Other factors associated with mor- 

 tality estimates.— Paloheimo(1961,1963) 

 discussed catchability coefficients in associa- 

 tion with population estimates for lobsters. He 

 demonstrated inconsistencies in the catchability 

 estimates when derived from temperature, catch, 

 and effort (trap-hauls per day fished per square 

 nautical mile). While his shortcut method ap- 

 pears reliable, we do not have a long enough 

 series of yearly catch and effort data to com- 

 plete the estimates. 



Because of the complexities of availability 

 and recruitment in the stock of lobsters along 

 the Maine coast, it appears futile to estimate 

 this catchability coefficient. However, if we 

 hold before us the goal of establishing precise 

 population parameters, in particular the esti- 

 mates of fishing and natural mortality from 

 the relationship F = qf, where q = catchability 

 coefficient and / = fishing intensity (weighted 

 effort), then the first approximation of the 

 catchability coefficient makes a step toward 

 this objective. Also, this coefficient might ex- 

 plain the deviation in the catch-per-unit-of- 

 effort values from the true density (provided 

 we use the effective effort term), and changes 

 in this coefficient from year to year or period 

 to period might indicate changes in availability. 



Armed with these concepts and the techniques 

 of Beverton and Holt (1957), and Cushing (1968), 

 we attempted estimates of the catchability co- 

 efficient from the survey data. In order to do 

 this, we compiled the required data by month 

 and year. 



The first attempt was made by plotting the 



instantaneous mortality (y) for May of 1968, 



1969, and 1970_(calculated from the equation 



N t 

 log e - 



of Z 



ty+1 



using 14% increments as 



assumed age or molt groups) against the cor- 

 responding number of THSOD (x). The linear 

 regression, solved by the method of least squares, 

 is: 



y = 0.079857 + 0.00016&r (r = 1.00). 



The estimate of the catchability coefficient 

 is 0.000168. In this case the total annual natural 

 mortality estimate is 7.7% . This estimate is 

 identical to two estimates that we made in the 

 mortality section. 



Next we plotted the instantaneous total 

 mortality (y) for May of 1968, 1969, and 1970 

 (calculated by the method of Robson and Chap- 

 man (1961) for all assumed age or molt groups 

 of 14% increments) against the corresponding 

 number of THSOD (x). The linear regression, 

 solved by the method of least squares, is: y = 

 0.579137 + 0.000168.V (r = 0.99). 



The estimate of the catchability coefficient 

 is 0.000168. In this case the total annual natural 

 mortality is 0.43962. The estimate of the catch- 

 ability coefficient is identical with the preceding 

 estimate. This higher natural mortality esti- 

 mate may be attributed to the different method- 

 ology for calculating the total mortality esti- 

 mates. 



Other attempts to calculate the catchability 

 coefficient led to negative natural mortality 

 estimates. It appears then that the best esti- 

 mate we can make of the catchability coefficient 

 is 0.000168 for the month of May in each year. 



Consideration of Trap Limitations 



Some legislators and fishermen have pro- 

 posed a trap limitation. The objective of their 

 proposal is not clearly defined. I believe the 

 general intent is to lessen the fishing pressure 

 (effective effort) on lobsters. 



Fisheries biologists deal with this concept 

 in a different way. We are concerned with the 

 effect of fishing effort on fishing mortality. This 

 report has dealt with such effort categories as 

 numbers of: (1) traps, (2) trap-hauls, and (3) 

 trap-haul-set-over-days. In the mortality sec- 

 tion, we demonstrated limited success in cor- 

 relating the trap-hauls, and trap-haul-set-over- 

 days with fishing mortality while Dow and 

 Trott (1956) had none at all with the number 

 of traps. In the case of the latter two categories, 

 I believe that the problem is due to the magni- 

 tude of the effective effort so that fairly large 

 changes in these categories have relatively little 

 effect on the actual fishing mortality. 



Then the proposal at hand should be con- 

 sidered with the coalescence of the concepts 

 of fishermen and biologists. This should enable 

 us to reach a determination as to whether these 

 proposals would accomplish the objective. The 

 united concept assumes that a limit on the 

 number of traps per boat will lessen the fishing 

 effort and therefore the fishing mortality. 



48 



