Season changes the risk of tilapia transport

Small changes in loading density do not, by themselves, make live-fish transport safe. A 2026 study in Nile tilapia found that physiological responses and fillet traits varied more consistently between summer and winter journeys than among three nearby commercial densities. The study does not identify an ideal density. It shows why temperature, oxygen supply, pH, nitrogenous waste, journey duration and biomass must be managed as one transport system before, during and after every trip.

Six commercial journeys in Brazil

Terto and colleagues studied market-size Nile tilapia, Oreochromis niloticus, reared in net pens in Paraná State, Brazil. Fish averaged 930 ± 150 g and 37 ± 3 cm. After a 24-hour fast, they travelled approximately 100 km to a slaughter facility, a journey of about 1.5 hours. The 1,000-litre fibreglass tanks were fitted with diffusers connected to pressurised oxygen cylinders.

The design crossed three loading biomasses — 375, 425 and 475 kg/m³ — with two periods. Three journeys took place in the local summer, in March, and three in winter, in August. Each journey used three tanks per density and carried 1,275 fish, giving 7,650 transported fish overall. Ten fish per density per journey were sampled, for a total of 180 analysed animals. Density assignments were rotated among tanks to reduce equipment-related bias.

Researchers recorded air temperature and humidity and measured water temperature, dissolved oxygen, pH and ammonia before and after transport. They also assessed plasma glucose and lactate, oxidative markers and fillet properties including muscle pH, colour, texture and water-holding capacity.

This design represents commercial handling more closely than a small-tank laboratory trial. Its scope is nevertheless narrow: one species, one source population, one short route, fish close to one kilogram and actively oxygenated transport tanks. “Summer” and “winter” describe two specific Brazilian periods, not every hot- and cold-season transport system.

Loading density did not explain the response on its own

Air temperature ranged from 23.6 to 33.2°C in summer and from 14.9 to 22.1°C in winter. Changes in water temperature, pH, dissolved oxygen and ammonia showed density-by-season interactions rather than a simple deterioration as biomass increased. Active oxygenation sometimes exceeded fish consumption, while dissolved oxygen fell in other density and season combinations.

Mean plasma glucose was higher in winter than in summer — 172.00 versus 126.89 mg/dL — while no statistically detectable density effect appeared across the tested range. Lactate showed a more complex interaction. The lowest value occurred at 375 kg/m³ in summer, whereas 375 kg/m³ in winter and 475 kg/m³ in summer were among the highest. There was no uniform decline in transport stress as density fell.

Fillet traits also followed non-linear patterns. Muscle pH and water-holding capacity depended on the season–density combination. Colour and several texture measures changed mainly by season or through irregular density effects. Reduced glutathione was higher at 475 kg/m³, which is consistent with a greater antioxidant challenge, but catalase and thiobarbituric acid reactive substances did not differ significantly.

The operational message is not that density is unimportant. Under these short, oxygenated journeys, moving 50 kg/m³ above or below the regional practice of 425 kg/m³ did not produce a stable response gradient. A longer duration, poor oxygenation, more extreme temperature, a different fish size or densities outside this range could yield a very different outcome.

Replace a fixed density with a transport-capacity decision

Loading density should be the output of a capacity assessment, not a stand-alone instruction. Before a transport campaign, operators should relate planned biomass to fish size and fitness, actual duration, initial water temperature, weather, expected CO₂ and ammonia production, oxygen reserve and the equipment available to stabilise water quality.

Chapter 7.2 of the World Organisation for Animal Health Aquatic Code calls for water quality appropriate to the species and transport method, including oxygen, CO₂, NH₃, pH, temperature and salinity. It also addresses suitable vehicles, oxygenation, competent personnel, checks during transport and contingency planning. It does not prescribe one biomass for all journeys: density should reflect available scientific evidence, the species and the situation.

The European good-practice code published in 2025 takes the same systems approach. Oxygen demand varies with biomass, species, fish size, temperature, stress, fasting history and journey time. The useful planning question is therefore: how much biomass can this vehicle keep within its agreed water-quality envelope until arrival, with enough reserve for a delay?

Build a journey record that supports decisions

A practical protocol can be structured around four stages.

Before loading, confirm fitness for transport, record the fasting protocol, compare source and tank water, and check diffusers, sensors, cylinders and backup capacity. In the study, gradual acclimation was used when differences exceeded 1°C, 0.3 pH units or 1 mg/L dissolved oxygen. Those figures describe the experimental protocol; they are not universal thresholds.

During loading, calculate the actual biomass and minimise handling. Tanks, pipes, pumps and nets should prevent injury. Initial temperature, oxygen, pH and fish behaviour provide a baseline for interpreting the journey.

During transport, the recording interval should detect a trend before it becomes an emergency. Dissolved oxygen alone is not enough. Highly oxygenated water may still accumulate CO₂ or ammonia, while pH changes the toxic fraction of total ammonia nitrogen. Responsibilities, available equipment and decision triggers should be agreed in advance for delays, oxygen-system failure, warming or abnormal fish behaviour.

At arrival, record time, water parameters, mortality, injury, ventilation and swimming before adding another handling stressor. Linking these observations with slaughter and fillet-quality data makes it possible to compare journeys, identify a higher-risk season and revise the vehicle’s authorised operating capacity.

Why six journeys cannot become a universal standard

The study included neither a non-transported control nor a loading density below 375 kg/m³. It tested one approximately 1.5-hour journey with active oxygenation, using one population of market-size tilapia. Seasonal effects may reflect temperature but also other differences between the March and August series. Individual measurements also came from fish sharing tanks and journeys, so modest contrasts deserve caution despite the balanced tank rotation.

The authors explicitly note that density may matter more during longer journeys, under inadequate oxygenation or at more extreme biomasses. They also call for evaluation below 300 kg/m³. None of the three tested values should therefore be copied into a different species, vehicle, climate or legal framework as a default target.

This distinction matters internationally. The work describes Brazilian production conditions and cites local operational guidance. Facilities elsewhere must apply their own legal duties and species-specific standards while using the study to improve risk assessment, not to import a number.

Conclusion: manage seasonal conditions as well as biomass

The study offers a useful warning for commercial transport. In short, adequately oxygenated Nile tilapia journeys, seasonal conditions and water-quality dynamics may shape welfare and product traits more strongly than small adjustments around a commercial loading density. Density still matters, but only in relation to temperature, duration, oxygen delivery and monitoring capacity.

Operators can act now by defining a seasonal capacity sheet for each vehicle, logging water quality before, during and after transport, and linking those records to mortality, welfare observations and product outcomes. Vetofish can support live-fish transport audits, monitoring plans, water-quality interpretation and team training while keeping recommendations specific to the species, system and applicable jurisdiction.