Scottish Marine and Freshwater Science Volume 6 Number 4: A Mapping Study of the Overlap of Potential Eurasian Beaver (Castor fiber) Habitat and Atlantic Salmon (Salmo salar) Distribution in Scotland

4. Discussion

The present study clearly demonstrates that a wide overlap currently exists between potential beaver habitat and known salmon distribution: in the order of 47-73% across six rivers of varied geographic location and topology. There was a consistent trend of more beaver-salmon habitat overlap in major than minor rivers. This variation largely reflected differences in the extent of riparian tree cover, and the prediction of little colonisation by beavers into steeper gradient streams at higher altitudes.

4.1 Biases in estimates

Any model of beaver distribution is speculative because the way that the animals actually use the Scottish landscape would only become clear should they be formally reintroduced and permitted to colonise suitable habitat. However, given assumptions, suitability of rivers for beavers can be estimated from knowledge of habitat use (Allen, 1983) and applied effectively using GIS (Anderson & Bonner, 2014) to predict the range of the animals. The putative suitable beaver woodland dataset used in this project is based on known parameters for the Eurasian beaver in a European context. However there are several potential biases. In practice, beaver distribution would be variable over time because colonies commonly abandon ponds before re-establishing years later (Pullen, 1971). This phenomenon would introduce a positive bias in the overlap estimates. The suitable beaver woodland dataset presently does not include woodland close to streams of greater than 2% gradient. Eurasian beavers have been observed to exist on stream gradients of up to 2.5% (Schulte, 1989), and North American beavers may use streams of up to a 6% gradient (Pollock et al., 2014). This factor may have resulted in systematic underestimation. Further, in future decades overlap will likely increase as riverine tree planting schemes generate bankside woodland in upland areas. Conversely, although beaver foraging activity may occur up to 100m from water, in Denmark for example, the majority (95%) occurs within 5m (Elmeros et al., 2003). Therefore, the beaver dataset, derived using a 200m buffer, may generate a systematic overestimate. Understanding of the present distribution of salmon is obtained from extensive field survey work, and can be expected to provide a good coverage of the distribution of salmon as of 2008. However, salmon distribution will change, for example as man-made barriers are removed. In view of these uncertainties the results should be considered indicative of the predicted general, rather than precise, level of overlap.

4.2 Scottish salmon populations: status and structuring

The relevance of overlap in salmon and beaver distributions can be understood through consideration of the state of salmon populations in Scotland and the mechanisms by which the two species may interact. The overall strength of the Scottish salmon stock (all populations combined) has declined markedly in the last fifty years due to increased mortality at sea (Anon, 2014). A reduction in international and coastal net fisheries has generally offset this change at the whole-Scotland scale to maintain or increase stocks entering rivers to support rod fisheries and provide a spawning escapement. However, little scope remains for further such compensation. Furthermore, serious reductions in populations have occurred at more local scales. Atlantic salmon are geographically structured within the larger rivers into genetically distinct groups with significantly different phenotypes (Stewart et al., 2006). Those salmon from upstream tributaries tend to leave and return earlier in the year and support a "spring fishery" which is both highly valued for sport and at highest conservation risk. There is evidence that some such populations have approached a level that is sufficiently low that further reduction in spawning will have a direct effect on the numbers of emigrants they produce and hence the next generation of returning adults (Anon, 2014; Jonsson et al., 1998). Salmon are protected at the local sub-stock level within SACs under the EC Habitats Directive. Hence, there would evidently be a need to manage any adverse effects of beavers on spring salmon. If mortality of salmon on the high seas increases further, then it is likely that adverse effects of beaver dams will exacerbate the situation across a wider range of tributaries and river main stem areas as summer and autumn stocks decline. Furthermore, reduction in the condition and fecundity of salmon due to high seas warming (Todd et al., 2008) can be expected to lead to an increase in the number of returning fish needed to attain a conservation limit.

