FRENCH and DUNN: LOSS FROM HIGH-SEAS GILLNETTING 



possibility exists that the presence of decoy 

 fish attached to the gill nets when they are set 

 may attract predators and carrion eaters and, 

 therefore, increase the incidence of predation. 



To study further the relation between decoy 

 losses and dropouts, we examined losses of 

 decoys and salmon marked for dropout studies 

 in 10 gill net sets in which dropout and decoy 

 loss studies were conducted simultaneously. In 

 these sets the decoys were generally attached 

 near the distal end of the net string from the 

 vessel, and salmon were marked for dropout 

 studies at the proximal end of the string. Of 

 these 10 sets, three showed no decoy loss but 

 did have a mean total dropout loss of 11.8% 

 (Table 14). The remaining seven sets had a 

 mean total dropout loss of 27.3% , but also had 

 a mean decoy loss rate of 36.4% (Table 14). 

 Thus, if we accept a zero loss of decoy fish as 

 evidence of no predation, we have evidence of 

 dropout loss independent of predation. 



Additionally, Thompson et al. (1971) 

 studied the viability of salmon that disentangled 

 from gill nets in a predator-free controlled 

 environment. In the 1968 studies, nearly 46% 

 of the 180 sockeye salmon exposed to the gill 

 nets became disentangled. 



Thompson et al. (1971) also studied the loss 

 of dead salmon which became unmeshed from 

 gill nets during high-seas fishing in 1968-69. 

 These studies involved the placement of 

 identifiable dead salmon in the net (i.e., "gill- 



ing" them in as natural a manner as possible) 

 at the time of set and counting the retained car- 

 casses when the net was hauled the following 

 morning. Although it was not possible to 

 establish whether a missing fish had dropped 

 out of the net passively or had been extracted 

 by an animal, these authors estimated the 

 passive loss by comparing the loss of dead 

 carcasses to the loss of decoy salmon during 

 the same sets. The conditional estimate of 

 passive loss (independent of predation) was 

 12.5%. 



Admittedly, this is not extensive evidence. 

 We must, therefore, attribute the loss of sal- 

 mon from gill nets to both predation and dis- 

 entanglement, and we cannot accurately 

 separate the two. 



It is pertinent, however, to draw analogies 

 between the Japanese mothership fisheiy and 

 our research vessel fishing. Both vessels fish 

 gill nets in a similar manner, and both types of 

 fishing would be subject to predation. The 

 presence of Stellar sea lions in abundance in 

 the area of the Japanese mothership fishery 

 (from the central Aleutians to the Kuril Islands) 

 has been documented (Mathisen, 1959; 

 Tikhomirov, 1964). Estimates of abundance of 

 Steller sea lions ranged up to 40,000 animals 

 in the western Aleutians and up to 20,000 in 

 the Kuril Islands (Kenyon and Rice, 1961; 

 Tikhomirov, 1964). 



Thus the estimates of loss of salmon from 



Table 14. — Summary of loss from gill nets when "decoy" and dropout studies were conducted 



concurrently. 



865 



