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Understanding Fish Migration and Barriers in Freshwater Environments

Learn about the challenges fish face in migrating between marine and freshwater habitats, tools for assessment, and the importance of addressing barriers. Explore key aspects like swimming abilities, behavior considerations, and the impact of various structures on fish movement.

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Understanding Fish Migration and Barriers in Freshwater Environments

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  1. Fish migration and barriers Anthony Watkins Fish ecologist SEPA (original by: Dr Alistair Duguid Senior fish ecologist, SEPA) Photos: Colin Bull, Stirling University

  2. Talk overview • What is the issue? • Key aspects of fish migration • Types of barrier • Tools for assessment

  3. Why are barriers important? • UK: High density of river obstacles • Highly managed river systems • A major impact on spatial & temporal distribution of fish • Secondary effects, including increased predation, exploitation, behavioural changes • Mitigation via action at obstacles is possible • Single biggest restoration and management potential for fish Photos: Colin Bull, Stirling University

  4. Fish need to migrate • Most fish species migrate • Between marine and freshwater environments: • Eels- from breeding grounds in the Caribbean sea to freshwater rivers across Europe • Salmon- from breeding grounds in freshwaters to seas off Greenland • Sea trout- between streams, rivers, estuaries and the open sea • Sea lamprey and river lamprey- from breeding grounds in rivers to estuaries and open sea • Between different freshwater habitats: • Brown trout - very flexible behaviour • Coarse fish (e.g. pike, perch, roach, grayling) Photos: Colin Bull, Stirling University

  5. Key issues: • Which species need to be considered? • Which life stages and sizes? • Which direction? Likely up and down! • Usually consider adults moving upstream to spawn but downstream movement equally important • Adults following spawning • Juveniles (e.g. smolts) • Seasonal - which times of year are important? • River discharge - elevated flows trigger some species (e.g. salmon); low flows needed by others (e.g. elvers) Photos: Colin Bull, Stirling University

  6. Key issues (continued): • What are their swimming abilities? • Burst swimming • Sustained swimming • Swimming form and behaviour – trout very different to eels, and lamprey! • Height fish swim in the water column – dictates where entrance to a fish pass should be.

  7. Swimming ability • Ability depends on fish size, species, health and reproductive status • Sufficient depth is critical for generating thrust (for salmonids) • Water temperature affects maximum speed (+ve) and endurance (-ve) (Beach, 1984)

  8. Swimming ability Source: WFD111 barrier assessment project

  9. Behavioural considerations • Fish also react to: • Light- particularly sharp contrast between light and dark; lighting in culverts can help; affects migration timing • Noise - piling/construction impacts • Hydraulic conditions (e.g. abrupt change at weir crests – fast, shallow water) • Cover – instream (e.g. rocks) and overhead (e.g. trees)

  10. Structure types• Culverts• Weirs • Fords / bridge footings• Abstraction off-takes• Dams• Sluices• Natural obstacles

  11. Culverts: key considerations • Slope, length and water velocity- determine how quickly and for how long a fish has to swim for • Depth- key for successful swimming/ leaping • Outlet design- elevated entrances require a leap • Light (?)- may affect fish behaviour • Debris • Very common- cumulative effect Photos: Colin Bull, Stirling University

  12. Fords and bridge footings • Velocities & depths • Length of swim required • Associated outlet drop characteristics Photos: Colin Bull, Stirling University

  13. Weirs: key considerations • Hydraulic head difference (usually require a leap) • Velocity and depth on weir face • Gradient and length • Presence of lip or step is particularly challenging Photos: Colin Bull, Stirling University

  14. Impact of weirs on downstream migrating salmon smolts Gauld, 2013

  15. Assessment tools • WFD111 (2a) Coarse resolution rapid-assessment methodology to assess obstacles to fish migration Photos: Colin Bull, Stirling University

  16. More assessment tools • Observation • Direct surveys • Fisheries records • Redd counts • DNA • Tagging and tracking • Fish counters – to monitor passage

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