Modelling the influence of flow and other environmental variables on the migration of atlantic salmon (Salmo salar) in a UK chalk stream

Abstract

The Atlantic salmon (Salmo salar L.) is a species of substantial cultural, economic and ecological importance. Named “Salmo”, meaning ‘leaper’, by the Romans (Stolte, 1981; Sutterby and Greenhalgh, 2005), the fish is synonymous with persistence and power. The unique nature of the species’ anadromous life cycle is perhaps why the Atlantic salmon is so iconic. Juveniles habituate the freshwater environment during their early development, then migrate to the marine environment to feed and grow, before returning to rivers as mature adults to spawn (Netboy, 1958; Jonsson et al., 1991; Aas et al., 2011). Adult migrations upstream have provided societies with sport, commerce and food for centuries (Hendry and Cragg-Hine, 2003; Susdorf et al., 2017), and are the foundation for the vastly popular salmon angling industry. There are estimated to be 843,000 game anglers in the UK, which contribute to the salmon fishery, which across England and Wales, is thought to be worth £10 million per year (Hinkley, 1995; Environment Agency, 2009). Fished salmon have historically provided communities with a commodity for trade, whilst also offering a proteinrich food source (Thorstad et al., 2008). Ecologically, salmon act as an indicator species for the assessment of riverine health (Parrish et al., 1998), and downstream migrating juveniles in particular provide food for a range of predators such as other freshwater fish species, land and aquatic mammals, and native birds (Metcalfe et al., 1987; Jepsen et al., 1998). Evidence for the protection of Atlantic salmon in legislation dates back to the 13th century, largely owing to recognition for its primary value both as a commodity and as a food source (Netboy, 1958). Modern conservation has evolved to protect the species’ cultural and ecological worth, as well as economic benefits. The Atlantic salmon is acknowledged as a ‘priority species’ for conservative action in the UK Biodiversity Action Plan, and is listed in annex II of the European Union’s Habitat Directive (Hampshire Biodiversity Partnership, 2000; Hendry and Cragg-Hine, 2003). Furthermore, the presence of Atlantic salmon in UK rivers heavily influences the selection of Special Areas of Conservation (SAC), where site management is obligated to comply with the specific ecological requirements of listed species. Due to the nature and scale of migrations, however, international cooperation is key to ensure effective management. The North Atlantic Salmon Conservation Organisation (NASCO) is an inter-governmental organisation that was founded for this purpose, and has greatly reduced marine harvests of the species through the implementation of prohibited fishing zones in large portions of the North Atlantic (NASCO, 2019). Despite protective efforts, however, populations of Atlantic salmon are in decline throughout their native range (Parrish et al., 1998; Windsor et al., 2012; Sundt-Hansen et al., 2018) (Figure 1). Catch-rates and the mean weights Chapter 1 2 of caught fish are deteriorating globally (Welton et al., 1999; ICES, 2015). In 2001, 57% of global salmon populations were classified as extinct, at risk of extinction, endangered, or vulnerable (WWF, 2001). The pattern of population decline is mirrored in UK, where between 1983 and 1998 the total declared salmon catch in England and Wales deteriorated by approximately 64% (Environment Agency, 1999), and the estimated total pre-fisheries abundance of salmon is estimated to have approximately halved since the early 1970s (Environment Agency, 2016). Populations in Southern and Central England are identified as the most endangered in the UK, considered extirpated, compared to those of Northern Ireland (stable), Scotland (stable), Wales (deteriorating) and Northern England (deteriorating) (Parrish et al., 1998). Figure 1 The endemic range of the Atlantic salmon (Source: Jonsson and Jonsson, 2009) One of the main challenges to the survival of Atlantic salmon are anthropogenic activities and their impact on freshwater environments. Humans have historically had a profoundly negative impact on the freshwater environment via; overexploitation of resources, water pollution, flow modification, the destruction and degradation of habitats, and the facilitation of invasive species (Revenga et al., 2005; Dudgeon et al., 2006; Thorstad et al., 2008). An increase of these pressures, specifically on freshwater ecosystems, has occurred over the last century, which echoes the decline in salmon populations for the same period. This increase is largely owing to rises in human populations, which have consequently lead to greater competition for freshwater resources and an increased demand for services such as hydropower, domestic water supply, flood control, irrigation and recreation (Arthington et al., 2006; Alcamo et al., 2007; Murchie et al., 2008; Godfray et al., 2010). These services directly impact aquatic fauna, such as