Quantification of the Response of Brown Trout (Salmo trutta) to Habitat Modification by Reintroduced Eurasian Beaver (Castor fiber): Implications for River Management in Great Britain

Abstract

The recovery, following widescale persecution, of the Eurasian beaver Castor fiber has been ongoing throughout Europe since the 1920’s with the first reintroduction occurring in Sweden with a pair of animals from neighbouring Norway. Great Britain was slow to follow with the first licenced ‘trial’ reintroduction occurring in Western Scotland in 2009. Since then, the impetus to reintroduce, deliberately and inadvertently, has increased with wild populations now established in Scotland, England and Wales. This thesis assessed the impacts of habitat modification by reintroduced Eurasian beaver on a population of brown trout, Salmo trutta, in Northern Scotland which is an economically important salmonid species in Great Britain. The study site was selected due to its established beaver territory with multiple dams and its recognised population of brown trout. The site consisted of two stream tributaries that fed a common lake, one modified by beavers (beaver modified) and one that remained unaltered during the study period (control). Electrofishing results demonstrated that trout in the beaver modified stream (n = 990) were larger, comprised a greater variety of age-classes and had a greater abundance of age 1+ trout (n = 842), whereas the control (n = 434) supported high densities of 0+ trout but held very few 1+ individuals (n = 84). Trout in the beaver modified stream exhibited positive growth during all seasons and exceed the predicted growth based on an optimal growth model during both winter periods. Using PIT telemetry installed at a series of four beaver dams on the same tributary as above, passage efficiency and migratory delay was quantified over two successive spawning season (2015 and 2016). Passage success varied between dams, was highly correlated to rainfall (24 hr lag applied and used as a proxy for discharge) and fork length, with larger fish passing more frequently. Predictive models indicate that migratory delay is reduced if the fish are larger and if an individual has previously passed a dam, passing much quicker on subsequent passage events. By means of remote cameras, piscivorous predator presence was assessed, using grey heron as the model predator. Heron images per riverbank length were greater in the beaver modified stream in comparison to control sites, varied seasonally with greatest images / riverbank metre observed during autumn and spring respectively and was positively correlated with age 1+ trout. The results of this study suggest that under certain circumstances and river systems the presence of beavers may enhance and support a greater diversity of brown trout life stages, through creation of habitat complexity and could be used as a natural management tool to enhance trout populations. Passage over beaver dams during average and high flows should not be a management concern, however passage may be hindered in periods of low flow and during these times management intervention may be required. The habitat changes brought about by the activities of beavers, may re-distribute piscivorous predators within a local area, but further research is required in order to quantify the impact, if any, on fish stocks. The findings presented in this thesis provide the first ever investigation into the influence of beaver habitat modification on brown trout in a GB context. They demonstrate a myriad of complex ecological interactions that occur following the activities of an ecosystem engineer and they significantly enhance our understanding of how fish respond to these habitat alterations and provide crucial insights into the possible issues which may arise and the possible management implications for the future.

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Identifiers

ORCID for Robert James Needham: orcid.org/0000-0003-4639-6031

ORCID for Paul Kemp: orcid.org/0000-0003-4470-0589

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