Best aquaculture practices to adapt to heat stress

Aquaculture production is expected to grow at a slower pace from 2022 to 2032, with an average annual growth rate of 1.6%. This represents a significant decline compared to the 4% growth rate observed between 2012 and 2022 (FAO, 2024). One of the main aquaculture problems contributing to this slowdown is climate change, which may impact water availability, suitable farming locations, disease prevalence, overall productivity, and the contribution of farmed seafood to global food production systems.

According to the Intergovernmental Panel on Climate Change (IPCC, 2014), global mean surface air temperatures are projected to increase by up to 4°C by the end of the century. Warmer regions, such as the Mediterranean in southern Europe and the tropics, are expected to experience even more intense temperature rises (Mugwanya et al., 2022). Over 90% of the excess atmospheric heat driven by rising greenhouse gas concentrations has been absorbed by the ocean, leading to ocean warming and an expected increase in the frequency, intensity and duration of extreme regional warming events, known as marine heat waves (Venegas et al., 2023). Climate change will impact not only marine environments but also freshwater and brackish ecosystems, posing a significant threat to the aquaculture industry. Since many aquaculture operations are located in open farming systems, these changes will also make aquaculture-dependent livelihoods increasingly vulnerable (Handisyde et al., 2017).

Among the various environmental impacts of climate change on aquaculture — such as rising sea levels, algal blooms, reduced oxygen levels, salinity fluctuations, disease outbreaks, fluctuations in wild fisheries production and potential disruptions in the supply of raw materials like fishmeal and fish oil — rising temperatures stand out as the primary driver of change (De Silva and Soto, 2009).

Since aquatic animals are ectothermic, their body temperature fluctuates with environmental changes and the temperatures of the surrounding water. As a result, temperature increases will likely expose farmed fish, crustaceans and mollusks to chronic and acute thermal stress (Fernandes et al., 2023). Anthropogenic climate warming may interfere with critical biological and physiological processes, including metabolic rate, oxygen demand, oxygen solubility, thermal stress tolerance, immune response, reproduction, growth, nutrition and disease resistance (Mugwanya et al., 2022). The impacts are expected to be severe for farmed aquatic species with narrow thermal tolerance ranges, particularly those already living near their upper thermal tolerance limits, such as freshwater tropical species, which may be especially vulnerable to the ongoing challenges posed by climate change (Nati et al., 2021).

Water temperatures exceeding the upper thermal tolerance limit of a species (Figure 1), particularly during heat waves, may surpass the critical threshold, ultimately leading to mortality (Cereja, 2020). Although there is no single mechanism explaining temperature-induced fish mortality, it may be caused by indirect effects (such as reductions in water oxygen levels or increased susceptibility to parasites and diseases) or at extreme temperatures by direct thermal effects on the fish’s physiology (Ern et al., 2020). For example, in midsummer, water temperatures in the southern Mediterranean (e.g., Turkey, Greece, Spain) can rise above 32°C, exceeding the upper tolerance limits of key farmed marine species in the region, such as European seabass and gilthead seabream, which grow optimally at temperatures around 25°C (Dülger et al., 2012). However, seabream appears to have a lower ability than seabass to acclimate and survive in the highly variable temperature conditions of the Mediterranean Sea (Kır, 2020). Studies on both species have shown antioxidant responses to thermal stress, but the specific antioxidant thresholds at which fish performance and fitness become compromised under climate change–induced temperature shifts remain unknown. Besides, models have shown that seabream would require more time to reach the minimum commercial size, where growth performance, harvest yield and return on investment may decline as climate change intensifies (Cubillo et al., 2021).

Rising temperatures due to climate change, particularly in antibiotic resistance “hot spots” such as the Mediterranean Sea, may alter bacterial physiology, potentially leading to increased resistance to antibiotics (Pepi and Focardi, 2021). The harmful synergy between climate change and antimicrobial resistance can further drive the emergence, transmission and spread of infectious diseases, posing an increasing threat to human health (van Bavel et al., 2024). Mediterranean aquaculture is just one example to discuss the effects of climate change and heat stress on aquaculture. Concerns about the potential impact of climate change have also been raised in salmon farming regions such as North America, Chile, Norway and Tasmania (Calado et al., 2021). Meanwhile, low-latitude countries such as those in the Asia-Pacific region, which have limited adaptive capacity, are expected to be extremely vulnerable under future climate change scenarios (Troell et al., 2023).

Figure 1. Maximum thermal tolerance limit for aquaculture species: Salmon and rainbow trout (16°C), common carp (21°C), turbot (23°C), seabass and seabream (30°C), Nile tilapia (31°C), catfish and shrimp (32°C). (Mugwanya et al., 2022; Kır, M., 2020).

Long-term resilience of the aquaculture industry to climate change can be achieved by adopting best aquaculture practices and certifications that align farms with blue farming strategies. These strategies primarily focus on reducing greenhouse gas emissions, capturing carbon and enhancing carbon sequestration (Bennett et al., 2023). Others could suggest the implementation of sustainable aquaculture systems such as aquaponics and recirculation aquaculture systems (RAS) for the control of environmental conditions in a changing climate. Genetic and selective breeding approaches could also develop heat-tolerant farmed strains (Yadav et al., 2024). While these options may prove viable in the long term, integrating nutritional technologies offers a practical and cost-effective means to meet animals’ nutritional requirements while enhancing resilience against heat stress and further supporting the industry’s ability to adapt to climate challenges (Pailan and Biswas, 2023).

