Research theme: Barriers to migration

River fragmentation is the breaking up of a river’s continuous flow from source to the sea and has been identified as one of the greatest global threats to freshwater ecosystems and their biodiversity.

Threats from barriers include interruptions to the movement of migratory species, especially those that need to move between fresh and saltwater; blockages to the exchange of individuals and genetic information between populations; and modifications to aquatic habitats, with changes in flow and sediment transport.

Dalligan barrier

The impact of instream structures on the movement and migration of fish was recognised as early as the 1830s. The collapse of salmonid stocks caused by dams and weirs built during the industrial revolution led to the construction of fish passages, but these solutions only attempted to cater for salmonids (salmon & trout), which have stronger swimming and leaping abilities than many other fish species.

As well as this boom in dam and weir construction, the 19th century was also a period of construction for roads, railways, canals, bridges and other structures, many of which were poorly designed and unable to facilitate fish passage. These historic pressures have resulted in the catchments we see today, with valuable habitat unoccupied due to barriers preventing fish migration.


Impacts on fish

All fish in Irish rivers and lakes move around to some extent according to season. Fish species move around the rivers, lakes or sea in which they live, using the available habitat for feeding and spawning.  These movements may be small, only hundreds of metres or a few kilometres, but some fish species migrate vast distances during their lifetime.

Atlantic salmon, for example, migrate from upland spawning streams, down into the sea and up into the North Atlantic Ocean towards Greenland. They eventually travel back down to Ireland and back up river, often to the same spawning site where they were born. The European eel, which hatches in the Sargasso Sea across the Atlantic Ocean near the Caribbean, drifts with ocean currents to Ireland before ascending our rivers to feed and grow. They may spend years in our freshwaters before returning to the Sargasso Sea to breed.

Man-made barriers may negatively affect fish migrations for feeding and breeding and can lead to population declines and the elimination of migratory fish species from a catchment.

Diagram of the effect of dams on rivers and fish movements.

Diagram of the effects of barriers on rivers and fish movements.

Obstructions to migration

  • Barriers directly block migratory routes to breeding and feeding habitats, which may cause localised extinctions. Atlantic salmon, for example, disappear completely upstream of some barriers.
  • Barriers increase the energy needed to migrate. Fish need to work harder to swim and leap these obstacles, and this can result in fish reaching their spawning sites in poor condition and affect their breeding success.
  • Barriers cause fish populations to become isolated. This reduces genetic mixing between populations, which increases inbreeding and reduces the potential of the fish population to adapt to changes in their environment.
  • Barriers reduce the amount of channel available to fish, which increases their to predators and which lowers their ability to escape incidents of pollution.

Click on the images above to view diagrams of the types of barriers found in rivers:

culverts, dams, fords, sluices & weirs

Lucan Weir on the River Liffey

Habitat impacts

  • Barriers block and slow the flow of water, resulting in ponding upstream. This creates deeper conditions, more akin to a pond or lake, and destroys shallow water habitats, such as riffles, which are shallow, bubbly areas that provide ideal spawning sites.
  • Standing water areas upstream of barriers behave like ponds and heat up in hot weather. Such circumstances provide poor conditions for our cold water species, including Atlantic salmon and brown trout.
  • Because barriers prevent the free movement of water downstream, they disrupt natural erosion and deposition within the channel.  Sediment and gravels that might accumulate downstream to create spawning beds can become disrupted.