4.3 Key interactions between Atlantic salmon, the fish community and the thermal, physical and chemical environment

Given the precarious state of some Atlantic salmon populations, it would be particularly welcome if presence of beavers resulted in overall beneficial effects. In reviewing effects of beavers on fish, Kemp et al. (2012) highlighted that generally a greater number of positive than negative factors had been identified and demonstrated. However, such a general assessment does not inform on likely effects of beavers on individual fish species in specific contexts. The Scottish native fish communities are of relatively low diversity compared with others in Europe due to isolation in the last Ice Age (Maitland & Campbell, 1992). In Scotland, Atlantic salmon co-occur across much of their range with the closely related brown trout, Salmo trutta L.. These two species differ in their morphology such that salmon have adapted to life in swift waters, whereas brown trout are dominant in pools (Armstrong, 2010). Both species are threatened by invasive fish species, notably the European minnow, Phoxinus phoxinus L and the northern pike, Esox Lucius L. Minnow are small shoaling cyprinid fish that occupy pools and may filter out food and hence exert competition on salmon and trout (Museth et al., 2007). Pike are predatory fish that also generally inhabit slower flowing areas and can have devastating consequences for salmon populations, particularly through consumption of smolts on their downstream emigration to sea (Kekalainen et al., 2008).

An increase in pool habitats may favour trout over salmon and increase habitat for minnows, pike and other competitors and predators. However, there may also be benefits for salmon of additional pool habitat, at least in some situations. For example, the interaction between trout and salmon is complex. The presence of trout may reduce dominance hierarchies among salmon of similar size (Höjesjö et al., 2010) with possible growth benefits for subordinate individuals. There is indeed some evidence of relatively fast summer growth among salmon parr in a beaver pond (Sigourney et al., 2006), although this has not been demonstrated on fish communities of Scotland. Pools may also modulate thermal properties of the water, with possible positive or negative consequences, depending on how the local temperature relates to optima and lethal levels for trout and salmon (Elliott & Hurley, 1997; Elliott, 1994). Beaver ponds may constitute nutrient sinks (Wilby et al., 2014) which may be beneficial in agricultural areas. However, this process may result in the stripping out of chemicals (Danell, 1996) that are essential for growth of salmon in the faster-flowing reaches they occupy in upland areas (Williams et al., 2009).

An increase in large woody debris due to beaver activities may benefit young salmon by increasing the availability of good foraging and hiding areas (Nislow et al., 1999). The importance of this effect in Scotland is unknown and would depend on local habitat structure and on which size-specific habitats limit overall production of salmon (Armstrong & Nislow, 2006).

Bankside tree cover affects light and temperature in streams (Malcolm et al., 2008; Finstad et al., 2010). In some circumstances a reduction in vegetation cover due to beaver activity would increase light and temperature, and thereby increase production at primary levels and up into higher trophic levels including salmon (Kemp et al., 2012). In other cases, reduction in the tree canopy could increase temperature above the optimum for growth (about 16 ^oC for salmon) with negative consequences. Indeed, in exposed upland moorland areas, daytime summer temperatures may already far exceed these optimal levels. Contemporary management of salmon involves modelling geographic variation in temperature regimes to guide the plantation of riparian vegetation specifically to protect against anticipated elevation in temperature if climate change continues. Clearly, beaver activities would need to be incorporated into such assessments and management plans.

It is evident that there is a complex range of mechanisms relating to light, temperature, nutrients, water depth and in-stream cover via which beavers may affect production of salmon. In some situations the balance may be positive (for example where competing fish and still water predators are absent) and in others they may be negative (for example where summer stream temperatures are high and pike are abundant in pools). The mapping work is important because it shows that the potential for those positive effects that are not directly related to damming activity is greatest in larger rivers where the degree of overlap is most extensive.

