Carbon footprint values as a climate change assessment criterion of the partial harvests of European seabass and meagre in earthen pond aquaculture

With the prediction that the world population is expected to reach around 10 billion by 2050, the need for animal proteins such as meat, milk and fish will rise due to the increase in the economic income levels of developing countries (Wu et al. 2014; NRC 2015; UN 2019). Fish and seafood products have become a factor that is carefully evaluated by policymakers and governments in meeting the world's food needs due to their high protein, low fat and saturated fat content and long-chain omega-3 polyunsaturated fatty acids, as well as their richness in essential mineral and trace element values compared to land-based meat and processed meat products (Tacon 2023). Aquaculture, which has become an important protein source for improving food security in preventing malnutrition because of the importance of animal protein, has become an important sector for achieving the United Nations Sustainable Development Goals (Pradeepkiran 2019; Naylor et al. 2021). This growing interest in aquaculture and the fact that fisheries resources have reached their limits have made aquaculture the fastest-growing animal food sector and investment instrument (Henriksson et al. 2012; Bohnes & Laurent 2019). World fisheries production, including aquatic plants, became an aquaculture-based sector in 2013 and beyond (FAO 2023a).

The 99.4 and 99.9% shares of Mediterranean countries in the world production of European seabass and meagre in 2000 and later, respectively, indicate that the sector is a Mediterranean-based industry (FAO 2023a). Since 2003, Turkey has become the world leader in European seabass production, and since 2018, it has been meeting more than half of the world production. Turkey, which follows Egypt in the world production of meagre, was the leading European country in 2020 and 2021 (FAO 2023a). Because the Mediterranean Basin countries' European seabass and gilthead seabream production, which have a steadily increasing trend, will continue in this way, sustainable management strategies of the aquaculture sector need to be addressed (Zoli et al. 2023). In addition, the Mediterranean Region requires the selection of aquaculture species and cultural methods that will adapt to environmental conditions because of climate change, especially in water availability and/or quality (Rosa et al. 2012).

Climate has changed in relation to the geological history of the world corresponding to natural conditions (Nica et al. 2019). With the Industrial Revolution in 1750, when humanity increased its impact on natural processes depending on the evolution and development process, the increase in the use of fossil fuels, where embedded carbon sources such as coal and petroleum derivatives are used more, and industrial activities based on deforestation increased the atmospheric concentrations of greenhouse gases such as carbon dioxide and methane and put our planet into the ‘6. Mass Extinction’ process into the anthropogenic era (Crutzen & Stoermer 2000; Shahid & Behnassi 2014; Barnosky 2015; Livi-Bacci 2017; Köse 2018; Srivastav 2019; Steffen 2020; UN 2021; Huang et al. 2022). The atmospheric carbon dioxide value, which was 277 ppm in 1750, increased by 52.4% in 274 years and reached 422.14 ppm in July 2023 (CO₂ Earth 2023). Climate change caused by the increase in anthropogenic greenhouse gas concentrations significantly affects agriculture and food security (IPCC 2014; Gul et al. 2020). The global impacts of climate change on agriculture, livestock and aquaculture production have been questioned from many perspectives (Phillips & Pérez-Ramírez 2017; Mubeen et al. 2020; Abisha et al. 2022; Froehlich et al. 2022; Kumari et al. 2022; Morgado et al. 2022; Park 2022). One of these assessments is global food security (D'Abramo 2021; FAO et al. 2022; Ahmed et al. 2023; Diken 2023). The carbon footprint (CF) resulting from fossil fuel-based greenhouse gas emissions, which is a criterion for these assessments, is calculated as CO₂e (CO₂ equivalent) per product and standardized with a value of kg CO₂e kg⁻¹ (Alley et al. 2007; Weidema et al. 2008; Winther et al. 2009; Shahid & Behnassi 2014; Liu et al. 2016; UN 2021; Islam et al. 2022; Jones et al. 2022; Diken 2023). The CF of aquaculture is lower than that of farm animal meat farming (beef, lamb, sheep, pig) and similar to poultry farming (Sonesson et al. 2010; MH 2017). Many seafood products with low emissions provide more nutrients than land animal proteins, especially red meat (Bianchi et al. 2022). Marine aquaculture, which is a high protein source, is a climate-friendly sector as a key to protection against greenhouse gas emissions as it has low greenhouse gas emissions compared to equivalent products grown on land (Jones et al. 2022).

