Facilitate downstream migration

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Facilitate downstream migration

Category 04. Longitudinal connectivity improvement

General description

A series of barriers and obstacles along the river continuum reduce the survival of downstream migrating fishes. A fish population can only survive, if a distinct percentage of the downstream migrating abundance survives.

Also, fish can get lost migrating downstream in rivers with numerous water intakes that delay them to their spawning or growing grounds. Also, these water intakes may lead them to dangerous channels, pipelines or turbines where injuries and mortalities are caused.

Therefore, the design and construction of a system to guide fishes in their way down through barriers, dams, and to avoid bypasses and water intakes is a measure to mitigate these impacts. These, guiding systems include deflectors, screens and electro-barriers that incentivize fish to go in the right channel, and avoid taking the dangerous ones. Main types are (Miranda, 2001):

  • Downstream fish passage
  • Spill flows: high flows when reservoir is full that transport fish over the dam
  • Screening devices: ‘Eicher’, ‘Louver’, ‘Johnson’, static, rotatory, boubles,..
  • Behavior-based guidance: by light (Strobe), underwater sounds
  • Electro-barriers


In view of the widely divergent conditions can be found in different dams, reservoirs and river typesobtaining at hydropower plants, no “all sizes fits all” bypass solution can be applied. So far, physical barriers that are designed with a specific flow velocity and flow angle in mind and with fish-friendly intake bar spacing in conjunction with an appropriate bypass have proven to be most effective where the hydromorphological and biological characteristics make them a suitable solution.

Many barriers that are currently under development select only certain species owing to the excessive flow velocities involved and the fact that various species exhibit widely divergent behaviors.

Hence, fish safeguards and bypasses should be built, according to existing studies demonstrating their abilities to meet future requirements and having the specific species characteristics, specific local conditions, specific management goals and a specific watercourse firmly in mind.

For surface water with water discharge < 20 m3/s the maximum screen bar distance should not exceed for adult potamodromus fish species 20 mm, for catadromus fish species (e.g. eel) 15 mm and for anadromus fish species (e.g. smolt of salmon and trout) 10 mm.

One way to improve downstream migration is by selecting or designing fish-friendly turbines. (see measure 4.6).

Expected effect of measure on (including literature citations):

Hydromorphology (general and specified per HYMO element)

  • No change

Physico-chemical parameters

  • No change

Biota (general and specified per Biological quality elements)

BQE Macroinvertebrates Fish Macrophytes Phytoplankton
Effect of measure no change migrant species no change no change

Temporal and spatial response

This measure should be tightly adjusted to the natural migration season of the target fish species, taking into account the peculiarities on temperature and floods cues that trigger downstream migration for each population.

Placement: instream where the bypass is located, or in the top of the dam at the spillway and sluiceways, or in the bypass channel just before reaching the turbines.

Pressures that can be addressed by this measure


This measure may be specially effective when the dowstream migration period of all the target species are short, and mainly simultaneous. Also, is much cost-effective when this migration period occurs during high flows season, or when water is demanded for other economic use.

The reservoirs and slow flowing zones upstream a dam have a “lake effect”, that cause delay of the downstream migration to the sea and increased predation of anadromous species. If the young migrants do not enter the sea at the right time it will not survive in the sea. In reservoirs and artificial lentic water bodies the community is often dominated by many ictiofagous species.

In deep reservoirs, with hypolimnethic bottom outlets, no migrant fish is able to find its way downstream, and the efficiency of possible guiding systems is very low.

Case studies where this measure has been applied

Useful references

Miranda, L.E. (2001). A review of Guidance and criteria for managing reservoirs and associated riverine environments to benefit fish and fisheries. In: G. Marmulla (ed.) Dams, fish and fisheries. Opportunities, challenges and conflict resolution. 91-137. FAO. Rome.

Other relevant information