SAMPLING GEAR 



Tow Net 



One of three types of sampling gear used 

 was a small high-speed tow net identical to 

 that used by Canadian biologists (Johnson, 

 1956). The net was a cone-shaped fine-mesh 

 nylon bag 9 feet long with a rigid ring 3 feet 

 in diameter holding the mouth open. It was 

 towed through the water as fast as possible 

 by two boats powered with large outboard 

 motors. 



Tows were made on 12 nights for a total of 

 7-1/2 hours of actual fishing. Only 10 sockeye 

 salmon were caught, and 5 of these were taken 

 in one 15-minute tow. Most tows were made 

 during twilight (the time when the Canadians 

 caught most fish) but were also made at other 

 times. Weather ranged from flat calm to very 

 rough, and lake areas fished ranged from 

 shoal to midlake. All tows were at the surface. 

 The catch of fish per hour was 1.3, compared 

 with a maximum of 384 per hour in the Cana- 

 dian studies. 



Further experimentation with modified tow 

 nets might produce better catches, but from 

 our experience it is apparent that catches are 

 affected by many variables. Often the lake 

 was too rough to operate boats, particularly 

 at night. Since our net was only 3 feet in 

 diameter, a slight shift in depth of concentra- 

 tion of fish could greatly affect the catch (it 

 was learned in gill net sampling that weather 

 affects vertical distribution). Probably the 

 major contributing factor to our lack of suc- 

 cess was the low density of resident sockeye 

 salmon at Brooks Lake in contrast to the high 

 density encountered by Johnson (1956). 



Only one beach seine haul resulted in a 

 large catch. A comparison of the length 

 frequencies of juveniles in this catch (fig. 18) 

 with those of juveniles in July gill net catches 

 (fig. 19) indicates considerable disparity, prob- 

 ably reflecting different growth from gill net- 

 caught fish because of their atypical diet in 

 the shallow (less than 3 feet) beach environ- 

 ment. Selectivity of gill nets undoubtedly ac- 

 counted for the lack of a mode at 50 mm., 

 representing the fry population (fig. 19). Beach 

 seine-caught fish had this 50-mm. mode, but 

 lacked a mode at 85 to 100 mm., probably 

 reflecting slower growth of beach-dwelling 

 yearlings (fig. 18). 



Gill Nets 



Gill netting proved to be the best method of 

 catching pelagic juvenile sockeye salmon, and 

 most of our data on abundance, size, and food 

 were from gill net samples. Similar nets had 

 been used successfully for sampling salmonids 

 by the Washington State Department of Fisher- 

 ies in forebays of high dams (Rees, 1957). 



The gill nets were of three basic types. By 

 modification and combination, seven types of 

 sets were possible (fig. 20): 



Type 1 — Floater, graduated mesh. Five 

 15-foot-square panels of white nylon mesh, 

 sizes 1/2, 3/4, 7/8, 1, and 1-1/8 inches, a 

 total length of 75 feet. 



Type II — Floater, graduated mesh. Six 

 15-foot-square panels of brown nylon mesh, 

 sizes 1/2, 3/4, 1, and 1-1/2, 2, and 3 inches 

 a total length of 90 feet. 



Type III — Floater, uniform mesh. A 50- 

 by 6-foot white nylon mesh net, 3/4 inch. 



Beach Seines 



A second sampling gear, used on 11 occa- 

 sions, was a 50- by 5-foot nylon beach seine 

 with 1/4-inch mesh (bar measure). The net 

 proved to be unsatisfactory as a sampling 

 tool because young-of-the-year were able to 

 swim through the mesh during the early part 

 of the season, and yearlings were seldom 

 present along the shoreline — the only place 

 a seine could be used. 



Type Illa — Same as type III, but sub- 

 merged, fishing on bottom. 



Type IV — Combination floater and sub- 

 merged. A type I and type II net attached end 

 to end and suspended vertically with one end 

 at the surface and the other submerged. Total 

 length, 165 feet. Used only in deep water. 



Type V — Combination floater and sub- 

 merged. A tandem arrangement of three type I 



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