4.4 Damming

A major clear potential negative effect of beavers is obstruction to migration of salmon due to damming, which is most likely to occur in relatively shallow and hence generally narrower tributary areas. Variation among tributaries in the area of overlap of distribution of salmon and beavers is not in itself relevant to the extent of such impacts since the whole area of tributary upstream may be affected by a dam. However, such overlap delimits the areas where obstructions would be most likely to be constructed should local conditions be suitable. In the context of identifying locations where dams would be built, it may in the future be useful to refine mapping procedures to identify core habitat in which beavers would be most likely to establish territories (Webb et al., 1997). However, in the absence of maps of water depths and channel morphology it is in any case not currently feasible to apply GIS meaningfully to predict likely dam locations.

It is well established that man-made dams and weirs may affect survival and energetics of salmon moving upstream (Gowans et al., 2003) and downstream (Gauld et al., 2013) even if those structures are passable. It is also clear that beaver dams in some contexts may be impassable and that the occurrence, stability and abilities of salmon to pass beaver dams would be likely to vary greatly depending on local geomorphological and hydrological conditions, which would influence river discharge characteristics (Mitchell & Cunjak, 2007). It seems likely that there would be strong interactions between beavers and man-made structures in rivers. For example, fish passes and culverts constitute structures against which beavers can engineer dams; furthermore, regulated hydro rivers may lack the seasonal high flows that would otherwise displace dams.

4.5 Population responses to beaver-induced impacts

There are often more adult salmon spawning than are required to replenish a stream. Reduction in the number of upstream migrating salmon would be of concern only if it brought the spawning stock below a critical level termed the conservation limit. Below this level, there is little scope for compensation from density-dependent processes and a reduction in smolt production would be expected, although the preceding and subsequent couple of cohorts may be enhanced as a consequence of a single weak year class (Einum et al., 2011; Gurney et al., 2008). Regardless of the year class strength, reduction in numbers of emigrating smolts due to obstruction of downstream migration would be likely to have a direct proportionate effect on returning adults since mortality at sea is thought to be density independent (Jonsson et al., 1998). Although dams may have these immediate negative effects, populations of salmon would also have substantial resilience to becoming extinct due to damming because in Scotland there may typically be two to four cohorts in fresh water and two at sea at any time. Hence, a tributary population might recover from several consecutive missing year classes in the scenario that beaver damming fully prevented upstream passage. If a tributary population is eliminated then it would likely be replaced over time should damming subside, but possibly with a loss of the locally-adapted gene pool and hence lower productive potential. Such change has conservation implication through reduction in genetic diversity.

4.6 General conclusions

Beavers are a natural component of Scotland's wildlife heritage that was lost due to man's activities. Atlantic salmon evolved with beavers over millennia and clearly the two species co-occurred in Scotland. There is little doubt that beavers can generally have overall positive effects on production of some species of salmonid fishes due to their role in engineering river habitats and influencing the chemical dynamics within the watercourse (Kemp et al., 2012). However, their influence on Atlantic salmon is more ambiguous, because this species of fish is specialised for swift waters, which would be reduced by extensive beaver damming. Furthermore, Atlantic salmon is highly migratory and hence vulnerable to obstruction of free passage. As with beavers many years ago, Atlantic salmon may be threatened now by human activities, but in this case through the current general effects of climate change on the high seas (Todd et al., 2008) combined with a range of local impacting factors. It is therefore by no means certain that salmon across their range can tolerate negative effects of beavers in the way that once they could. It is likely that beavers would need to be managed to avoid negative effects and if done so carefully then any positive effects may be harnessed for the good of salmon. In this regard, the mapping work in this study provides a foundation for planning effective management strategies and can usefully be extended more widely. If beavers expand their range in Scotland and more is understood of their detailed biology in this new landscape, then GIS might usefully be applied to predict damming points (Dryburgh, 2009). GIS might also be used to estimate the linear extent of tributaries in which dams might be constructed as an aid to management surveys for identifying and breeching beaver dams to protect spring salmon in upper river tributaries.