ON THE ESTIMATION OF NUMBERS OF 

 NORTHERN FUR SEAL, CALLORHINUS URSINUS, PUPS BORN ON 



ST. PAUL ISLAND, 1980-86 



Anne E. Yorri and Patrick Kozloff^ 



ABSTRACT 



Since 1962, the numbers of northern fur seal, Callorhimis ursinus, pups born on St. Paul Island have 

 been determined using a mark-recaptiu-e procedure. We investigate the feasibility of determining estimates 

 of the total pup population on the 14 rookeries of St. Paul Island from subsamples of rookeries. Estimates 

 are derived from simple random sampling and stratified (by rookery size) random sampling using stan- 

 dard ("blow up") estimation procedure, and ratio and regression estimates (based on the same sampling 

 procedure but taking advantage of a strong relationship between numbers of breeding males and live 

 pups on the various rookeries). Evaluation of the sampling schemes and estimation methods is based 

 on the performance of the estimators for 3 years (1965, 1970, 1975) of data for which the mark-recapture 

 estimates from all 14 rookeries were available. Ratio estimates are preferred to estimates obtained from 

 the standard procedure for both simple random sampling and stratified random sampling. Furthermore, 

 estimates from sampling plans based on three strata proved more satisfactory than those based on either 

 unstratified or two-strata sampling. The ratio methods are applied to data collected during 1980-86. The 

 number of northern fur seal pups born on St. Paul Island decreased at approximately 7.5% per year 

 during 1975-81. There was no statistically detectable trend in numbers born during 1981-86. 



The number of northern fur seals, Callorhinus 

 ursinus, born on St. Paul Island (approximately 80% 

 of the total Pribilof Islands herd production) has 

 been determined in a variety of ways since the 

 United States assumed direct management of the 

 fur seal herd in 1910 (Parker 1946). The history of 

 northern fur seal population estimation during 

 1912-47 and analyses of the reliability of methods 

 then proposed for estimating numbers of pups are 

 presented in Kenyon et al. (1954). The evolution of 

 the "shearing-sampling" method, a variant of the 

 mark-recapture technique, is discussed in Chapman 

 (1964) and Chapman and Johnson (1968). 



Since 1962, the estimate of the size of the pup 

 population has been obtained using the "shearing- 

 sampling" method. The safety of the crew, the 

 accuracy of the estimate, and the minimization of 

 disturbance to rookeries are major concerns; hence, 

 the work is done as the breeding structure breaks 

 up, but before pups spend most of their time in the 

 water. During early August, a large number of pups 

 (approximately 10% of the population) are marked 

 by shearing a small patch of hair from the top of 



'National Marine Mammal Laboratory, Northwest and Alaska 

 Fisheries Center, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way N.E., Seattle, WA 98115. 



^National Marine Mammal Laboratory, Northwest and Alaska 

 Fisheries Center, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way N.E., Seattle, WA 98115; present address: 

 Tanadgusik Cooperation, St. Paul, AK 99660. 



their heads; this exposes the pale underfur and pro- 

 duces an easily identifiable mark. The marking ef- 

 fort is allocated throughout the rookery so that each 

 pup has an approximately equal chance of being 

 marked. A few days later, each rookery is sampled 

 twice during different periods to estimate the pro- 

 portion of marked animals on the rookery. Thus, 

 estimates of the population size and its variance can 

 be calculated for each rookery. The estimate of the 

 population present at the time of shearing is the 

 number of sheared animals divided by the propor- 

 tion of sheared pups among all those resighted— 

 the normal Petersen estimate. The variance of this 

 estimate is one-fourth the squared difference of the 

 two estimates. 



The purpose of this paper is to demonstrate the 

 feasibility of obtaining accurate estimates of the 

 total pup population on St. Paul Island from 

 "shearing-sampling" estimates on a few sample 

 rookeries. The advantages of obtaining estimates of 

 the population from a subsample of rookeries in- 

 clude 1) less disturbance on the total northern fur 

 seal population (each season that pup production is 

 estimated on a particular rookery, crews must 

 traverse the rookery four times— once to do the 

 marking, twice to estimate the proportion of marked 

 pups among the population, and once to count the 

 number of dead pups); and 2) considerable savings 

 in time, energy, and funds. 



Manuscript accepted February 1987. 

 FISHERY BULLETIN; VOL. 85, NO. 2, 1987. 



367 



