114 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



different portions of the salmon runs and upon matters important to their conserva- 

 tion. This section deals with the methods used in forming these modified tables and 

 the reasons for the various modifications that have been introduced. The chief 

 purpose in the original report was to show the contribution that the Rock Island runs 

 make to the commercial catch for different periods and also the intensity with which 

 the run as a whole, and particularly tbe Rock Island component, is being exploited. 

 In that report only the spring runs of chinook and blueback salmon and the steelhead 

 trout were considered. In the present report all of the species of salmon found in 

 commercial quantities in the Columbia River have been included and the data covering 

 the fall season to the end of the year have been considered. Information not available 

 at the time the original report was prepared has, we believe, made possible an improved 

 analysis. Additional facts not pertinent to the original report but bearing on the 

 more general problems of the depletion and conservation of these fishery resources 

 have been introduced. 



Primarily for the purpose of comparing commercial catch with escapement of fish 

 to the spawning grounds, it has been necessary to convert the catch as given in pounds 

 into numbers of fish. Entirely satisfactory conversion factors (average weights) are 

 not available, so that the estimated numbers as given in the following tables cannot 

 be considered as anything more than reasonable approximations. The terminal digits 

 in the figures as given are not, therefore, to be taken as significant. 



In the original report the following conversion factors were used in converting the 

 catch, given as poundage landed, into numbers of fish: For chinook salmon 2 systems 

 were used; (1) an average weight throughout the season of 22 pounds, and (2) an 

 average of 15 pounds during May, 20 pounds during June, and 25 pounds during July 

 and August. For bluebacks also 2 systems were used; (1) an average of 3 pounds 

 throughout the season in all zones, and (2) an average of 3 pounds throughout the 

 season below Bonneville (Zones 1 to 5) and 2}i pounds above Bonneville (Zone 6). 

 For steelhead trout an average weight of 10 pounds throughout the season in all zones 

 was assumed. In general these were in accord with accepted figures. In the present 

 report we introduce no change in respect to the figures used for bluebacks and steel- 

 heads, but have considerably modified our treatment of the ckinooks. 



In another paper (Rich 1940a) the writer has described the seasonal changes in 

 weight of chinook salmon in the commercial catch on the Columbia River during the 

 season of 1939, and the estimated weekly average weights given in that paper have 

 been used in this report to convert poundage to number of fish. The validity of 

 applying the 1939 averages to the 1938 run is perhaps questionable, but appears to 

 us to be by far the most acceptable procedure available. 



It was shown in the paper just mentioned that a satisfactory empirical graduation 

 of the observed weekly mean weights in 1939 is given by the use of two linear equations. 

 Letting ?/=weekly mean weight, z=the week, with origin at the week of July 9, the 

 data for the first part of the season, up to and including the week ending July 9, are 

 fitted by the equation ?/= 30 +1.782, and those for tbe last part of the season, including 

 again the week of July 9, are fitted by the equation y=Z0— 0.55i. Table 11, gives 

 the estimated weights for each week of the spring season as determined from these 

 equations. For this present report, estimated average weights for the weeks previous 

 to the opening of the fishing season on May 1 and for the fall season have also been 

 determined by the dubious method of extrapolation. We fully recognize the dangers 

 of this procedure but, in the absence of any better objective basis for estimate, believe 



