15.9 Status of Dolly Varden and Bull Trout in a Changing Water Regime

Last Updated: 
June 10, 2015

This indicator tracks the status of riverine chars, dolly varden (Salvelinus malma) and bull trout (S. confluentus) using changes in abundance and population structure, such as length and age. Hybridization is also monitored, as the occurrence of hybrids may signal significant environmental change. These two species differ from the Arctic char (Salvelinus alpinus) mainly found east of the Mackenzie Delta but most local users in the NWT call them char.

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Bull trout
Bull trout, Salvelinus confluentus. Credit: P. Vescei.

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Dolly varden
Dolly varden, Salvelinus malma. Credit: P. Vescei

                                                             

 

 

 

 

 

 

 

 

Information and analysis for this indicator was provided by C.D. Sawatzky, N.J. Mochnacz, and J.D. Reist. Fisheries and Oceans Canada. Winnipeg, MB based on studies done for the International Polar Year - Climate Change Effects on Char in the Arctic Project. Information was updated based on COSEWIC status reports1,2.

NWT Focus

Char species, such as dolly varden and bull trout, can be used to monitor changes in both the Arctic Ocean and freshwater ecosystems. Dolly varden and bull trout are particularly good fish species to monitor because they have life history stages that occupy different habitats during their lifetime, such as sea-run fish link lakes, rivers, estuaries and near shore habitats. Each species’ life stage is exposed to the same environmental stressors, but may respond differently to these stressors thus increasing our understanding of cumulative effects. Possible responses of char to environmental change include:


i) loss of local biodiversity (e.g. hybridization/introgression would lead to a loss of genetic integrity)
ii) shift in biodiversity (e.g. sea-run fish may switch to freshwater only if local production increases)
iii) shifts in fish population parameters (e.g. growth rate, age-at-maturity, life history, reproductive frequency shift)
iv) loss of local populations due to a) habitat change exceeding thresholds, or b) impact of colonizing species via competition, predation, parasitism or disease

Current status and trend

a. Distribution

Distribution - Bull trout and Dolly varden

Dolly Varden 

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Gayna River, NWT
Gayna River, NWT. Credit: DFO/N. Mochnacz.
Dolly varden are present as anadromous (i.e. sea-run) fish in coastal waters of the Beaufort Sea in the western Arctic primarily west of the Mackenzie Delta during ice-off periods (i.e. summer and early autumn). Offshore distribution is poorly known mostly due to poor sampling efforts. However, our present understanding suggests adults and juveniles mainly occur in nearshore waters in marine areas within the 0-10 m isobaths. Dolly varden use habitat in freshwater streams for spawning, overwintering, rearing, feeding and resting. This species is primarily adapted to flowing waters and is often found in high-gradient rivers associated with perennial groundwater springs. These groundwater sources are essential for northern populations because they maintain winter habitat and provide high-quality spawning habitat. However, these areas are spatially limiting in most streams. In the NWT, anadromous dolly varden occupies drainages of the Big Fish, Babbage, Rat and Vittrekwa Rivers. The furthest known upstream location (and currently the only known site upstream of Point Separation in the Mackenzie Delta) where this species is found in the Mackenzie River basin is in the Gayna River3,4.

Bull Trout

Bull trout from interior North America (i.e. Idaho to the NWT) only use freshwater environments to carry out their life cycle. They are distributed in the western portion of the NWT in Mackenzie River drainages north to the central Sahtu Settlement Area. To date, the northernmost record of bull trout is in the Gayna River. This is the only location where bull trout and northern dolly varden are known to be sympatric. Additional sampling in suitable habitats surrounding this area is required to confirm and extend this understanding3,4. This species utilizes similar habitats to those of dolly varden in high-gradient freshwater streams, however, populations are also found in larger slow-flowing rivers such as the Mackenzie mainstem, and occasionally in lakes in the NWT.

b. Hybridization/Introgression

There is presently no evidence (genetic or morphological) for hybridization of bull trout and dolly varden in the NWT, which suggests if this event occurs it is very infrequent3. Accordingly, the genetic integrity of the two species is intact in this area and occurrence of hybrids might signal significant environmental change.

c. Fluctuating Asymmetry

Fluctuating asymmetry (FA) in paired structures (e.g. different counts for left- and right-side fin rays) has been noted as a potential, but somewhat controversial, indicator of environmental and genetic stress in fish populations5. FA may occur when development is disrupted as a result of either environmental change, genetic stress or a combination of both. The frequency of occurrence of this event is currently being investigated in northern Canadian populations of dolly varden and bull trout. Both low asymmetry and stable temporal patterns of symmetry/asymmetry suggest minimal environmental stress on the fish.

d. Abundance

Dolly Varden
 

The Rat River (67.62°N, 134.87°W) is the only population that has been periodically studied for population size during the past 10 years. Mark-recapture population estimates exist for between 1989 and 19986. Population estimates up to 2007 are forthcoming.