Aquaculture has opportunities for adaptation to climate change, which has fragile effects on the growth of aquaculture, and is dependent on environmental conditions (Reid et al. 2019; Pernet & Browman 2021; Mugwanya et al. 2022). Aquaculture, which has activities and potential impacts based on extremely diverse and complex systems and species differences, is a potential and competitive production area in terms of sustainability with low CF value and energy use (Boyd et al. 2007, 2020; Winther et al. 2009; Angel et al. 2019; Macleod et al. 2020; Bianchi et al. 2022; Jones et al. 2022). In the face of these evaluations, it is difficult to find reliable figures on the CF of aquaculture species due to the diversity of production and different culture systems (Raul et al. 2020; Lutz 2021). In the estimates made on this subject, it was reported that the ratio of aquaculture to the amount of greenhouse gas emissions in 2017 was estimated as 263 megatonnes (Mt) CO₂e, and 0.49% (263 Mt/53.5 Gt, gigatonnes) of the CO₂e anthropogenic emission value. It was reported that the total climate change value of 291.2 Mt CO₂e in 2008 will increase by 132% and reach 674.6 Mt CO₂e in 2030 (Hall et al. 2011).

The CF value of aquaculture per product varies largely depending on the feed ingredient and compound diet, transport, investment-based system differences, energy value and project capacity (Henriksson et al. 2015; Diken et al. 2022; Diken 2023). In this direction, it has been reported that innovation planning in feed, transport and operations of salmon farms will be effective in reducing carbon emissions in terms of combating climate change (Hogan 2021). In line with these objectives, sectoral studies are also being carried out. These include feed production using low-emission feed ingredients, production of feed ingredients that enable the conversion of methane emissions into protein-based solutions, feed formulation studies with low CF value, planning a feed production facility on the farm site to reduce the logistics and production-based CF values of salmon farming, and targets to reduce the CF of aquaculture by 30% by 2030 (HatcheryFeedManagement 2021; HatcheryInternationalPersonnel 2021, 2022; Cargill 2022; EFA 2022). In addition to applications such as eco-labelling and/or carbon labelling per product in sustainable production planning of aquaculture systems, the calculation of the CF value according to the edible food value of the harvested product can be considered as a quantitative criterion in the evaluation of sustainability, food and climate policies at the national and international levels to determine food security (Ziegler et al. 2013; Parker et al. 2018; Bohnes & Laurent 2019; OECD/FAO 2021; Diken 2022). Empirical studies within the scope of primary data from feed companies and producers are important for the accurate calculation of emissions (MacLeod et al. 2020). Empirical studies, modelling and farm observations should be used to investigate the impacts of climate change on aquaculture (Reid et al. 2019). Methodological choices in practice will provide important clues to the industry and policymakers working on sustainability (Henriksson et al. 2013). The results obtained should be adapted towards species and production methods with improved nutritional and climate performance, considering nutritional characteristics and climate impact, nutritional needs of production and consumption patterns and emission reduction targets (Bianchi et al. 2022).

Consequently, the adaptation processes of aquaculture to climate change need the special attention of policymakers and planners, as they require the collective integration of many exogenous factors from an ecosystem perspective (Rosa et al. 2012). Sustainable animal production with different nutritional characteristics is critical for designing effective food policies (Kuempel et al. 2023). The future of the aquaculture sector and its role and policies in global food and nutrition security evaluated in terms of climate change and policies that consider approaches that prioritize the reduction of carbon emissions should be determined (Gephart et al. 2020; Naylor et al. 2023). In short, more information is needed on the carbon emissions of aquaculture as a climate-friendly food source (Henriksson et al. 2013). Within the scope of these evaluations, the CF of aquaculture based on species differences at the micro level was evaluated. In addition, the CF of aquaculture based on harvesting processes, which is neglected in the literature, was analysed. For this purpose, the CF expended values of European seabass and meagre in earthen pond aquaculture harvested at four different stages were evaluated in relation to species differences. Assessments were made for policymakers in terms of food security, climate impact and sustainability of species differences based on partial harvesting.

