North Saskatchewan and Ram Rivers Bull Trout Spawning Stock Assessment
Mike Rodtka, Chad Judd, and Kevin Fitzsimmons
Bull Trout (Salvelinus confluentus), Alberta’s provincial fish and a native sport species, have declined significantly in abundance and distribution throughout Alberta. Despite being a priority species for management, critical Bull Trout habitats and migratory behavior of the species in the North Saskatchewan River drainage is relatively unknown. Our study area included the North Saskatchewan River from the Bighorn Dam downstream to the community of Rocky Mountain House, as well as along the lower reaches of the Bighorn, Clearwater and Ram rivers. In a 2007 pilot study we confirmed angler reports of large, migratory Bull Trout using Fall Creek, a Ram River
tributary, to spawn. Following this, our study objectives were to determine the timing, magnitude and location of Bull Trout spawning in Fall Creek. Additional objectives included ascertaining the overwintering location(s) of the stock, assessing the use of Fall Creek by juvenile Bull Trout and examining the relatedness of the stock to putative migratory stocks in the Bighorn, North Saskatchewan and Ram rivers.
We used a combination of migration trapping, redd surveys, telemetry, abundance estimation and genetic techniques to achieve our study objectives. We operated a fish fence and trap at the mouth of Fall Creek to capture out‐migrating Bull Trout. We performed redd surveys on the length of Fall Creek that is available to migrants. A total of 27 migrants captured in the Fall Creek fish trap were implanted with radiotransmitters and manually relocated monthly using a rotary wing aircraft. We electrofished nine 250‐m sites systematically distributed along Fall Creek and modeled abundance of juvenile (70‐300 mm fork length; FL) Bull Trout in the stream. In addition to our catch at the trap, angling and electrofishing gear was used to capture Bull Trout ≥200 mm FL for collection of tissue samples (adipose fin clip) for microsatellite DNA analysis.
Bull Trout spawning activity in Fall Creek peaked around the third week of September and was essentially complete by the first week of October. We observed 30, 55 and 58 Bull Trout redds in Fall Creek in 2007, 2008 and 2009, respectively. The majority of redds (≥85%) were consistently observed in the uppermost 1.5 km of stream that is available to migrating fish. The 25 fish consistently located using telemetry gear moved directly to overwintering locations in the Clearwater (n=2), North Saskatchewan (n=7) iv and Ram (n=13) rivers, or their confluence (n=3). Overall the distance fish migrated was relatively short (2‐74 km) and movement over the winter months (November to March) was minimal (0‐3 km).
We estimated that 3,981 (95% CI=1,331‐13,206) juvenile Bull Trout inhabit Fall Creek. Size distribution ranged from 41‐680 mm FL and most likely included young‐of‐theyear, juvenile and adult fish. Tissue samples collected from Bull Trout in Fall Creek and the lower Ram River were sufficient (n>30) for population‐level analysis of genetic diversity and divergence. Samples obtained from these streams were significantly different from one another genetically (Ө=0.038, P<0.001) indicating to a large extent that these are demographically independent populations. Of these populations only
one fish could be excluded from the Fall Creek population and it was inferred to be an immigrant from the lower Ram River population, indicating a high level of spawning site fidelity. Despite considerable effort, Bull Trout catch rates in the Bighorn and North Saskatchewan rivers were very low and only 8 (includes fish < 200 mm FL) and 6 tissue samples were obtained from these streams, respectively.
Study results clearly indicate that Fall Creek is a key spawning and rearing stream for Bull Trout in the North Saskatchewan River drainage. Industrial and recreational activities are pervasive throughout much of the Fall Creek watershed. While it is unclear what impact these activities have had on the population, it is clear that the opportunity for future negative impacts is considerable and continued monitoring of the Bull Trout population is advised. Based on the results of our tissue sampling effort it appears that Bull Trout may be less abundant and more restricted in distribution in
the study area than is commonly assumed although a more comprehensive evaluation is necessary to confirm this. Overwintering habitat in the North Saskatchewan River upstream of the mouth of the Ram River is limited by shallow water and anchor ice but increases downstream toward Rocky Mountain House. As a broad diversity of factors limit the maintenance and recovery of Bull Trout within the study area, basic life history and habitat‐use information collected during studies like this is essential for the sound management of the species.