by Dr Kiranpreet Kaur, Director R&D Aquaculture, Aker QRILL, Norway and Dr Jorge Pino-Marambio, Product Director Aquaculture, Aker QRILL, the Netherlands
During the summer months, water temperatures are increasingly reaching levels that challenge farmed fish. As temperatures increase, oxygen availability decreases and metabolic demand rises, fish exhibit diminished capacity to cope with additional stressors.
Temperature strongly influences fish health and performance. Because fish cannot regulate their own body temperature, even minor deviations from an optimal range can disrupt metabolism, suppress the immune system and hinder growth. These signs tend to manifest gradually, with the process occurring over the summer season rather than occurring all at once.
While farmers have limited control over the seasonal water temperature, nutrition is one area where practical adjustments can be made. Functional nutrition has become an important tool for managing the physiological consequences of the warm summer season.
The Temperature Challenge
Fish are adapted to live within relatively narrow temperature ranges. When water temperatures rise above this comfort zone, metabolic activity increases. Fish burn energy faster, require more oxygen and generate more internal stress at cellular level.
Research shows that both heat and cold stress can disrupt normal body functions in fish, which alters how energy is used and increases the internal damage caused by oxidative stress. While heat stress accelerates metabolism and increases oxygen demand, prolonged exposure to low temperatures have the opposite effect, slowing metabolism, reducing feed intake and weakening immune responses. Both extremes can slow growth and increase vulnerability to disease.
At the farm level, the manifestation of heat stress frequently occurs through subtle yet persistent indications, including but not limited to: reduced or inconsistent feed intake, diminished growth rate, diminished feed conversion efficiency and heightened sensitivity to routine activities such as grading or handling. Internally, prolonged exposure to warm water forces fish to redirect energy away from growth and instead towards basic survival and repair.
One of the primary consequences of heat stress is its effect on the immune system. Sustained oxidative pressure and elevated stress hormone levels can suppress immune responses, which increases the fish’s vulnerability to pathogens. Evidence shows that temperature-related stress directly affects tolerance and survival, even in otherwise healthy fish.
Implications for Farmers and Production Systems
From a production point of view, temperature stress rarely causes immediate failure. Instead, it can diminish performance over time.
Feed conversion ratios are known to decline as fish utilise more energy to cope with stress. This results in growth slowing down even when feed intake appears relatively stable. Mortality may increase later in the season, often during periods of additional stress such as handling, low oxygen events or minor disease outbreaks that the fish would normally tolerate.
More importantly, heat stress reduces the fish’s ability to cope with other challenges. Conditions like crowding, fluctuating water quality and routine management practices all become more difficult when fish are already stressed by temperature. This explains why many producers experience ongoing problems through the summer rather than a single heat event. Effective management focuses less on avoiding warm conditions entirely and more on helping fish cope with those conditions over time.
Role of Functional Nutrition
In aquaculture, functional nutrition refers to the use of specific nutrients or ingredients designed to support resilience during stress, for example by strengthening antioxidant capacity, supporting immune function or maintaining gut integrity rather than simply meeting basic growth requirements. During warm periods, nutritional priorities shift toward helping fish manage stress, maintain immune function and protect basic physiological processes.
When temperatures rise, fish require more energy for maintenance and repair. Their increased metabolic rate, oxidative pressure and immune suppression all place higher demand on nutritional systems. Evidence shows that targeted nutritional strategies can help moderate stress responses and maintain performance that is closer to the baseline, even in periods of higher temperatures.
Nutritional Strategies for Heat Stress
Functional feeds are considered to be one of the most effective ways to anticipate stress rather than respond when it is already too late. By supporting antioxidant systems, gut function and immune responses, functional nutrition helps fish to maintain physiological balance even in less-than-ideal environmental conditions.
When water temperatures rise, nutritional priorities shift from maximising performance to maintaining physiological stability. Functional nutrition strategies during these periods are typically designed to support the systems most affected by heat stress, including antioxidant defense, immune response, metabolic regulation and gut function.
Supporting Antioxidant Defense
As water temperatures rise, fish metabolism speeds up, which increases oxygen use and leads to higher production of harmful oxidative by-products that can damage cells. That’s why functional nutrition often prioritises antioxidant support. Evidence shows that antioxidant vitamins and trace minerals can help control oxidative stress and protect cells during heat exposure.
Maintaining Immune Function
Heat stress places continuous pressure on the immune system, which increases the likelihood of infections over time. Functional nutritional approaches aim to support immune readiness while avoiding the inflammatory responses that drain energy reserves. Certain bioactive compounds have been shown to help maintain immune function during periods of prolonged stress.
Managing Metabolic Stress
As temperatures rise, fish need more energy just to maintain basic functions, leaving less energy available for growth. During these periods, certain amino acids become more important because they help fish cope with stress, support immune function and limit internal damaging, which improves their ability to handle prolonged warm conditions.
Protecting Gut Function
Heat stress can disrupt digestion and nutrient absorption, reduce feed efficiency and increase health risks. Nutritional strategies that support gut integrity and microbial balance help maintain feed utilisation and overall robustness at a time when fish are already under pressure.
Practical Considerations for Use
Functional feeds are most effective when introduced before the warmest part of the season. Early use helps prepare fish for prolonged exposure to elevated temperatures rather than trying to correct issues once performance has already declined.
Nutrition should always be combined with good farm management. Oxygen control, stocking density and careful handling remain essential. Functional nutrition does not replace these measures but supports them, helping fish cope better with unavoidable environmental stress.
Role of Krill Meal Under Heat Stress
Krill meal has earned recognition as a functional ingredient for aquaculture feed because it provides several supportive benefits at the same time. Rather than addressing a single nutritional need, it delivers a combination of components that are all relevant for heat stress resilience.
Evidence from high-temperature challenge trials shows that fish fed krill-containing diets have exhibited improved tolerance to warm conditions compared to those fed the control diets. Additional studies show improved antioxidant capacity and reduced internal stress in fish receiving the krill-based feed.
Research on gilthead seabream juveniles also shows improved responses to oxidative stress when krill meal is included in the diet. Additional work has shown improvements in stress-related biomarkers where krill meals are included at levels between 3-7 percent, supporting a potential benefit under challenging conditions.
Benefits Linked to Krill’s Composition
The phospholipid-bound EPA and DHA found in krill support cell membrane stability under heat stress, while astaxanthin and omega-3 fatty acids help limit the internal damage caused by oxidative stress. Together with other bioactive compounds such as choline, these nutrients work to improve immune resilience and are associated with higher tolerance of elevated water temperatures.
As European aquaculture continues to adapt to warmer summers, functional nutrition has an increasingly important role to play. Krill meal is not a standalone solution but as part of a broader nutritional strategy, it can help support fish resilience at times when there is little else farmers can do to change the water environment.
