FISHERY BULLETIN: VOL. 69, NO. 4 



used. Since the fish in the escapement average 

 smaller than in the catch, it will result in an 

 overestimate of the total run for this year. 



Unquestionably, the computed escapements 

 are subject to many sources of error, and they 

 reflect only the general magnitude of the escape- 

 ments. In genera! there are some measurements 

 from each fishing period that can be weighted by 

 the corresponding catches, and any unrepresent- 

 ativeness of the sampling was in part corrected. 

 It therefore appears that the greatest bias arises 

 from the way in which fish were selected and 

 measured. In 1930, for example, there were few 

 measurements taken, and they included a rather 

 high proportion of suspiciously small 2-ocean fe- 

 males, which resulted in the rather large esti- 

 mated total escapement. Almost 12,000 mea- 

 surements were made in 1937, but largely of fish 

 from the resident set net fishery near the upper 

 fishing boundary. The result is an underestimate 

 of the mean average length in the commercial 

 catches, since the run at this point had been sub- 

 jected to the selection of the drift net fishery; 

 the calculated escapement is substantially in- 

 flated. In 1939 no 2-oceaii fish were measured 

 in drift net catches, and therefore the low rate 

 of escapement may be substantially correct. 



The International North Pacific Fisheries 

 Commission (1962) has published estimates of 

 Nushagak escapements for the period considered 

 here. A fishing rate common for all size groups, 

 and with no distinction between males and fe- 

 males, was computed from Bristol Bay catch and 

 escapement data for 1955-1957. Furthermore, 

 nylon gill nets were used and were operated 

 from power boats. Because of the selective ac- 

 tion of the gill nets for males and females, and 

 for 2- and 3-ocean fish, it is easy to understand 

 that these estimates are entirely different from 

 those presented here. 



SUMMARY OF RESULTS 



On the previous pages, escapement levels were 

 calculated for the three distinct periods of the 

 Nushagak fishery shown by the catches on Fig- 

 ure 2. These results have been summarized in 

 Table 8. 



Table 8. — Rate of exploitation in three periods of the 

 Nushagak fishery. 



Average 

 escapement 



Average 

 catch 



Exploitation 



1908-1919 

 1925-1945 

 1946-1966 



Thouiandl 

 1,126 

 1.812 

 1,385 



Thoujattds 

 A.TX, 

 2,905 

 1,183 



% 

 81 

 62 

 48 



During the early period of the fishery, the 

 runs sustained a fishing mortality of more than 

 80''7 until 1919 when all runs to Bristol Bay 

 suff'ered a drastic decline. The universality of 

 this decline in many sockeye salmon systems 

 suggests that the causes must be sought in 

 changes in the environment and not in the mode 

 of fishing operation. The Nushagak runs never 

 returned to their former level, in contrast to 

 those of the other systems in Bristol Bay, notably 

 those to the Kvichak River. 



During the middle period, here defined as the 

 time from 1925 to 1945, the amplitude of the 

 year-to-year oscillation increased (Figure 2). 



Following the last World War, not only did 

 the Nushagak and other Bristol Bay sockeye 

 salmon runs decline, but many of the Kamchatka 

 .salmon runs did too (Krogius and Krokhin, 

 1956). The widespread decline suggests again 

 that environmental and probably oceanographic 

 conditions not related to fishing depressed the 

 survival. In the third period of the Nushagak 

 fishery the runs remained at a very low level, 

 compared with levels of the two previous periods. 



Concomitant with this stepwi.se decline in aver- 

 age yield, there has been a decrease of the re- 

 productive potential of the Nushagak sockeye 

 salmon runs. Whereas during the early period 

 of the Nushagak fishery, the runs were exposed 

 to an ex|)!oitation rate of nearly BO'';, during 

 the middle period of the Nushagak fishery, the 

 runs were exposed to an exploitation rate of 

 around 60%. During the last period, the exploi- 

 tation rate was around 50''; . largely set by the 

 regulation. The runs are maintaining them- 

 selves, but so far no substantial increase is 

 apparent. 



Thus the rate of return per sjiawner has fallen 

 from five to less than three and finally to two 

 mature fish. As a result, there has been no in- 

 crease to former run levels in spite of the reduced 



758 