This study was based on an inventory of earthen pond marine fish farms in Milas-Muğla, Türkiye. Aquaculture and management criteria for European seabass in earthen pond aquaculture (EPES) and meagre in earthen pond aquaculture (EPM) are given in Tables 1 and 2 respectively. The water temperature of the earthen ponds was 13–27 °C and the oxygen level was 3–9 ppm. The compound diet used in earthen pond marine fish farming was obtained from the aquafeed factory, which was 18 km away, and the juveniles were obtained from the marine finfish hatchery at 0.6 km. The physical structure of the enterprise consisted of earthen ponds, a main aquafeed warehouse, container aquafeed warehouses, a resting-office area and a dining hall. There were two paddlewheel aerators (four paddles) for each earthen pond and submersible wells of different kWh were shared by earthen ponds. For water distribution, 300 m long 175 mm U-PVC and 400 mm U-PVC pipes were used. The earthen ponds were built by taking out 1,890 m³ excavation volume from each earthen pond and ponds were strengthened with gravel and concrete processes. Each earthen pond had an electrical control panel. There was one tractor shared by two separate earthen pond facilities, one broodstock facility and one adaptation facility.

Four partial harvests were carried out in EPES and EPM (Tables 1 and 2). Based on the unit values (kg CO₂e) of each partial harvest given in Table 3, the carbon footprint consumed for compound diet (CFCD), CF for general management (CFGM), CF for transport (CFT) and CF for machinery, equipment and construction (CFMEC) of EPES and EPM were calculated according to Diken (2023).

Diet formulations were created according to the label values of the compound diets given in Table 4, and the CF expended values of the compound diets were determined (Table 5) by considering the CF values of feed ingredients (Table 3). In the calculations, the carcass ratios of European seabass at the harvest 1, 2, 3 and final were taken as 89.02, 88.86, 86.77 and 84.68% and fillet ratios as 50.32, 51.57, 57.01 and 59.96%, respectively (Poli et al. 2001). When calculating the energy deposited in the fillet of harvest 1, 2, 3 and final of the European seabass, it was assumed that the dressing percentage was 50.32, 51.57, 57.01 and 59.96 (Poli et al. 2001) and that the juvenile fish would contain 17.1% crude protein and 8.57% crude fat (Lupatsch et al. 2001) and the fillet would have 20.04% crude protein and 5.86% crude fat (Trocino et al. 2012). In the calculations, the carcass ratios of meagre at the harvest 1, 2, 3 and final were taken as 94.78, 94.44, 93.97 and 94.5% and fillet ratios as 42.77, 43.74, 44.28 and 43.51%, respectively (Poli et al. 2003). When calculating the energy deposited in the fillet of harvest 1, 2, 3 and final of the meagre, it was assumed that the dressing percentage was 42.77, 43.74, 44.28 and 43.51 (Poli et al. 2003) and that the juvenile fish would contain 18.58% crude protein and 5.46% crude fat (El-Dahhar et al. 2021) and the fillet would have 20.50% crude protein and 2.12% crude fat (Grigorakis 2017). Energy values were taken as 5.7 and 9.4 kcal for 1 g protein and fat, respectively.

Meagre, which is a species that is suitable for earthen pond aquaculture as an alternative to marine systems, can be cultivated in all kinds of facilities, and is essential for the species diversity of Mediterranean aquaculture, FCR can reach 1.7–1.8 (1.7–1.8 for earthen ponds), and even 0.9–1.1, the specific growth rate is 1.49 and tolerant to stress and environmental conditions (Queméner 2002; Gamsız & Neke 2008; Monfort 2010; Duncan & Myrseth 2011; Duncan et al. 2013; Gracia & Jofre 2013; Bodur et al. 2014; Parisi et al. 2014; Vargas-Chacoff et al. 2014). In addition, the cortisol stress values of meagre are lower than European seabass (Samaras et al. 2015). Meagre, which has the advantages of physiological development, FCR and abiotic and biotic conditions of cultivation, has low CF values (Tables 2 and 6).

