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Scottish Marine and Freshwater Science Reports

Formal report series, containing results of research and monitoring carried out by Marine Scotland Science


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Spatio-temporal variability in Scottish smolt emigration times and sizes

Scottish Marine and Freshwater Science Vol 6 No 2

The Scottish Government has set ambitious targets for renewable power generation. Marine Renewable Energy, including wind, wave and tidal power are expected to contribute significantly to these targets. Information on the timing of Atlantic salmon smolt migrations is required for planning coastal development and the timing of particular activities. Information on smolt sizes is required to plan acoustic tagging studies aiming to improve understanding of fish movement and behaviour in the coastal zone. This report assessed pre-existing information on smolt migration timing and sizes. The commencement of migration was characterised as the day of the year (DoY) on which 25% of smolts had migrated (S25); the end of the migration period was characterised as the DoY on which 75% of smolts had emigrated (S75). The start and end of the migration period varied with elevation and over time, but not as a predictable function of geographical location (latitude and longitude). Thus, a single coastal migration DoY range (S25 to S75) was estimated for the whole of Scotland. By including uncertainty in the predictions it was possible to identify a “sensitive window” for development, where large numbers of migrating salmon smolts could be expected in the coastal zone. This window covers the period DoY 103-145. This envelope does not consider the time that smolts will spend in the coastal zone, so further work is required to adjust this window appropriately. It was not possible to model the spatiotemporal variability in smolt sizes given available data and resources. However, considerable inter-site and inter-annual variability was observed. The data presented here could be used to inform future acoustic tagging studies at existing sites, although it is suggested that pilot data should be obtained where work is to be carried out at new sites.

Malcolm, I.A., Millar, C.P., Millidine, K.J. 2015. Spatio-temporal variability in Scottish smolt emigration times and sizes. Scottish Marine and Freshwater Science Vol 6 No 2. Edinburgh: Scottish Government, 15pp.
Release Date
Spatial / Geographical Coverage Location
Temporal Coverage
1966-01-01 to 2014-12-31
UK Open Government Licence (OGL)
Iain Malcolm
Data Dictionary

Data on smolt emigration times and sizes were obtained from fisheries trusts, fisheries boards, universities and private companies and compiled under a Marine Scotland contract to the Scottish Fisheries Coordination Centre (SFCC, 2014). This was supplemented by monitoring data from Marine Scotland Science Freshwater Fisheries Laboratory. A map showing the locations of the sampling sites in the final dataset is shown in Figure 1. Smolt data were collected by a wide range of methods including fixed and temporary, partial and full river traps and rotary screw traps. It is important to note that these data were collected for varying purposes, but that the objectives of these studies were not necessarily the same as those of this 5 report. Although this is inevitable in any large scale opportunistic meta-data analysis, it should be borne in mind. To facilitate analysis, a number of assumptions were made about the data. Firstly, it is assumed that fish are caught in proportion to the numbers of emigrating fish and thus that capture efficiency remains constant within years. Secondly it is assumed that the captured fish provide a representative sample of migrating fish and there is no size selection in the capture process.

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Marine Scotland Science
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