Effects of barriers on migratory fish

There is increasing interest in understanding and managing the effects of barriers on migratory fish species including Atlantic salmon. The removal or easement of barriers is often a preferred management action because unlike many other fisheries management activities it is associated with unambiguous benefits. As a result, considerable national and local funding is spent each year on barrier improvement works. It is therefore important that this work is prioritised appropriately based on expected benefits.

Recently, scientists from Marine Scotland Science, the University of Aberdeen, and the James Hutton Institute undertook a study to investigate the impacts of barriers (e.g. dams, weirs and other in-river structures) on river connectivity for Atlantic salmon. The study estimated the value of habitat in Scotland's rivers using the MSS national juvenile salmon density model and then calculated the potential loss of connectivity for Atlantic salmon (i.e. the loss of juvenile salmon production) resulting from the presence of impassable manmade barriers. The effects of each barrier on connectivity were estimated as a percentage of connectivity across the whole of Scotland so that the impact of individual barriers can be ranked at both national and local scales.

This information provides a valuable resource to inform and prioritise river restoration efforts and is a substantial methodological improvement on previous assessments that estimate the value of habitat only from river length or area.

Shiny app

A shiny app has been developed to allow users to understand and explore the effects of impassable man-made barriers on juvenile Atlantic salmon production in Scotland. Barrier locations are displayed on a map and coloured according to their level of impact (Delta DCIScot(%)).  Impact is assessed by combining river connectivity models that indicate the opportunity for fish to migrate to and from the sea from particular sections of river (which is impeded by barriers) and juvenile density models that assess the importance of different sections of river for juvenile salmon production. Delta DCIScot(%) indicates the percentage increase in national connectivity that would result from removing or easing a barrier assuming all downstream barriers have also been removed. Cumulative gain is the potential gain in Delta DCIScot if a barrier and all downstream IMBs are removed. By ranking barriers according to Delta DCIScot(%) it is possible to prioritise the management  of barriers based on environmental gain at both local and national scales.

Data availability

Underlying data on predicted barrier impacts can be downloaded from both the shiny app or from the Marine Scotland Data webpage.

The passability of barriers was informed by the Scottish Obstacles to Fish Migration dataset. The barriers dataset used in this study is being constantly updated as barriers are added, altered or removed. However, not all barriers may be included. In particular, natural impassable barriers are likely to be underestimated. If you have any further information relating to barriers (e.g. removed barriers, barriers absent from the dataset, barrier passability information for different species) please pass this information onto SEPA so that the dataset can be updated.

Further Information

• The assessment of barrier impacts: see Willem B. Buddendorf, Faye L. Jackson, Iain A. Malcolm, Karen J. Millidine, Josie Geris, Mark E. Wilkinson, Chris Soulsby, (2019) Integration of juvenile habitat quality and river connectivity models to understand and prioritise the management of barriers for Atlantic salmon populations across spatial scales. STOTEN 655, 557-566 where these data underpin Figure 3, Table 3 and Appendix B.
• The assessment of juvenile habitat quality: see Iain A. Malcolm, Karen J. Millidine, Ross S. Glover, Faye L. Jackson, Colin P. Millar, Robert J. Fryer (2019) Development of a large-scale juvenile density model to inform the assessment and management of Atlantic salmon (Salmo salar) populations in Scotland Ecological Indicators, Volume 96, Part 1, 303-316.