Rat River (Fish Creek), NWT
Rat River (Fish Creek), NWT. Credit: DFO/N Mochnacz

The dolly varden in the Rat River is the only population that has been periodically studied for population size during the past 10 years. In 1998, the population numbered about 15,000. Prior to 1999, there was evidence from the fisheries data at Rat River of loss of larger/older fish, suggesting population-level effects. Recovery in later years (e.g. 1999 and 2000) was thought to be related to higher growth rates due to above average productivity in 1998 and/or the effect of decreased fishing pressure.

Population estimates
Mark-recapture population estimates (with 95% confidence limits) for the Rat River dolly varden population. Source: DFO.

Bull Trout

Preliminary work suggests that most populations are relatively small, widespread, and perhaps fragmented3.

Density and growth estimates for juvenile bull trout in Funeral Creek (~ 61.60°N, 124.82°W) have been calculated. The estimated average annual growth of juveniles captured (age 0-5 years) was 27.9 mm/yr in length and 14.2 g/yr in weight. However, after age five the average annual weight gain increased to approximately 60 g/yr. The density of juveniles captured in two different reaches from Funeral Creek during September 2001 was 5.6 and 6.7 fish//100 m2, respectively8. This would be a suitable location for future monitoring.

e. Population Structure (Length & Age)

Dolly Varden

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Length-frequency distribution
Length-frequency distribution of current year non-spawning Rat River dolly varden caught in subsistence fisheries at Destruction City (on the Rat River) from 1986-1994, and at five monitoring sites (including Destruction City) from 1995-2000. Source: DFO.

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Age-frequency distribution
Age-frequency distribution of Rat River dolly varden caught in the subsistence fisheries at Destruction City from 1986-1994, and at five monitoring sites (including Destruction City) from 1995-2000.

Truncation in size and/or age structure, particularly loss of larger/older fish (e.g. 1997) suggests population-level effects. Recovery in later years (e.g. 1999 and 2000) suggests relaxation of perturbing factors.

Bull Trout

Although sample sizes are low, data suggest that non-migratory (i.e. stream-resident), and migratory (adfluvial and fluvial) populations occur in the area1. Non-migratory fish are smaller once sexually mature and grow slower than migratory fish.

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Length-at-age plot of non-migratory and migratory bull trout from the NWT. Points to the right of the straight and dashed lines represent mature non-migratory and migratory fish respectively.

Looking Around

The southern taxon of dolly varden (S. malma lordi) is secure throughout its range (southern Alaska, British Columbia to Washington), and the northern taxon (S. malma malma) is secure throughout Alaska, however, it appears to be stressed in north-western Arctic Canada1.

According to NatureServe10 rankings within the continental USA, all five distinct populations of bull trout (ranked as vulnerable as a species) rank as critically imperiled (n=1) or imperiled (n=4). The status of bull trout populations in the NWT is poorly understood, although most populations appear to be small and have specific habitat requirements (e.g. perennial groundwater springs) associated with higher order stream reaches. COSEWIC recently designated the Alberta-NWT unit of bull trout and the Yukon-NWT unit of dolly varden as Special Concern (COSEWIC 2012).  

Virtually all stressors that are known to affect fish populations generally have been documented as affecting chars, a group which appears to be particularly susceptible to both local (e.g. exploitation) and pervasive (e.g. climate change) stressors as well as individual and cumulative effects of stressors. Additional stressors include habitat degradation, industrial development, and fragmentation of watercourses and species introductions. Southern populations (south of 60°N) of chars appear to be at greater risk overall as evidenced by higher levels of conservation concern. Two conclusions result: i) southern populations of chars, particularly those isolated in lakes or requiring unperturbed river habitats are at acute risk, and given their probable evolutionary history represent an irreplaceable component of biodiversity of the char group; and ii) southern populations are useful proxies of potential future effects and issues facing northern chars. Accordingly, appropriate care in addressing conservation, management and stressors of both chars and their ecosystems is required, particularly as wide-reaching changes occur throughout the north12.

