ings." The selected years were 1942-1943 and 

 1946-1947. The estimates of total mortality 

 for each of these periods are 58.0% for 1942- 

 1943 and 83.0% for 1946-1947. The assumed 

 constant annual natural mortality between 

 these periods is 22.9%. The total mortality 

 estimates are logical, whereas the natural mor- 

 tality is suspect because it is more dependent 

 on an effective effort determination. The esti- 

 mate of the number of traps fished in a year 

 without trap-hauls or trap-haul-set-over-days 

 should not satisfy the requirement for estimat- 

 ing natural mortality with this method. 



The next series of estimates were made by 

 earlier investigators who used or modified exist- 

 ing methodologies. 



(8) Dow et al. (1953) estimated an annual 

 natural mortality of 7 to 8% for the years 1949 

 through 1952. Their modification of a catch 

 curve was unique and entailed considerable 

 assumptions; nevertheless, their estimate still 

 could be correct. 



(9) Dow (1964) used these and more recent 

 length frequencies, organized in a different 

 manner, but still essentially a catch curve, to 

 estimate a total annual mortality of 83 to 86% 

 with a natural mortality of 28 to 33% from 

 1948 through 1963. This range of total mor- 

 tality estimates closely approximates the esti- 

 mate from Silliman's method of 83% for 1946- 

 1947. 



(10) Skud (1969), with the use of our length- 

 frequency data, estimated a total annual mor- 

 tality of 90% . This estimate, based upon the 

 method of Thomas (1955), requires the use of 

 the ratio of the number of females to males 

 by length. The ratios that Skud used are biased 

 because there are no subtotals by size and sex 

 of the total number of lobsters that we counted 

 by boat. As described earlier, the length-fre- 

 quency data in this report are from the cluster 

 samples of 10 lobsters per boat. This situation 

 could seriously affect Skud's estimate. 



Substantiation of certain estimates 

 of natural mortality. — I believe the 

 lower natural mortality estimates are closer 

 to the actual value for the following reasons: 



(1) A low fc value from the von Bertalanffy 

 Growth Equation indicates a low M as 

 explained by Gulland (1965). 



(2) From the preceding sections, the more 

 acceptable methods yield consistently 

 lower estimates of M. 



(3) The greater average size and magnitude 

 in numbers of lobsters that have been 

 lightly exploited from some canyon areas 

 offshore might indicate a low natural 

 mortality (Skud and Perkins, 1969). 



(4) D. G. Wilder (personal communication) 

 estimated an annual natural mortality 

 of 10% in at least one district in the 

 Maritime Provinces. 



(5) R. A. Cooper (personal communication) 

 estimated an annual natural mortality 

 of 6% from his tagging work on lobsters 

 near Monhegan Island. 



Fishing mortality estimates. — The 

 combination of estimates of total instantaneous 

 mortality (Z) and instantaneous natural mor- 

 tality (M) led to a simple solution for estimating 

 instantaneous fishing mortality (F) as follows: 



Z = F + M 



so that F = Z - M . 



A more desirable method for estimating in- 

 stantaneous fishing mortality involves the rela- 

 tionship between the catchability coefficient (q) 

 and fishing intensity (/). This equation is 



F= qf. 



Because of the discussed problems in estimat- 

 ing the catchability coefficient, a reasonable 

 alternative is to use the first procedure. To do 

 this, we used estimates of instantaneous total 

 mortality which range from 1.1363 (67.9%) to 

 2.9188 (94.6%) and the estimates of instantan- 

 eous natural mortality which range from 0.0202 

 (2% ) to 0.3467 (29.3% ). Therefore, the estimates 

 of the instantaneous fishing mortality range 

 from 0.7896 (54.6% ) to 2.8986 (94.5% ). 



Again, all of the data from the survey of 

 this commercial fishery overwhelmingly sup- 

 ports the higher estimates of fishing mortality. 



47 



