AN ACOUSTIC METHOD FOR THE HIGH-SEAS ASSESSMENT OF 



MIGRATING SALMON ^ 



Gary Lord,^ William C. Acker,^ Allan C. Hartt,^ and Brian J. Rothschild" 



ABSTRACT 



A system of free-floating acoustic buoys with upward-looking transducers has been developed for use in 

 assessing high-seas salmon stocks. The transducers, operating at 120 kHz, are suspended 46 m below 

 the surface. The fish counts and the range to each fish are obtained in digital form, and the data are 

 radioed from each buoy to the tending vessel where the data are decoded and recorded on magnetic tape. 

 The present system consists of four buoys although the receiver-decoder system can accommodate up to 

 10 buoys operating synchronously. 



The assessment of fish stocks is of obvious impor- 

 tance to all segments of the fishing industry in 

 planning their respective operations. A problem 

 of particular interest to the United States Section 

 of the International North Pacific Fisheries 

 Commission has been the assessment of imma- 

 ture sockeye salmon, Oncorhynchus nerka, which 

 occur in abundance each summer south of the 

 central Aleutian Islands in the North Pacific 

 Ocean. It has been found (Hartt 1962, 1966) that 

 immature sockeye salmon, mainly of Bristol Bay 

 origin, migrate westward through this area in 

 summer and that their relative abundance is re- 

 lated to the number of mature fish returning to 

 Bristol Bay the following year (Fisheries Re- 

 search Institute Staff 1960; Rogers 1972, 1973, 

 1974). This information has been used since 1960 

 as a means of forecasting the Bristol Bay run. 

 Because the size of the run may vary by a factor of 

 10, an accurate forecast with a lead time of nearly 

 a year is of obvious importance to the fishing and 

 canning segments of the industry. Mathews 

 (1966) has shown, by means of a comprehensive 

 model simulating the cannery portion of the 

 fishery, the relative value of run forecasts of vary- 

 ing precision. Run forecasts are also of value to 

 the fishery management agencies in setting pre- 

 liminary escapement goals and in planning their 



'Contribution No. 438, College of Fisheries, University of 

 Washington, Seattle, WA. 



^Fisheries Research Institute, University of Washington, 

 Seattle, WA 98195. 



^Applied Physics Laboratory, University of Washington, 

 Seattle, WA 98195. 



"Southwest Fishery Center, National Marine Fisheries Ser- 

 vice, NOAA, P.O. Box 271, La Jolla, CA 92038. 



early season strategies to meet these anticipated 

 goals. 



The assessment of immature fish at Adak Is- 

 land has been done by the Fisheries Research In- 

 stitute using a fine-mesh purse seine 400 fathoms 

 (730 m) long at a series of stations from 5 to 50 

 nautical miles off the southern shore of Adak Is- 

 land. From 1956 through 1967, no station pattern 

 was followed — purse seine sets were made ran- 

 domly, mainly in an area within 20 nautical miles 

 of shore. Since 1968, the fishing has been con- 

 ducted uniformly at five stations spaced at ap- 

 proximately 10 nautical mile intervals between 5 

 and 50 nautical miles offshore. 



Although the purse seine is a useful tool for 

 providing information on abundance, species 

 composition, and age composition of the stocks 

 present, it suffers from several disadvantages as 

 a research tool. Its use is limited to periods of 

 moderate sea conditions resulting in significant 

 gaps in the time-space coverage in this particu- 

 larly stormy region. A maximum of five sets can 

 be made in a day under ideal conditions which 

 yields only 2y2 h of actual fishing. Also, seines 

 give no direct information on depth stratification 

 or schooling of the fish, and in areas where the 

 direction of migration of the fish is not uniform, 

 multiple sets are required to sample all of the 

 stocks present. Variability in direction of migra- 

 tion is not a serious problem in the area south of 

 the central Aleutian Islands because the direc- 

 tion of migration of immature salmon is uni- 

 formly westward (Hartt in press). In an effort to 

 overcome the sampling limitations of the purse 

 seine, the Fisheries Research Institute and the 

 Applied Physics Laboratory jointly developed an 



Manuscript accepted August 1975. 

 FISHERY BULLETIN; VOL. 74, NO. 1, 1976. 



104 



