fishery by partitioning the size distribution into 

 approximate year classes based on the average 

 annual growth increment. If the natural loga- 

 rithms of numbers are plotted against size, a line 

 whose slope is proportional to the total mortality 

 rate is obtained over the size range where growth 

 is constant. Annala (1977) also used this method 

 for estimating the total mortality rate of J. ecl- 

 wcnxlsii in the Otago fishery of New Zealand. 



The average annual growth increment of 4.8 

 mm was rounded to the nearest millimeter, and 

 the seasonal size-frequency distribution of Table 1 

 and Figure 2 partitioned into 5 mm size classes. 

 The results are shown in Table 4, with the annual 

 instantaneous total mortality rate estimated to be 

 1.07. The estimates from the monthly samples 

 (Table 5) ranged from 0.78 to 1.25, with a weighted 

 mean of 1.11 and 959^ confidence limits of 0.97 and 

 1.25. 



Analyses of Tag Return Data 



Mortality rates were also estmiated from the 

 rate of return of tagged male rock lobsters released 

 in the Gisborne local area in July 1976 and recap- 

 tured during the following 12 mo. The instanta- 

 neous total mortality rate was estimated using the 

 method derived by Robson and Chapman (1961) 

 for analyzing a segment of the catch curve. The 



Table 4. — Estimation of the annual instantaneous total mortal- 

 ity rate iZ) from the 1976-77 weighted seasonal size-frequency 

 distribution of male New Zealand rock lobsters from the Gis- 

 borne local area using the average annual growth increment 

 method of Hancock (1965). 



Size class (mm) N, £_ 



100 0-104 9 446 



1050-1099 154 



110 0-114 9 52 



Table 5. — Estimates of the annual instantaneous total mortal- 

 ity rate from the monthly size-frequency distributions of male 

 New Zealand rock lobsters landed from the Gisbome local area 

 during the 1976-77 season. The methods used were Method iv of 

 Bhattacharya (19671. Method 2 of Van Sickle (19771, and the 

 average annual growth increment method of Hancock ( 1965), AT 

 = sample size. 



Average 



1 06 



1 09 

 1 07 



instantaneous fishing mortality rate was esti- 

 mated by ] ) a method developed by Paulik ( 1963) 

 for use with recaptures grouped into time inter- 

 vals, and 2) a method described by Ricker (1975) 

 where estimates are available for the instanta- 

 neous total mortality rate and rate of exploitation. 



A total of 444 male rock lobsters were caught by 

 pots, tagged using the western rock lobster tag 

 (Chittleborough 1974), and released on the fishing 

 grounds. All of the returned rock lobsters were 

 taken in pots by commercial fishermen. Fishing 

 effort was not constant throughout the 1976-77 

 season, so the rate of return of tags was adjusted by 

 the effort expended in each month. The best mea- 

 sure of effort available was the average number of 

 days fished per month per boat for 12 selected 

 boats in the Gisborne local area. 



The average number of days fished in June 1976 

 (9.9 days/boat) was used as the basis for determin- 

 ing relative effort. The number of recaptures for 

 July 1976 was not included in the analysis because 

 tags were not returned over the entire month. The 

 number of males recaptured, the relative effort, 

 and the number of recaptures per unit of relative 

 effort for each month are shown in Table 6. 



The method of Robson and Chapman (1961) 

 used for estimating the total mortality rate de- 

 pends on determining a mean coded age,.r accord- 

 ing to the terminology of Jones (1976), wherex = 

 X/1\\ . The total coded age iX ) was calculated from 



X = il; = l)y, for i = 1,2 J. where J = the 



number of samples, and y, = the number of recap- 

 tures per sample. 



Using the number of monthly recaptures per 

 unit relative effort from August 1976 through 

 April 1977 is shown in Table 6 as an example, 



475 