the Atlantic salmon, in a variety of ways. Dams and low head weirs which allow for the provision of hydropower often create channel Chapter 1 3 obstructions with no available means for passage (Poff and Hart, 2006; De Leaniz, 2008). Channel modification for the establishment of water abstraction plants and other infrastructures can drastically alter the physical environment and degrade habitats (Petts, 1996; Ward et al., 1999). Furthermore, increases in human population leads to greater stress on aquatic fauna through more frequent recreation and leisure purposes, such as fishing and water sports. (Parrish et al., 1998; Lackey, 2005). The UK salmon population comprises a significant proportion of the total European stock (JNCC, 2019). Of the 49 rivers in England that support ‘major’ Atlantic salmon populations, 5 are chalk streams (Environment Agency, 2018; Ikediashi, 2018). The chalk streams of southern England are stable, with small annual variations in the physical and chemical environment, resulting in highly productive settings for aquatic fauna and flora (Solomon, 1978a; Welton et al., 2002; Riley et al., 2002; Grapes et al., 2005). Whilst beneficial for fauna such as Atlantic salmon, chalk streams are attractive for a wide range of human activities that can result in ecological damage. The characteristics of chalk geology render groundwater aquifers highly important across northern Europe (Edmunds et al., 1987), and the most important in the UK (MacDonald and Allen, 2001). Both surface and ground water abstractions from chalk rivers are key for domestic demand across the south of England, and have been extensively developed for public water supply (Edmunds et al., 1987; Macdonald and Allen, 2001; Environment Agency, 2004), and consequently, conflicts between land drainage, land use and ecological requirements are commonplace (Mann, 1989). Moreover, recreational fishing is highly popular on chalk streams due to the prevalence of desirable game and coarse fish. During the close season for salmon and trout (Salmo trutta), species such as the European eel (Anguilla Anguilla), grayling (Thymallus thymallus) and pike (Esox lucius) are often targeted by anglers (Mann, 1989). Salmon are negatively impacted upon through illegal stocking, negative habitat management for the benefit of anglers, and poor handling and angling practice (Netboy, 1958; Mann, 1989). In addition to increasing human population exerting stress on freshwater ecosystems, anthropogenically driven climate change stands to exacerbate these pressures (Whitehead et al., 2009; Vörösmarty et al., 2010). Increasing global temperatures and alterations to precipitation and runoff will inevitably both increase the demand for water whilst simultaneously reducing the available supply. The influence of climate change on freshwater ecosystems is thought to be more severe in the South and East of England, as they are the most populated and most intensely-farmed regions of England, resulting in larger competition for water resources (WWF, 2017). In addition, the temperate climate of the South of England is predicted to experience a greater impact from increasing summer temperatures, than the comparatively cooler North (Watts and Anderson, 2016). Groundwater-fed streams, such as chalk streams, are particularly sensitive to extended Chapter 1 4 periods of drought, given that available water is vastly dependent on aquifer levels (Wood and Petts, 1999). Chalk aquifers account for 60% of the groundwater and 20% of the total water used in England and Wales (UK Groundwater Forum, 1998), and 70% of the public drinking water supply for the south-east region (Stewart and Smedley, 2009; WWF-UK, 2014). As such, chalk environments and the services derived from them are considered particularly vulnerable to the forecast changes in climate. Understanding the ecological requirements of river flora and fauna is a fundamental prerequisite for setting conservation objectives (Hendry and Cragg-Hine, 2003).The mechanics by which environmental factors can influence the migrations of fish are broadly understood, though precise effects will differ between rivers and specific reaches (Thorstad et al., 2008). The physical characteristics of a gravel bed river in Scotland, for instance, will have considerably different characteristics in terms of hydrological lag time following precipitation events, channel morphology and temperature regimes, when compared with the typically stable nature of chalk streams. Given the pressures associated with increasing human population and impacts deriving from anthropogenic climate change, improved understanding of the environmental variables that influence migrations of Atlantic salmon would be of significant value (Hodgson and Quinn, 2002). Such understanding is imperative for ensuring the effective conservation of the species, through the development of scientifically-supported legislation (Hodgson and Quinn, 2002).

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

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