Pre-transport fasting: no universal duration

Withholding feed before transport can reduce waste and metabolic demand, but a duration copied from another stock may be inappropriate. A review commissioned by the World Organisation for Animal Health (WOAH) recommends targeting the minimum period required for gut evacuation while accounting for species, life stage, fish size and water temperature. Published by WOAH in 2026, this proposal informs an ongoing review of the international transport chapter. It is not yet a newly adopted standard or a universal timetable.

What pre-transport feed withdrawal is meant to achieve

Digestion raises oxygen consumption and the production of carbon dioxide and nitrogenous waste. Inside a transport tank carrying substantial biomass, those outputs can accelerate deterioration of water quality. Feed withdrawal is therefore used to reduce faecal loading and avoid moving fish while they are actively digesting. The operational objective is gut evacuation, not conditioning fish through deprivation.

WOAH Aquatic Code Chapter 7.2 already recommends withholding feed before transport, taking account of species and life stage. It also asks operators to assess whether fish can cope with the journey by considering health, previous handling and recent transport history. Insufficient or excessive fasting is listed among the possible reasons why fish may be unfit for transport.

The technical review dated 28 August 2025 and released on the WOAH website in 2026 proposes more explicit wording. A future provision would include species, life stage, size and water temperature and would aim for the minimum period necessary for gut evacuation. The report states that more detailed guidance cannot be justified at a general level. Numerical limits used in particular assurance schemes may support decisions within their own scope, but they are not universal biological thresholds.

Why 48 or 72 hours is not a complete protocol

The review by Hvas and colleagues focuses mainly on Atlantic salmon (Salmo salar). It describes feed withdrawal of two to three days as common before transport, delousing and other operations. That range offers context for some salmon production stages; it is not a prescription for trout, seabass, carp, fry or fish held at a different temperature.

Gut evacuation slows as water becomes colder and as fish become larger. The authors report that Atlantic salmon weighing approximately 5.3 kg required seven days for complete gut evacuation at 4°C. Applying a standard short window may therefore miss the intended endpoint in large cold-water fish. Extending the same window automatically creates a different problem in early stages. Salmon fry and parr are more vulnerable to feed withdrawal than post-smolts and adults, owing partly to lower reserves and the potential for aggressive interactions.

Temperature affects digestion, metabolism and oxygen availability simultaneously. Degree-days — temperature multiplied by duration — can describe exposure, but only a validated species-, stage- and operation-specific framework turns that value into a useful decision. A calculation cannot compensate for a sick stock or a poorly designed journey.

Voluntary appetite loss must also be separated from planned feed withdrawal. Fish that have stopped feeding because of hypoxia, thermal stress, disease or recent handling have not conveniently completed the fasting protocol. Loss of appetite is first a welfare and health warning. The cause should be investigated and fitness for transport reassessed.

Building a stock-specific fasting window

A practical procedure begins with species, life stage, mean body mass and size distribution. It records recent water temperatures, feed type, time of the last meal and the intended endpoint: reducing faecal output, lowering digestive metabolic demand or reaching a defined degree of gut evacuation.

Routine evidence can include feeding behaviour before withdrawal, body condition, mortality, clinical observations, faecal appearance and solids accumulation in the holding unit. Where risk justifies it, a veterinary protocol may use a representative sample to validate a local rule. This does not mean sacrificing fish without a plan before every trip. It means establishing a defensible relationship between local conditions and the selected window, then refining it with results.

The procedure should set minimum and maximum durations, identify who may change them and list events that stop dispatch. A longer withdrawal period needs a documented reason, such as slower digestion under cold conditions. An unexpected transport delay triggers a new review of total time without feed, stock condition, oxygen, temperature, loading time and the feasibility of postponement.

Fasting cannot repair an inadequate transport plan

Lower waste production never replaces calculations for biomass, journey duration and oxygen demand. The plan should cover loading, transit and unloading, including oxygenation capacity, carbon dioxide control, temperature, pH, ammonia, circulation, stocking density, vibration, stops, equipment autonomy and emergency actions.

Crowding before loading should be kept as short as possible. Pumps, pipes and fittings must suit fish size and avoid sharp edges, constrictions and damaging drops. Fish showing abnormal ventilation, disorganised swimming, important injuries or compromised health should not be loaded. Feed withdrawal does not make an unfit fish fit.

Evaluation continues after arrival. Resumption of feeding, behaviour, fin condition and delayed losses provide more information than immediate mortality alone. These outcomes should feed back into the protocol. A longer fasting period is not the logical correction when the failure arose from crowding, density, oxygen supply, handling or acclimation.

Evidence limits and regulatory caution

Detailed evidence is concentrated in a limited range of salmonid species and production contexts. The Hvas review integrates many studies but remains centred on Atlantic salmon. Its conclusion that large salmon tolerate extended periods without feed cannot be extrapolated to early stages, tropical species or fish already in poor body condition.

The report hosted by WOAH is an expert review proposing changes. Its suggested wording must be distinguished from the currently adopted Aquatic Code. Likewise, hour- or degree-day limits in private or national standards apply within their stated systems. Operators still need to check the animal-health, welfare, trade and legal requirements relevant to each route and destination.

Turn each journey into evidence for the next one

For every dispatch, record the last feeding time, water temperature, biomass, stage, health status, water parameters at loading and arrival, journey duration and incidents. Adding time to resume feeding and observations over subsequent days makes it possible to compare journeys rather than repeating a traditional number of hours.

The most defensible rule is therefore a documented, species- and system-specific window: long enough to reach its digestive objective and no longer than necessary. Vetofish can audit transport preparation, develop fitness-to-travel checks, validate monitoring points and connect feed withdrawal with loading, water-quality and contingency procedures. The aim is a repeatable decision that can be reviewed when evidence changes, not a promise that one duration will make every journey safe.