Scottish Marine and Freshwater Science Volume 6 Number 2: Spatio-temporal variability in Scottish smolt emigration times and sizes

Introduction

The Scottish Government has set ambitious targets to meet Scotland's entire electricity needs from renewable power generation by 2020 (Scottish Government, 2011). Marine Renewable Energy, including wind, wave and tidal power are expected to contribute significantly to these targets. However, it is important that these industries are developed sustainably and that potential interactions with wildlife are assessed and mitigated where required.

Malcolm et al. (2010, 2013) scoped the research requirements for diadromous fish in the context of offshore renewables. This identified the need for greater information on the migratory timing and sizes of migrating salmon smolts to identify sensitive periods and locations for development, and to inform future acoustic tagging studies that aim to improve understanding of coastal movements and behaviour.

Previous research has investigated environmental controls on the timing of smolt migrations at a range of spatial and temporal scales from individual river systems (Youngson et al., 1983; Jonsson & Ruud-Hansen, 1985; Byrne et al., 2003; Kennedy & Crozier, 2010), to national (Antonsson & Gudjonsson, 2002) and international scales (Otero et al., 2013). Within individual river systems, the timing of migration (also known as run timing) has been attributed to river and ocean temperature, discharge and lunar cycle. At larger spatial scales, the onset of smolt migration has been modelled as a function of latitude, longitude, river and ocean temperature and year (Otero, et al., 2013). Although there have been previous investigations of smolt migration in Scotland these studies have either been focussed at a local level, in particular river catchments (Youngson et al., 1983, Stewart et al., 2006; Todd et al., 2012) or at very large spatial scales across the whole Atlantic region (Otero et al. 2013). To date there has been no analysis of the timing of smolt migration at a national scale (Scotland) that could be used to inform risk assessments of development in near-shore areas.

There have also been a number of studies of smolt size at emigration. Differences in smolt size are often related to the age of emigrating fish (e.g. Gurney et al., 2008; Todd et al., 2012). However, size-at-age can also vary depending on changes in environmental conditions and competition (e.g. Gurney et al., 2008). It has been hypothesised that salmon parr attain a critical size prior to emigration (Gurney et al., 2008); in which case it could be expected that smolt sizes would be similar among rivers. However, other studies have suggested that size alone does not control the timing of emigration and that rates of juvenile growth also have an influence, which could result in spatial and temporal variability in size at emigration (Okland et al., 1993). Regardless of the precise mechanisms, there are relatively few studies that have investigated large scale spatial variability in the size of salmon smolts (Okland et al., 1993), and none within Scotland that could be used to inform the likelihood of tagging opportunities, which require larger smolt sizes (Lacroix et al., 2004).

This report aims to characterise and model the spatial and temporal variability in (1) smolt migration times and (2) sizes, at the national (Scotland) scale using simple spatial predictors that can be readily obtained from a Geographical Information System ( GIS) thereby negating the need for local site specific information which is frequently unavailable (e.g. river temperature, sea surface temperature, discharge, etc.). The start and end of the smolt migration period are considered with the aim of identifying a "window of sensitivity" for activities that have the potential to harm migrating smolts. The analysis of smolt sizes focusses on identifying the proportion of emigrants exceeding a size threshold of 135mm, which corresponds to the recommended size for tagging using VEMCO V7 acoustic tags (Lacroix et al. 2004; Middlemas et al., 2009). This information can subsequently be used to identify opportunities for studies of smolt movements and behaviour in the coastal zone. The information in this report contributes towards the National Research and Monitoring Strategy for Diadromous fish ( NRMSD; http://www.scotland.gov.uk/Topics/marine/marineenergy/Research/NatStrat).