the cooperation of the U. S. Forest Service, provides 

 discharge rates and stream drainage areas, and other 

 information about a number of important salmon 

 streams. 



U. S. Forest Service. Data on stream characteristics 

 and salmon escapements are available on several 

 streams in records of studies conducted by this agency 

 on the effects of logging on the physical makeup of 

 streams. 



U. S. Geological Survey. Aerial photographs from 

 this agency are the primary source for measurements 

 of stream distances and areas and for valley features 

 not visible from the ground. These photographs, 

 which are of most of the streams in Southeastern 

 Alaska, were made by the U. S. Navy (Patrol Squad- 

 ron Four) in 1948. 



Local residents. Another source is the descriptive in- 

 formation on several major streams provided by local 

 residents. 



UMITATION OF DATA 



Escapement estimates obtained by visual 

 means are often limited in accuracy because fish 

 are not seen in turbid water, under overhanging 

 streambanks, or in areas inaccessible to observers. 

 Actual counts throughout the duration of the salmon 

 run past a counting weir or tower are relatively ac- 

 curate estimates of total escapement. However, it 

 is not economically feasible to establish a weir on 

 each stream, and escapement surveys are the only 

 source of information for a large part of the area 

 which must be covered. The value of the catalog 

 as a history of the salmon escapements can be real- 

 ized only if its limitations are fully known. 



Escapement Estimates 



Escapement estimates do not indicate the 

 actual total escapement. At no time are all the 

 salmon in the stream simultaneously since the 

 spawning run extends over a period of weeks. There- 

 fore, each escapement estimate is an index of the 

 relative abundance at the time of survey. 



The maximum estimate determined by survey 

 methods at about the peak of the run is used as an es- 

 timate of the relative abundance of the total escape- 

 ment. Reliable indices of relative abundance from 

 year to year can be made only if the surveys are com- 

 parable. Evaluation of the following factors is 

 necessary to determine the accuracy of the escape- 

 ment estimates. 



Observers. --The escapement records are 

 from many different observers. Variability in 

 estimating the number of salmon in a given area by 

 different observers should be considered in judging 

 the accuracy of the data. In general, with more 

 observers variability increases. 



Survey systems. — Different survey systems 

 have been used by the various agencies. Reliability 

 of the escapement estimates varies with the systems 

 used. 



Survey systems that employ standard count- 

 ing techniques over standard distances are the most 

 reliable method now available for comparison of 

 abundance between years, particularly when 

 streams can be only partially covered. Standard 

 survey distances in comparatively long streams 

 were not widely used prior to 1949. 



Survey systems requiring periodic visits 

 to each stream during the spawning period are more 

 reliable for estimates of peak abundance than sys- 

 tems requiring only one or two visits. The peak 

 period of abundance in a salmon stream is usually 

 relatively short, and one or two visits may miss 

 the peak. 



Type of survey. — Two basic methods for 

 covering the streams during escapement surveys 

 are being used. 



The oldest method is the ground survey in 

 which the observer follows the stream course on foot 

 or in a skiff with an outboard motor. Most parts of 

 the stream can be closely observed by this type of 

 survey. 



The newer method is aerial survey. This is 

 a fast, economical means of covering a large num- 

 ber of streams in a short period of time over stream 

 distances greater than is possible on the ground. This 

 method requires experienced personnel familiar with 

 ground surveys as well. 



Aerial surveys are best suited for large 

 rivers and streams where ground coverage is limited 

 usually to the lower portion of the stream near the 

 banks. Ground surveys are more reliable than aerial 

 surveys on small streams that offer poor visibility 

 from the air. 



Observation conditions. --Weather is an 

 extremely important factor in the reliability of 

 escapement estimates. During flooding, ground 

 surveys can be made only with great difficulty. 



