EVALUATION OF VARIABILITY IN 



SABLEFISH, ANOPLOPOMA FIMBRIA, ABUNDANCE INDICES IN 



THE GULF OF ALASKA USING THE BOOTSTRAP METHOD 



Michael F. Sigler and Jeffrey T. Fujioka^ 



ABSTRACT 



Relative population numbers (RPN's) and length compositions were computed for sablefish, Anoplopoma 

 fimbria, in the Gulf of Alaska from the results of the Japan-United States cooperative longline survey 

 from 1979 to 1986. A statistical evaluation of annual changes in the RPN's using the bootstrap method 

 is demonstrated and showed that sablefish abundance increased significantly from 1979 to 1986, an in- 

 crease likely due to recruitment of two strong year classes. The effect of missing data on the bootstrap 

 calculations was examined and found to be negligible. 



Early in this century, United States and Canadian 

 fishermen began harvesting sablefish, Anoplopoma 

 fimbria, in nearshore waters from California north 

 to southeastern Alaska, but sablefish were not 

 heavily exploited until Japanese longline vessels 

 began fishing in the Bering Sea in 1958. Japanese 

 catches off both the U.S. and Canadian coasts rose 

 dramatically in the following two decades. After 

 passage of the Fishery Conservation and Manage- 

 ment Act, foreign catches were reduced and the 

 domestic allocation was increased. The domestic 

 catch eventually increased nearly fivefold, thereby 

 replacing the foreign fishery, and in 1985, sablefish 

 in the Gulf of Alaska were harvested entirely by 

 domestic fishermen. 



Before the reduction of the foreign fishery, infor- 

 mation on sablefish abundance consisted of statistics 

 on catch per unit effort from the Japanese longline 

 fishery. In 1978, the Fisheries Agency of Japan and 

 the U.S. National Marine Fisheries Service (NMFS) 

 began a cooperative longline survey along the con- 

 tinental slope of Alaska to assess the abundance of 

 sablefish and Pacific cod, Gadus macrocephalus. The 

 survey, conducted annually, has provided eight con- 

 secutive years (1979-86) of data for the Gulf of 

 Alaska, seven years (1980-86) of data for the Aleu- 

 tian region, and five years (1982-86) of data for the 

 eastern Bering Sea. The first year of the survey, 

 1978, was experimental. 



Relative population numbers (RPN's) and length 

 compositions from the longline survey results from 



'Northwest and Alaska Fisheries Center Auke Bay Laboratory, 

 National Marine Fisheries Service, NOAA, P.O. Box 210155, Auke 

 Bay, AK 99821. 



1979 to 1985 have been estimated previously by 

 Sasaki (1986). In this study, the observed increase 

 in the RPN's for sablefish in the Gulf of Alaska is 

 evaluated statistically and an explanation of the 

 probable source of the increase is discussed. 



Statistical analysis of the survey results is based 

 on the bootstrap method (Efron 1982; Efron and 

 Gong 1983). This method is a relatively new statis- 

 tical procedure that has been little used in fisheries 

 analysis. Thus, this paper also demonstrates an 

 application of the bootstrap method to statistical 

 evaluation of fishery survey data. 



SURVEY METHODS 



The Gulf of Alaska portion of the Japan-U.S. 

 cooperative longline survey, conducted annually 

 each summer from 1979 to 1986, covered five 

 International North Pacific Fisheries Commission 

 (INPFC) statistical areas: Shumagin, Chirikof, 

 Kodiak, Yakutat, and Southeastern (Fig. 1). One of 

 a total of 47 stations each ranging in depth from 

 about 100 to 1,000 m was sampled daily by longline. 

 The longline was 16 km long and consisted of 160 

 hachis (the Japanese word for "skate" or length of 

 longline); each hachi was 100 m long and consisted 

 of 45 hooks baited with squid. Soak time, the time 

 between setting and retrieval, varied from 3 hours 

 at the beginning of the longline gear to 7 or 8 hours 

 at its end. The depth at which the fish were caught 

 was estimated by measuring the depth of water 

 under the vessel with an echo sounder every fifth 

 hachi. The fish caught were tallied by species and 

 hachi as the longline was brought aboard, then they 

 were weighed and their length was measured. Most 



Manuscript accepted May 1988. 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. 



445 



