Continental aquaculture faces a challenge that is no longer theoretical: Heat waves are putting stress on rainbow trout In the middle of the fattening phase, with direct implications for animal welfare and the economic viability of farms. In recent summers, warm spells have been longer and more intense and coincide with lower flow rates and less dissolved oxygen, a particularly delicate combination for this cold-weather species.
A team from the University of León, with support from the University of Murcia and the Agrarian Technological Institute of Castilla y León, has thoroughly analyzed this phenomenon and proposes adaptation strategies for the sector. The work, planned for publication in Aquaculture, focuses on how to measure heat stress in time and what operational decisions can help cushion the blow in fish farms.
What's happening in rainbow trout farms?
Castile and León, which accounts for more than a quarter of national production, is a prime indicator of what's to come. The data collected indicates increasingly longer heat waves., with episodes lasting up to 71 days and peaks of almost 9°C above the historical average in some places.
These conditions do not only affect temperature: The flow rate and available oxygen also change., factors that, combined, reduce the trout's physiological safety margin and affect daily management (feeding, densities or water renewals).
Five years of data: how the study was structured
The research followed 17 reference locations in the community for four years. Temperature, dissolved oxygen, flow rate and water level were monitored., which made it possible to accurately describe how habitats transform during a warm spell and in which time windows the greatest risks are concentrated.
With this environmental foundation, a 19-day experimental model was designed that reproduces a real-life scenario recorded in nature. We worked with commercial size specimens so that the results could be extrapolated to normal farm conditions.
A practical and minimally invasive indicator
One of the methodological advances is to evaluate stress without sacrificing animals. Quantification of malondialdehyde (MDA) in skin mucus It has proven useful for repeatedly measuring oxidative damage, allowing real-time management decisions.
This approach reduces costs and speeds up responses on the farm: If the biomarker shoots up, it is a sign of heat stress which advises adjusting feed, densities, shading or water renewals before losses materialize.
What happens to trout during heat waves?
In the controlled trial, the fish showed a tendency to lose weight under the sustained heat regime, a classic when the metabolism is unbalanced and intake is reduced.
In addition, a increased indicators of oxidative stress both in skin and blood plasma, along with an upregulation of genes related to heat response, confirming physiological overload.
In parallel, no significant changes were detected in genes linked to redox balance in gills and liver, a relevant nuance that defines where the earliest alterations under thermal stress are concentrated.
Mitigation measures that work
The team emphasizes that adaptation is not a single action, but rather a package that combines planning and management. These are the levers that show the most traction in hot scenarios:
- Enhanced environmental monitoring and alert systems with weather forecasts.
- Selection of locations with lower incidence, intensity and duration of heat waves.
- Handling settings: feed reformulated, density reduction and fine control of oxygen and flow.
- Implementation of more tolerant genetic lines and better adapted breeding systems.
In the short term, measuring and anticipating is key: Incorporating sensors, integrating climate data and defining tiered protocols based on thresholds (temperature, MDA, oxygen) accelerates the response and reduces mortality.
Choosing the right location is half the solution.
Intuition can fail: Not all river headwaters are suitableThe research highlights that there are springs that are highly vulnerable to prolonged thermal episodes or summer flow limitations, so deciding where to locate them requires thorough and comparative monitoring.
It is also recommended consider the life stage (embryos, juveniles, commercial size and breeders), because heat sensitivity and oxygen requirements change, and with them the management strategy and infrastructure.
Who's behind it and what's next?
The work is led by Marta Riesco and Ignacio Fernández, with the participation of Paulino de Paz Cabello, Laura Calvo-Rodríguez, and María Ángeles Esteban, among other collaborators. The initiative is part of Repheat 2022-2025, focused on monitoring these events and their effects on different phases of cultivation.
The agenda continues with Mitheat 2025‑2028, which will explore mitigation strategies and forecasting tools applied to farm situations. With Castile and León as a productive hub (more than 25% of the Spanish total), the potential for transfer is immediate for Spain and other countries where rainbow trout is a mainstay of aquaculture.
The findings paint a challenging but manageable picture: Longer and more intense heat waves that compromise performance and well-being, a practical biomarker for monitoring them without harming the fish, and a range of measures—from location to genetics and management—that can build resilience and sustain productivity in a changing climate.