Mitigating and adapting to heat stress can be achieved by supplementing readily available nutritional and non-nutritional feed additives for the aquaculture market. Nutritional solutions such as essential amino acids, omega-3 fatty acids, vitamins, minerals, antioxidants and nucleotides, along with non-nutritional solutions including bacterial and yeast derivatives, can play a crucial role in reducing stress challenges (Ciji and Akhtar, 2021). Furthermore, stressful environmental conditions, such as temperatures outside a species’ optimal range, trigger the release of cortisol and oxidative stress, which can inhibit feeding (Volkoff and Rønnestad, 2020). Therefore, farmers may need to adjust feeding schedules during heat waves.

Under prolonged heat stress, oxidative stress becomes a significant concern in aquaculture, as endogenous antioxidant systems may be overwhelmed. Therefore, external nutritional interventions are essential to restore oxidative balance in aquatic farmed animals (Madeira et al., 2016). Selenium, one of the essential trace minerals, is involved in various metabolic, biological and physiological functions, such as acting as a precursor for antioxidative enzyme synthesis leading to high total antioxidative capacity (Dawood et al., 2021). Heat stress increases oxidative stress, potentially elevating selenium demand. Therefore, optimizing selenium levels in aqua feeds, alongside essential antioxidants such as vitamins C and E, is recommended to enhance stress resistance and overall performance (El‐Sayed and Izquierdo, 2022). Alltech’s selenium-enriched Saccharomyces cerevisiae yeast (Sel-Plex®) was the first selenized yeast to receive EU authorization as a nutritional additive for all species and was the first FDA-reviewed organic selenium source. Its superior bioavailability, compared to inorganic sources, is attributed to selenomethionine.

Several studies have already investigated the potential effects of specific organic-selenium-enriched dietary strategies on oxidative stress and thermal stress responses in aquatic species (Figure 2). For example, research by Elfadadny et al. (2024) investigated antioxidant enzyme expression (GPx, GST and SOD) in all tissues of P. vannamei after heat stress (33°C). The same study reported variations in GSH levels and MDA content between treatment groups. Similarly, Ilham and Fotedar (2016), examining the effects of dietary strategy changes on yellowtail kingfish (Seriola lalandi) under elevated temperatures (26°C), observed variations in red blood cell glutathione peroxidase (GPx) activity and muscle selenium levels. In addition, recent research on rainbow trout suggested that fish acclimated to higher temperatures and receiving organic selenium-specific dietary interventions displayed differences in stress-related responses, including changes in expression levels of heat-shock proteins and cortisol levels (Hosseinpour et al., 2024).

Figure 2. Reduced glutathione (GSH) and malondialdehyde (MDA) content in the gill, muscle and hepatopancreas of Penaeus vannamei shrimp fed control and organic selenium (Se) diets under induced heat stress challenge for 6 h at 33°C (Elfadadny et al., 2024), and red blood cell glutathione peroxidase (GPx) activity and muscle selenium concentration in yellowtail kingfish fed control and organic selenium (Se) diets under elevated temperatures at 26°C (Ilham and Fotedar, 2016).

Apart from oxidative stress, temperature fluctuations can disrupt the intestinal microbiome’s equilibrium, promote the overgrowth of pathogenic bacteria, and weaken the intestinal barrier (Zhao et al., 2024). Therefore, maintaining gut health is crucial under heat stress conditions. Alltech’s mannan-rich fraction (MRF), a refined form of mannan oligosaccharides from a select strain of Saccharomyces cerevisiae, has been widely investigated as a functional additive in diets of aquaculture species. Alltech’s MRF, when supplemented in diets of the gilthead seabream during thermal stress (34°C), enhanced the immune health status of the animal (Figure 3), as shown by higher lysozyme content, lower levels of cortisol, and expression levels of il-1β, ghr-1, igf-1 and tnf-α in the gut compared to the control (Khosravi‐Katuli et al., 2021).

Figure 3. Plasma cortisol and serum lysozyme levels, relative gene expression of il-1β, tnf-α ghr-1, igf-1 in intestine of seabream fed a control and MRF-supplemented diet after 10 days of heat stress at 34°C (Khosravi‐Katuli et al., 2021).

Ultimately, global warming poses multifaceted challenges to aquaculture, affecting marine and freshwater environments, multiple regions and diverse species. These changes necessitate adaptive nutritional strategies to ensure the sustainability and resilience of aquaculture practices in a changing climate.

References are available upon request.

About the author:

Dr. Vivi Koletsi is a global technical support specialist within Alltech’s Technology Group. She collaborates with the company’s global Aqua team regarding all technologies on the aquatic species side.   

Dr. Koletsi, a native of Ioannina, Greece, first became interested in aquaculture while completing her undergraduate studies in biology at the Aristotle University of Thessaloniki. She began focusing on fish nutrition in earnest while pursuing her master’s degree in aquaculture and marine resource management at Wageningen University & Research in the Netherlands. This interest led her to complete an internship with Alltech Coppens, during which she established a protocol to help prevent mycotoxin contamination in aqua feeds. 

Upon earning her master’s degree, Dr. Koletsi continued her mycotoxin research at the doctoral level with support from Alltech in collaboration with the Aquaculture and Fisheries Group at Wageningen University & Research. While completing her doctoral studies, Dr. Koletsi conducted trials at Alltech Coppens’ facilities while continuing laboratory work at Wageningen. Her focus was on mycotoxins’ impact on rainbow trout.  

Dr. Koletsi joined Alltech as a team member upon completion of her Ph.D. in 2023. 

I want to learn more about nutrition for aquaculture.