112 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



was actively in charge of the count. This method assumes that the fish are passing 

 over the ladders for 12 hours per day at the same average rate as observed during the 

 period of the count and has been applied to each ladder separately and the sum is the 

 estimated total for the day. For periods during which no count was made a linear 

 interpolation between the preceding and the following days' estimated counts was 

 used. Although not comparable in accuracy to the actual count, these estimates 

 appear to give a reasonable basis for further calculations. 



A chief source of error in these counts and estimates is undoubtedly the identifi- 

 cation of species as the fish were passing up the ladders. After May 7, when the actual 

 count began, the fish were forced to pass through a small opening in a weir placed 

 across each fish ladder and over a submerged platform painted white. Identification 

 of species under these conditions can be made with some accuracy by careful observers 

 and, in general, reasonable confidence can be placed in the identifications so made. 

 Those made under less favorable conditions must, necessarily, be accepted as the best 

 available. Circumstances may arise in which a particular misidentification is espe- 

 cially likely to occur, in which case it may be recognized and steps taken either to 

 improve the identification or to determine its influence and allow for it in the estimates 

 of the number of fish of the species confused. 



It is apparent that one such particular case of misidentification might easily 

 arise during the time when the blueback run is at its peak. Grilse, which are approxi- 

 mately the same size as the bluebacks, are among the chinooks and run at the same 

 time, and it has seemed likely that bluebacks might be mistaken for grilse or grilse for 

 bluebacks. An analysis has been made in which the correlation was determined be- 

 tween the percentage of grilse in the total count of chinooks and the number of 

 bluebacks for the 10 weeks of the blueback run — June 11 to August 13. 



The Pearsonian coefficient of correlation is —0.72. Using the standard procedure 

 the probability of chance occurrence of a coefficient of correlation as high as this is 

 only 0.03, so that the observed negative correlation between the percentage of grilse 

 and the number of bluebacks can be accepted as significant. Furthermore, it seems 

 likely that the relationship between these two variables is curvilinear rather than 

 rectilinear, as assumed by the Pearsonian coefficient, and that a true measure of the 

 correlation would be even higher than that calculated. Our measure is, therefore, 

 conservative. It seems quite likely that this negative correlation can be ascribed to 

 a tendency on the part of the observers to mistake grilse for bluebacks when the blue- 

 backs are numerous. 



This raises the question as to what other errors there may be in the counts. It is 

 certainly difficult to distinguish species under the conditions of counting unless there 

 is a fairly well marked difference in size, shape, or markings, especially if light condi- 

 tions are not favorable. Observers should not be blamed for making errors under 

 these conditions, but, in view of the evidence of error in identification just given, it 

 would seem proper to investigate carefully to see how extensive these errors may be. 

 The importance of having properly trained and experienced observers is obvious. 



