River works: planning fish rescue without improvisation

A fish rescue is not an automatic preliminary to every river project. It is a controlled operation for a defined risk, such as temporary dewatering, diversion, local dredging or a cofferdam likely to strand fish. Poorly planned rescue adds capture, handling and relocation to an already disturbed environment. Properly integrated into the work sequence, it can reduce immediate mortality without replacing avoidance and mitigation.

The 2025 guidance from the French Biodiversity Agency (OFB) and Cerema places rescue within overall worksite management. Decisions should connect authorisations, hydrology, species, capture method, receiving-site capacity and holding conditions. Success cannot be measured only by the number of fish moved.

Avoid creating the trap

Before capture, teams should ask whether timing, footprint, duration or isolation method can reduce exposure. Spawning and migration periods, high temperatures, low flows and floods change vulnerability. A cofferdam installed without an escape route may concentrate animals in water that warms and loses oxygen.

The initial assessment covers expected species, habitats, access, turbidity, conductivity, depth and obstacles. It identifies protected, non-native or health-sensitive species that may require a specific response. A receiving site must be selected in advance, with suitable habitat, compatible water, ecological connection and enough capacity to avoid local crowding.

In France, Article L436-9 of the Environmental Code provides for administrative authorisation to capture and transport fish for purposes including health, science, ecology and rescue. An order dated 6 August 2013 defines application requirements. Project owners must check the applicable authority, species, gears, periods, destinations and reporting duties. Outside France, equivalent permits and animal-welfare rules must be identified locally rather than inferred from this framework.

Choose and adjust the capture method

Electrofishing is commonly used in shallow fresh water, but requires trained personnel, risk assessment and suitable equipment. Conductivity, temperature, channel shape, body size and species influence both efficiency and effects. Settings should remain at the minimum effective level, be monitored and adjusted; a single recipe is not defensible.

An experimental study in juvenile salmonids observed more injuries at some pulse frequencies, while short-term mortality remained low under the tested conditions. The older study does not establish a universal threshold. It demonstrates why visible survival alone is an incomplete welfare measure. OFB and INRAE’s 2022 technical guide describes the principles, safety requirements and precautions for scientific electrofishing.

Seines, hand nets and gradual drawdown may complement capture. Repeated chasing, contact with dry surfaces and compression in nets should be minimised. Passes should follow a defined stopping criterion while avoiding a prolonged effort whose catch declines as animal costs rise.

Control holding and transport

Captured fish should move promptly into clean containers, darkened where practical, at suitable density. Dissolved oxygen, temperature and behaviour require monitoring. Aeration or oxygenation should be sized and backed up; it cannot compensate for overcrowding or excessive waiting.

Groups may need separation by size, species or predation risk. Water renewal should avoid thermal shock and minimise transfer of sediment or unwanted organisms. Loss of balance, intense ventilation or injury calls for immediate reassessment of density, water and duration.

Counting and identification should be proportionate to the objective. Detailed, lengthy manipulation adds stress. Abundance classes, counting in small batches or standardised photographs may document some operations with less time away from free water.

Release into a genuinely suitable site

Release should avoid drops and use water with compatible conditions and habitat providing the right flow, depth and shelter. Temperature differences are checked and reduced progressively where needed. Fish should not be placed immediately beside machinery, pollution, a likely future dry reach or an already overloaded refuge.

Relocation is never biologically neutral. It can move infectious agents, alter populations or expose fish to unsuitable habitat. A 2023 European feasibility study on fish translocation emphasises evaluation of recipient sites and biological risks. In worksite rescue, that supports a local, authorised release and argues against choosing a destination merely because it is convenient.

Record outcomes and improve the next operation

The report connects hydrology, water measurements, capture effort, settings, species, numbers, incidents, mortality and release location. It distinguishes fish seen, caught, transported and released. Proportionate follow-up may reveal delayed losses or renewed exposure to works.

Lessons should feed back into project planning. High capture effort in one habitat may justify an earlier isolation method next time. Repeated low oxygen may require shorter holding cycles or additional capacity. Injuries must be recorded, not hidden in an aggregate count.

Vetofish can contribute to health and welfare planning, parameter selection and incident management. Rescue remains the final link in a sequence: avoid, reduce, prepare, capture with restraint, hold briefly and release into compatible habitat.