CARAPACE LENGTH IN INCHES 



sible, from different techniques of other investi- 

 gations on lobsters. 



In accordance with this reasoning, we listed 

 the methods and reported all estimates in 

 annual rates (Table 9) as follows: 



60 80 100 



CARAPACE LENGTH IN MILLIMETERS 



Figure 16. — The weight-length relationship of lobsters 

 (sexes combined), W = .001682 L 2 - 82826 . 



Mortality Rates 



The implications from the length frequency 

 section concerning age or molt groups create 

 some imponderables for estimating survival 

 or mortality. A reasonable alternative would 

 be to estimate the desired parameters by 14% 

 groupings and then by selected size modes 

 from the probability analysis, realizing the dis- 

 cussed assumptions in each category. With 

 this approach, we can compare estimates and 

 then, in certain situations, explain why there 

 are differences or similarities. Of course, even 

 if these estimates were similar, they would 

 be tentative because of the uncertainties con- 

 cerning the age composition of the catch. To 

 circumvent this situation to some extent, we 

 present corroborative estimates, whenever pos- 



(1) We used the method of Robson and 

 Chapman (1961) with 14% increments of growth 

 for the commercial-sized lobsters within cal- 

 endar years. The estimates are: 90.0% (1967), 

 91.4% (1968), 92.2% (1969), and 92.9% (1970). 

 These authors explained that the method is 

 not adequate when estimating survival by age 

 class between years because it does not con- 

 sider effort. Nevertheless, the authors devised 

 an unbiased estimate of survival and mortality 

 within years if the age and growth considera- 

 tions were correct. R. A. Cooper (personal 

 communication) estimated approximately the 

 same total annual mortality from a tagging 

 study off Monhegan Island, Maine. In my opin- 

 ion, it is unlikely that the two separate tech- 

 niques and data sources would approximate 

 each other by coincidence. 



(2) Cushing (1968) described a method which 

 does incorporate effort with assumed age 

 classes. However, Beverton and Holt (1957) 

 maintained that it is seldom efficient to estimate 

 an index of instantaneous abundance as is 



required in Z = log e 



N t 



. Nevertheless, 



this equation (after conversion) represents the 

 usual method of estimating total annual mor- 

 tality. We used it with two different types of 

 effort: (1) trap-hauls-set-over-days and (2) trap- 

 hauls. With the first effort term the estimates 

 are: 87.0% (»i/h 2 between May 1968 and May 

 1969) and 83.5% {;n 2 ln z for the same time period). 

 With the second effort term the estimates are: 

 85.8% (?hln 2 between May 1967 and May 1968) 

 and 90.8% (ii 2 ln s for the same time period) ; 74.8% 

 (toi/to 2 between May 1968 and May 1969) and 

 68.1% (n 2 ln 3 for the same time period); 64.6% 

 (wi/to 2 between May 1969 and May 1970) and 

 94.1% (« 2 /»3 for the same time period). 



Also, we used this method of Beverton and 

 Holt (1957) to estimate the annual natural mor- 

 tality for the prerecruit sizes of lobsters. These 

 estimates are: 29.3% (»i/»2 between May 1968 

 and May 1969) and 19.2% (»i/»2 for May 1969 

 and 1970). 



43 



