SUMMARY 



This report focusses on several aspects of the Arctic 

 grayling ( Thymallus arcticus ) population inhabiting the upper Big 

 Hole River drainage in southwestern Montana. The four main 

 aspects include: predation on young of year (YOY) grayling by 

 introduced brook trout ( Salvelinus fontinalis ) , analysis of the 

 substrate composition of six known Arctic grayling spawning 

 sites, use of irrigation ditches in the Wisdom, Montana area by 

 grayling (YOY in particular) , and distribution of Arctic grayling 

 in the drainage. Other aspects noted in the report include: 

 movement of YOY grayling in the stretch of river below the Wisdom 

 cemetery; relative abundance of YOY this summer; and growth of 

 YOY grayling. 



Two types of fry traps (drift and emergence) were placed at 

 the spawning sites in order to provide general data on time of 

 emergence and number of emerging grayling fry. Unfortunately, no 

 fry were captured by any of the traps. 



Arctic grayling YOY had a mean total length of 26.50 mm on 

 June 18, 1990. This rapidly increased to 97.89 mm by the middle 

 of August. YOY mountain whitefish and brook trout had greater 

 mean total lengths than YOY grayling until the end of July when 

 grayling surpassed both other species. 



Stomach samples were obtained through gastric levage from 

 brook trout (177) , burbot (15) , and rainbow trout (5) to 

 determine whether piscivory of YOY grayling was acting as a 

 limiting factor in this system. An Index of Relative Importance 

 (IRI) (George and Haley, 1979) was calculated from the stomach 

 contents data for each of the above three species. The IRIs 

 calculated from the stomach contents indicated that while 

 grayling were not an important food item to any of the three 

 species sampled, fish in general were quite important. This 

 allows for the possibility of predation- induced effects on YOY 

 grayling populations. However, only one YOY grayling was found 

 in any of the stomach samples, so the potential effect is likely 

 small . 



The substrates of the six spawning sites are composed 

 primarily of sand (14% ± 4.2) and gravel (81.5% ± 3.2). There 

 was little correlation between YOY numbers at the spawning sites 

 and the site's substrate composition. 



Two of the three irrigation ditches sampled showed no use by 

 either YOY or adult grayling. The large ditch on Cal Erb's Ranch 

 near Wisdom, did, however, have some use by both adult and YOY 

 grayling. The ditch also supported some larger brook trout. The 

 only incidence of predation by brook trout on YOY grayling was 

 observed in stomach contents of fish captured from this ditch. 



High numbers of YOY grayling were noted in several spots. 

 Swamp Creek, Erb's Ditch, and a double channel below Wisdom (near 

 the mouth of Pintlar Creek) all contained numerous YOY. Adult 

 grayling were most numerous in Swamp Creek and in the east 013 



