SPECIAL STUDIES 



This section includes short-term and other 

 studies of various aspects of fur seal biology 

 that are carried on in addition to the con- 

 tinuing studies described in the main body of 

 this report. 



AGE COMPOSITION, MEASUREMENTS, 



AND MORTALITY OF TERRITORIAL 



MALE FUR SEALS 



Biologists and managers working with fur 

 seals have frequently discussed the number 

 of male seals that must be spared in the kill 

 to nnaintain the breeding population. The fol- 

 lowing report on a study of 250 territorial 

 males killed in 1965 gives evidence of the 

 current age connposition, annual mortality, 

 and rate of replacement of these territorial 

 males (Johnson, 1968). 



Territorial males used for this study were 

 collected from two areas on each of two 

 rookeries on St. Paul Island; the testes of all 

 (250) were collected for studies of reproduc- 

 tion, and the bacula from 250 and canine teeth 

 from 249 of the males were saved for studies 

 of age and growth. 



Age Composition of Territorial Males 



The age composition of territorial males 

 collected from the four areas and that of 

 157 males found dead on St. Paul Island were 



similar (table 30). The high value (0.3<P<0.5) 

 of P in a Chi square test indicated that the 

 samples could have come from the same 

 population. The territorial males were 7 to 

 17 years old and 71 percent were 10 to 1 3 

 years old; the dominant age was 10 years. 



Estimate of Annual Mortality of 

 Territorial Males 



The rate of replacement (a) was estimated 

 by using the equation of Chapman-Robson 

 (I960): 



a = l-x/(l+x-l/n) 



where n = number of animals in the sample 



X = mean age; ages coded beginning 

 with age of full recruitment as 

 zero 



If age 10, the most frequently assigned age 

 among the samples, is considered as the age 

 of full recruitment, the estimated annual 

 replacement based on all males 10 years old 

 and older is 0.38. 



An additional estimate was calculated by 

 regressing the natural logarithm of the num- 

 ber in each age on age. This method, which 

 required neither constant recruitment nor 

 constant survival, produced an estimate of 

 replacement of 0.37. 



The annual estimated replacement of ter- 

 ritorial males (0.38) agrees with an estimate 



Table 30. --Age composition of territorial male seals killed for study and mature male seals 



found dead, St. Paul Island, 1965 



1/ Northeast Point. 

 11 Sea Lion Rock. 

 ?/ Males that occupied areas where the original territorial males were killed. 



31 



