Adjust for recovery efficiency . - This adjustment applies the magnet 

 efficiency records and also incorporates a calculation taking into account 

 any amounts of fish run through plants when or where magnets were not in 

 operationo To some extent a differential adjustment for reooveries from 

 reduction of whole fish and reduction of canning offal is feasible from 

 a comparison of returns from the two categories wherever plant records per- 

 mit segregationo Since canning is not practiced in T/ashington and Oregon, 

 this feature is not a problem there, but it exists in British Columbia as 

 ■well as in California. 



Adjust for intensity of fishing . - One essential statistic sought 

 through the tagging technique is the catch-mortali tvo If it were possible 

 to release all the tagged fish immediately prior to the fishing season 

 and if they were immediately distributed at random through the commercially- 

 fished population, this statistic would be the ratio of first-season re- 

 turns to the number tagged. Another essential statistic, total stock, 

 could then be computed simply by the proportionality? 



number of fish tagged = nximber of returns 

 ' toTal stocTE totaT~catch 



but tagging can be done only at intervals during the fishing season and 

 the tagged fish probably diffuse only gradually through the general stocko 

 Hence, first season's returns are almost useless and it is necessary to 

 deduce from subseauent years' returns what the first year's returns would 

 have been under the simple conditions described aboveo That deduction 

 is made by extrapolating back to the first year a line representing the 

 ar-nual rate of decline of tag returns. Since the annual returns depend 

 in part on the amount of fishing done, some adjustment must be made when 

 the amount of fishing changes during the series of years included in the 

 tagging experimento 



The adjustment used for the California statistics has been to 'oom- 

 pute for each season the number of returns per unit number of fish caughto 

 An identical adjustment would be feasible also for the Pacific Northwest 

 tagging returns. While this adjustment may suffice for useful approxima= 

 tions, it obviously gives identical treatment to a fluctuation in catch 

 whether due to a change in the amount of fishing or in abundance of the 

 sardines. Yet these two phenomena have different effects on the returns. 

 Furthermore, the fishing in a current year^ by removing a certain number 

 of tagged fish, influences the returns of subseouent years. Except 

 therefore, in situations where it may be safely assumed that the fluctua- 

 tions in catch have arisen only from moderate and random changes either 

 in intensity of fishing or in abundance, this adjustment may requir® re^- 

 vision. Where a trend exists in fishing intensity or in abundance there 

 would be particular likelihood of erroneous results. Such cases would 

 require additional adjustments appropriate to the particular circumstances 

 and possibly materials from the vital statistics methods could be drawn 

 upon for such analyses o This has not been indicated in the diagrsimo 



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