Brook Trout; A Management Conundrum in Swedish Streams
With their bright red markings with blue borders the brook trout (Salvelinus fontinalis) is a charismatic fish popular with Swedish anglers.
Brook trout are also often a species of interest with SEG and for good reason. Brook trout inhabit a variety of habitats from small streams and ponds to larger rivers and lakes favouring the colder waters of eastern North America. The wide habitat preference and diverse life cycles have arisen from several distinct genetic lineages of the species including some anadromous populations colloquially called “salters” in the eastern United States. Most populations will remain in freshwater systems; however, growth and development can vary greatly with habitat size and water temperature. Brook trout inhabiting warmer smaller streams are smaller (maxing out at 300mm) and typically mature in 2 years but as little as 1 year with an average life span of 5-7 years. Larger colder lakes and rivers have been found supporting larger brook trout (4-4.5 kg and up to 9-10 years). Spawning occurs in gravel beds of streams with ground water percolation or spring fed areas of lakes. After emerging in earl spring (February to April) young brook trout will progress from feeding on plankton to insects and just about anything smaller that drifts by them. Their diet is wide due to drift feeding and contains a significant terrestrial component from insects falling into streams.
Brook trout in their native range represent a conservation concern with human modifications to habitats, unregulated harvest in the late 1800’s, dams preventing instream migration, siltation of gravel beds and warming water temperatures has resulted in widespread decline in brook trout populations. Conservation of native populations is fraught with competition with invasive species, further fragmentation of populations and habitat, range contraction through climate change and difficulties with brook trout historical range covering multijurisdictional boundaries.
In contrast to the decline in their native range, the North American native is a relative newcomer being introduced to Swedish and European water ways. Introduced in the late 1800’s, they have proliferated to the point of being considered one of Europe’s top invasive aquatic species (Savini et al. 2010). Of special concern is the effect on native brown trout having replaced brown trout in several boreal lakes of northern Sweden and reducing brown trout populations in many southern streams and lakes. This competition with brown trout impacts not only brown trout and their structural role in fresh and saltwater systems, but also the endangered freshwater pearl mussel (Margaritifera margaritifera). Freshwater pearl mussels in Swedish water ways require brown trout hosts for glochidia to parasitize for development and dispersal. Freshwater pearl mussel glochidia are not able to effectively parasitize the new (in evolutionary terms) brook trout and studies have found that encystment on brook trout gills often fails or results in smaller less viable juvenile mussels.
Success of brook trout in Swedish water ways is attributed to numerous factors such as early maturation, larger size at hatching and behavioral changes such as increased aggression and piscivory in nonnative systems. Further complications come from a ‘habitat disturbance paradox’, niche convergence resulting in more direct competition with brown trout and evolutionary mechanisms such as the enemy release hypothesis. The habitat disturbance paradox seen with brook trout is nonnative populations exhibiting increased tolerances to temperature changes and habitat disturbance that have resulted in extirpation in their native range. Enemy release hypothesis is the idea that invasive species are less impacted by enemies than native species; the parasitism of brown trout by freshwater pearl mussel but not brook trout frees brook trout from the fitness cost of co-existing with the mussel.
The way that brook trout management represents two very different aspects of conservation biology shows the many dilemmas biologists and stakeholders face. Outside of their native range brook trout represent an invader; highly resilient to removal and impacting multiple species and many stakeholder interests. Inside their historical range they are a species pushed to extirpation across vast areas due to a plethora of invasive species, human modification to habitat, and climate change.
Written by Mitch Olson, Master’s student with SEG
Mitch is a Canadian biologist who has previously worked across western Canada for 4 years as an environmental consultant before attending the University of Gothenburg to further his education. He is currently a second-year maters student in conservation biology in the Department of Biology and Environmental Sciences at the University of Gothenburg. He is currently completing a masters thesis under the supervision of Johan Höjesjö. His thesis is attempting to correlate brook trout removal method with behavioral and phenotypic characteristics.