Marine Research Report: Hydrodynamic models to understand salmon migration in Scotland

Scotland has set the goal of producing the equivalent of 100% of its electricity demand from renewables by 2020; this will involve so-called ‘marine renewables’ – offshore wind, wave and tidal power. Since these technologies are still under development, the potential environmental implications of their deployment are unclear.

The Atlantic salmon (Salmo salar) is a migratory species of significant conservation and socio-economic importance around the coasts of Scotland, and in many Scottish rivers. Improving our understanding of the biology and migratory behaviour of these iconic and economically valuable fish is particularly important because of apparent long-term population declines and the possibility that migrating salmon may pass through, or close to, marine renewable energy development sites.

This report is concerned with methods for assessing the likelihood that salmon originating from Scottish rivers will pass close enough to an installation to experience potential effects from, for example, noise, electromagnetic emissions or physical interaction with devices. The report does not make any attempt to consider the nature or consequences of these effects. Consequently, the report focuses on migratory behaviour in coastal waters during the outward and return migrations. This information is required for robust and defensible prediction of the risk of interactions between salmon and any proposed development. Regulators and other stakeholders also require accurate information in order to make strategic assessments of the risks to migratory fish in Scotland and beyond. At present, meeting these requirements is challenging because little relevant information exists. Gathering the relevant data empirically would be very difficult, because studies of salmon movements in open water are technically challenging and expensive. One alternative or complementary approach is to adapt hydrodynamic models for prediction of salmon migration under a range of specified conditions.

Hydrodynamic models are regularly used to assess the availability of tidal and wave energy resources and to predict potential changes to tidal flows. To understand migration, ‘particle tracking models’ (PTMs) can be used to simulate the trajectories of hypothetical particles within hydrodynamic models; these ‘particles’ can be imparted with simulated biological behaviours in order to approximate the expected behaviour of salmon.

The main objective of this report is to assess whether PTMs represent a viable means of providing estimates of the potential for interactions between salmon and renewable energy developments. The report synthesises existing information on salmon migration in the Scottish context and explores PTM methods for simulating salmon migration. Additionally, the results of a disparate historical set of experiments on the movements of tagged salmon in Scotland have been formalised, compiled and scrutinised to establish whether the resulting data set constitutes an informative resource for understanding salmon movements in Scottish waters.

We conclude that hydrodynamic models have several advantages for estimating likelihood of passage through array locations, particularly in the case of the Pentland Firth:

A range of underlying hydrodynamic models may already be available.In the absence of empirical data, modelling would allow exploration of the effects of different behaviours and tidal or meteorological conditions on encounter rate.Modelling may allow hydrodynamic changes resulting from turbine operation to be predicted.The approach has potential to generate hypotheses for testing in the field.

In order to facilitate the development of suitable particle tracking models, we suggest three activities:

Implementation of a high resolution particle-tracking model for the Pentland Firth.Implementation of a larger-scale particle-tracking model covering near-coastal areas of the North Sea and the North Atlantic, in order to simulate outward migration of post-smolts and return migration of adult salmon through Scottish coastal waters. Collection of empirical data to support the above modelling activities.

For A, an illustration of one potential application of this approach is provided for migrating salmon (it should be noted that this is not intended to represent an actual impact assessment). For this initial demonstration, most attention was given to eastward movement through the Pentland Firth, representing the return migration of adult salmon along the north coast of Scotland and into the North Sea. The outputs demonstrate that it is possible to use hydrodynamic models to produce i) information on the probability of migrating salmonids passing through specific development sites; ii) estimates of the relative number of potential encounters with arrays positioned in a variety of locations; and iii) estimates of cumulative encounter rates where there are multiple arrays. Several improvements are suggested that would be required to develop this approach into a useful tool for impact assessment. The report also makes recommendations for methods and topics to be addressed under activities B and C.

A broad literature review identifies the most likely mechanisms underlying the migration of Atlantic salmon in Scottish coastal waters. Based on this review, a parsimonious conceptual model of coastal migration is provided, although since there is little direct evidence, it must be regarded as provisional, to be refined or replaced according to any new evidence.

The conceptual model requires that:

Migrating smolts have an innate directional preference that is approximately reversed in returning adults.Salmon have an approximate sense of latitude based on a geomagnetic sense.Smolts imprint to the latitudinal position of their home river and also to their point of departure from the coast.Salmon have a sense of the coastal profile and align to it in the coastal domain.Smolts imprint to home river odours.

A series of historical reports on the results of tagging of adult salmon, containing a large number of observations, have been digitised and geo-referenced. Overall, the geographical coverage of the data is extensive. These data are potentially a valuable resource for assessing migratory movements. In addition, data on recaptures of adults from tagging of smolts carried out in a number of key rivers on the east coast of Scotland (Rivers Tay, N. Esk and Dee) have been compiled in the same way. Together, these datasets constitute approximately 3400 records for the recapture of tagged fish.

These data are now available to develop a fuller understanding of migratory routes. This can be approached in two ways.

Visual assessment of patterns of distribution and movement of tagged adult salmon.Numbers of fish caught in different fishery districts could be incorporated in a probabilistic, cellular model for spatial distribution which has also been devised.

The latter approach will support i) improved knowledge of salmon migration, ii) hypothesis formulation and iii) more robust understanding of potential interactions between salmon and marine developments.

These outputs are likely to be of immediate value to a diverse set of stakeholders, including technology developers, conservation agencies and regulators. Furthermore, any new biological insights from empirical studies will strengthen the modelling approach and extend the range of contexts to which it can be applied with confidence.