Scottish Marine and Freshwater Science Volume 5 Number 18: Depth use and movements of homing Atlantic salmon (Salmo salar) in Scottish coastal waters in relation to marine renewable energy development

Introduction

Scottish Government aims to generate the equivalent of 100% of its electricity requirements from renewable sources by 2020 (Anon 2011). Marine Renewable Energy ( MRE) may be developed from wave, wind, and tidal currents, and concerns have been raised over the potential interaction between MRE installations and marine animals, including migratory fish such as Atlantic salmon ( Salmo salar). The salmon is of great cultural and conservation importance across north and west Europe (Hindar et al., 2010) and forms an important component of the Scottish rural economy (Radford et al., 2004). Salmon stocks have been in decline over much of their range, partly due to reduced marine survival ( ICES, 2014).

Malcolm et al. (2010) reviewed the potential for interaction between MRE development and Atlantic salmon in Scotland, and identified a number of potential interactions and knowledge gaps. Deleterious consequences could result directly from collisions with devices, at close range via the influence of electromagnetic fields ( EMFs) associated with cabling for the devices, and over wide areas via noise generated during construction and operation (Dolman & Simmonds, 2010; Slabbekoorn et al., 2010; Gill et al., 2012). To assess the potential for these impacts it is, in the first instance, necessary to know the spatial overlap between salmon and the proposed devices. The lack of information on the swimming depths of homing salmon in Scottish coastal waters is therefore an important knowledge gap in assessing the potential impact of renewables (Malcolm et al., 2010). Godfrey et al. (2014) presented information on the depth use of salmon tagged on the north coast of Scotland in 2013 . This study was continued in 2014 and the results of both years are described in this report.

In Scotland, salmon spawn in rivers, where they generally grow for 1-3 years before smolting and migrating to the sea. They then typically spend a further 1‑3 years feeding at sea before returning, most commonly to their natal river, to breed. Many salmon die after spawning, but some survive and return to sea as kelts, where they may regain condition and again return to rivers to spawn (Mills, 1989). Thus salmon are potentially vulnerable to MRE developments in coastal waters as post-smolts, post-kelts, and as maturing adults returning to home rivers to breed.

Studies of swimming behaviour have been conducted on post-smolts (Økland et al., 2006; Davidsen et al., 2008; Hedger et al., 2008; Dempson et al., 2011; Thorstad et al., 2012), post-kelts (Hedger et al., 2009; Halttunen et al., 2009; Reddin et al., 2011; Lacroix, 2013) and to a lesser extent maiden salmon (those returning to rivers for the first time to spawn) (Sturlaugsson, 1995; Holm et al., 2006; Davidsen et al., 2013). The depth use of maiden salmon is of particular interest as they comprise the majority of the breeding population (Shearer, 1992). However, studies examining the swimming depths of maiden salmon have sometimes been constrained by topography, and sample sizes have generally been small. Godfrey et al. (2014), using pop up satellite tags ( PSATs), recorded the swimming depth, over 1-10 days, of 47 salmon returning to northern Scottish coastal waters, of which 45 were maiden fish. Results indicated that these individuals were highly surface-orientated although they also appeared to use all the available water column at times.

In addition to swimming depths, information is required on the migration routes of returning salmon in Scottish waters. Information was limited to the recovery and reporting of conventional tags at coastal netting stations (Shearer, 1992; Malcolm et al., 2010), until Godfrey et al. (2014) reported the locations of 34 salmon, via pop-up satellite telemetry, 1-10 days after initial capture on Scotland's north coast. The salmon tracked by Godfrey et al. (2014) were shown to travel up to 56 km per day, and their migrations were not strictly linked to the coast, with pop-up locations up to 100 km offshore.

The present study expands on the work of Godfrey et al. (2014), focussing on the area around the Pentland Firth, between Orkney and the Scottish mainland, which has been identified both as a region for the development of tidal energy (Shields et al., 2009) and as an area which a large proportion of homing Scottish salmon may traverse (Malcolm et al., 2010). Here we bring together both the initial results of Godfrey et al. (2014) from the summer of 2013 and the additional data obtained during the summer of 2014, providing detailed information on the swimming depths and pop-up locations of salmon in open coastal water predominantly on their first homing migration.