11 



Estimates of Trout Population and Biomass 



Between August 1 and September 8, 1982, electrofishing for mark-recapture 

 estimates of trout populations was done in each of the 30 stations selected 

 for study. The electrofishing unit consisted of a 240-volt, 3500-watt alternator 

 and Coffelt control box (150 to 600 volts DC) mounted in a canoe and towed 

 upstream by a crew using two positive electrodes ahead of the unit, a negative 

 electrode trailing in the water behind the canoe — except in very shallow 

 stations, where long-line gear was used instead of the canoe-mounted unit. 

 Collected fish were anesthetized in MS-222 (Tricane Methanesulfonate ) , weighed 

 to the nearest 2 grams, and measured to the nearest 1 mm in length. The lower 

 caudal fin was clipped; when two 300-meter sections were contingous, the upper 

 caudal fin of the upstream section trout was clipped. Fish were carried in 

 large frame nets downstream to the bottom of the station from which they had 

 been caught and released. Two weeks were allowed between the marking and 

 recapture runs. 



The population estimate for each station (and 3-station section) was 

 calculated by the modified Petersen mark-recapture formula of Ricker (1975). 

 Fish were grouped into 20-mm size classes, 100 to 119 mm being the smallest. 

 Where numbers were too low to calculate a reliable pouplation estimate for each 

 species within a station, data were lumped either by species or by combining 

 several stations, and the estimates calculated then reapportioned according 

 to total of fish marked on first run plus the unmarked (new) fish caught on the 

 second run. 



Biomass was calculated by multiplying length-group estimates of trout 

 numbers by mean weight for the group. Mean weight of length group was 

 determined from length-weight regression equations for each species in each 



