Percent Number 



Coastrange sculpin ... 34 14,348 



Prickly sculpin 18 7,596 



Reticulate sculpin .... 18 7,596 



Torrent sculpin 30 12,660 



Total 42,200 



Estimated number of 

 salmon released .... 



Percentage of salmon 

 release eaten 



Percent 

 •) 



83 

 37 

 39 



Numbe 

 1.8 

 2.6 

 [.4 



1.7 



Number Percent 



1,033 



33,179 



7,869 



16,787 



1 



37 

 13 



Number 

 1.0 

 2.S 

 1.0 

 1.2 



287 



21,077 



2,960 



3,645 



Number 

 1.0 

 1.2 

 1.3 

 1.3 



Number 

 287 



1,640 

 988 



1,646 



58,868 

 1,493,483 

 3.9 



27,969 

 2,235,475 

 1.3 



4,561 

 127,305 

 3.6 



all the requirements; except for number of 

 predators, this number was estimated from 

 catches by electrofishing. 



On the basis of the number of sculpins 

 collected and observed during electrofishing, 

 I estimated that the minimum number of 

 sculpins over 65 mm. long per 30 m. of stream 

 was 100. Thus, the total population of preda- 

 tory sculpins for the lower 13 km. of the Elo- 

 komin River was about 42,000. 



The number of chinook salmon consumed by 

 sculpins in the Elokomin River is estimated 

 (table 9) from the product of the estimated 

 sculpin population, the average daily incidence 

 of predation (table 6), the average number of 

 salmon eaten by sculpins (table 7), and the 

 duration of predation. 



The estimated loss is roughly indicative of 

 salmon mortalities from sculpin predation, al- 

 though it is minimal for at least two reasons. 

 First, populations of sculpins of predatory size 

 are probably greater than the estimate (electro- 

 fishing caught slightly more than 1,000 sculpins 

 over 65 mm. long in 1963). This catch indi- 

 cates that 1/42 of the sculpins of a predatory 

 size were collected. Considering the small 

 area shocked, the short duration of shocking 

 (electric current was on a total duration of 1 

 hour and 50 minutes for the 15 collections), 

 and the inefficiency of shocking for sculpins 

 (Patten and Gillaspie, 1966), it is inconceiv- 

 able this effort removed 1/42 of the larger 

 sculpins from 13 km. of a stream as large as 

 the Elokomin River. Second, predation on a 



few salmon fry continued beyond the 48- or 

 72-hour period of observation because some 

 fry remained in the Elokomin River beyond 

 this time. This study indicates only the losses 

 during the period of the heaviest mortalities. 



Total loss estimates for the respective re- 

 leases of salmon are not directly comparable 

 because of differences in numbers released. 

 A percentage loss computation provides a bet- 

 ter evaluation. Sculpins ate an estimated 3.9 

 percent of the 1962 chinook salmon release, 

 1.3 percent of the 1963A release, and 3.6 per- 

 cent of the 1963B release. 



Prey-dependent factors. — The variation in 

 losses of salmon to sculpins for the different 

 releases (table 9) suggests that prey-dependent 

 factors are important. The factors that should 

 be considered, in addition to the obvious one 

 of rate of downstream migration, are: (1) 

 density, (2) size, and (3) ability to escape 

 predators. 



The intensity of predation by sculpins for 

 the three releases (table 9) showed no con- 

 sistent relation to the density of salmon (num- 

 bers released on a given date). The total 

 mortalities in 1962 were twice those for 1963A 

 despite the longer observation period in 1963. 

 Also, the percentage loss was three times 

 greater for the 1962 release than for 1963A. 

 The difference between the 1963A and 1963B 

 releases was also large. Although the total 

 numbers of fry eaten by sculpins reflected the 



10 