Looking forward

Possible responses of char to environmental change include:

i) loss of local biodiversity (e.g. hybridization/introgression would lead to a loss of genetic integrity)
ii) shift in biodiversity (e.g. sea-run fish may switch to freshwater only if local production increases)
iii) shifts in fish population parameters (e.g. growth rate, age-at-maturity, life history, reproductive frequency shift)
iv) loss of local populations due to a) habitat change exceeding thresholds, or b) impact of colonizing species via competition, predation, parasitism or disease

Find Out More

Other focal points

  • See WATER for indicators on water quality and quantity. 

Technical notes

  • Methods for determining the distributions of these species are outlined in Sawatzky et al. (2007)11 and Mochnacz et al. (2013)3.
  • Rat River population estimates (Petersen method) were based on tagging dolly varden that were collected by seining at the overwintering area and recapturing fish the following year through the subsistence fishery, with the exception of 1989 when electro-shocking was used to recapture the fish6.
  • Funeral Creek abundance estimates were based on the Zippin three-removal method1. Three consecutive electrofishing passes were performed in an upstream manner and the number of bull trout captured during each pass was recorded. The maximum-likelihood population size was calculated based on the number of fish captured on each electrofishing pass1.

Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


References:

Ref. 1. COSEWIC. 2011. COSEWIC Assessment and Status Report on the Dolly Varden Salvelinus malma (Western Arctic population) in Canada. 65pp.

Ref. 2.  COSEWIC 2013. COSEWIC Assessment and Status Report on the Bull Trout Salvelinus confluentus in Canada. 103pp.

Ref. 3. Mochnacz, N.J., R. Bajno, J.D. Reist, G. Low and J.A. Babaluk. 2013. Distribution and Biology of Bull Trout (Salvelinus confluentus) in the Mackenzie Valley, Northwest Territories, with Notes on Sympatry with Dolly Varden (Salvelinus malma). Arctic 66(1): 79-93.

Ref. 4. Reist, J.D., and C.D. Sawatzky. 2010. Diversity and Distribution of Chars, Genus Slavelinus, in Northwestern North America in the Context of Northern Dolly Varden (Salvelinus malma malma (Walbaum 1792). Canadian Science Advisory Secretariat (CSAS) Research Document.

Ref. 5. Leary, R.F., and F.W. Allendorf. 1989. Fluctuating Asymmetry as an Indicator of Stress: Implications for Conservation Biology. Trends in Ecology and Evolution 4:214-217.

Ref. 6. Fisheries and Oceans Canada. 2001. Rat River Dolly Varden.

Ref. 7. Harwood, L.A. 2001. Status of Anadromous Dolly Varden (Salvelinus malma) of the Rat River, Northwest Territories, as Assessed Through Community-based Sampling of the Subsistence Fishery, August-September 1989-2000.

Ref. 8. Leary, R.F. and F.W. Allendorf. 1989. Fluctuating Asymmetry as an Indicator of Stress: Implications for Conservation Biology. Trends in Ecology and Evolution 4:214-217.

Ref. 9. Monchnacz, N.J., J.D. Reist, G. Low, R. Bajno, and J.A. Babluk. 2010. Sympatric Bull Trout (Salvelinus confluentus) and Dolly Varden (Salvelinus malma) in the Mackenzie Mountains, Northwest Territories, with updated notes on distirbution and biology of both species. Arctic (In Prep).

Ref. 10. NatureServe. 2008. NatureServe Explorer: An Online Encyclopedia of Life [web application]. NatureServe (7.0). Arlington, Virginia. 

Ref. 11. Reist, J.D. and C.D. Sawatzky. 2009. Diveristy and Distribution of Chars, genus Salvelinus, in northwestern North American in the context of northern dolly varden (Salvelinus malma malma) (Walbaum 1792) In Prep. Canadian Science Advisory Secretariat (CSAS) Research Document.

Ref. 12. Reist, J.D., and C.D. Sawatzky. 2009. The State of Char in the Arctic. Arctic Report Card 2009.

Ref. 13. Sawatzky, C.D. et al. 2007. Distributions of Freshwater and Anadromous Fishes from the Mainland Northwest Territories.

Ref. 14. Zippin, C. 1959. The Removal Method of Population Estimation. Journal of Wildlife Management 22:82-90.