Compared to EPES, the daily weight gains of the second and third partial harvests of EPM from the beginning of May to the end of October increased further with the rise in water temperature (Tables 1, 2 and 6). This can be explained by the fact that European seabass is an eurythermic species (5–28 °C), whereas meagre is a fast-growing species in summer with an optimum water temperature of 19–24 °C (10–32 °C) and a significant decrease in feeding activity when the seawater temperature drops below 13–15 °C (Parisi et al. 2014; Pereira et al. 2015; FAO 2023b, 2023c). Similarly, the European seabass reached a weight of about 1,500 g in 1,061 days and the meagre in 633 days which can be explained by the species difference related to the effect of water temperature on fish growth.

In the comparative LCA analysis of meagre and European seabass from feeding to harvesting, feed has the highest share (Konstantinidis et al. 2021). This high share of feed is like the high share of CFCD in total CF expended in the present study. This superiority of meagre is due to its low FCR value and being a fast-growing marine teleost (Konstantinidis et al. 2021; Pfalzgraff et al. 2023). It is seen that different kg CO₂e values of crop-derived or fish-derived ingredients affect the kg CO₂e value of compound diet depending on their ratios in the formulation (Pelletier & Tyedmers 2007, 2010). Very different kg CO₂e values of crop-derived ingredients were used to substitute these fish meal, fish-derived and poultry-derived ingredients or used in omnivorous diets that will have significant effects on compound diets' kg CO₂e values (Pelletier & Tyedmers 2007, 2010). For example, Indian major carp and other cyprinids have low CF values, while trout values are more affected by soybean emissions from fishmeal production and land use changes (Lutz 2021). But although freshwater fish such as carp, catfish and tilapia are omnivorous or herbivorous species, they require relatively low levels of protein and fishmeal in their feed (MacLeod et al. 2020), species differences associated with omnivorous and carnivorous diets, cages and land-based aquaculture systems such as RAS and concrete ponds, which are more energy-dependent, have a significant impact on the CF of aquaculture (Ziegler et al. 2022; Diken 2023; Diken et al. 2022). The high FCR value should also be considered due to the impact of feed on greenhouse gas emission values, which leads to the most significant emission difference between striped catfish, Nile tilapia and Indian major carp aquaculture systems (Robb et al. 2017). In this report, the emission value of Indian major carp was also higher due to the high FCR value. However, the greenhouse gas emission values of all three feed species are lower than the results of the present study. The value of 0.787 kg CO₂e of tilapia feed fed with plant-based feed ingredients was lower than the values of carnivore ration feeds used in the study (Pelletier & Tyedmers 2010). Similarly, feed material production EI (kg CO₂e/kg live weight gain) values of 0.72, 0.74 and 0.91 for Nile tilapia, striped catfish and carp, respectively, were lower than the CFCD results of the present study, indicating species differences (Robb et al. 2017). Boissy et al. (2011) reported that the climate change potential (kg CO₂) of a plant-based low-aquaculture diet in trout feeds was 6% lower than that of a fishmeal-based diet. This situation reveals the arrangements and relationship of feed formulations in the CF value of feed. MacLeod et al. (2020) reported that a similar relationship between feed ingredient preferences of fishmeal with high protein value and feed ingredient preferences with high emission values due to land use on the emission increase of salmonid feeds varied depending on the ingredient utilization rates of compound diet used in European seabass and meagre feeding (Table 5). The 85% feed share of the total CF in cage salmon production was higher than the feed share of EPES and EPM (Ziegler et al. 2022). In contrast, the 66.42–79.59% and 59.64–61.85% CFCD shares of rainbow trout cage and concrete pond farming, respectively, which were harvested around 250–300 g, were more similar to the CFCD shares of EPES and EPM (Diken 2023; Diken et al. 2022). Among the average CFGM values per kg of harvested fish, the value of the first three harvests of meagre aquaculture was more similar to the value of rainbow trout farmed in concrete ponds than cage rainbow trout (Diken 2023; Diken et al. 2022). The results reveal differences in compound diet CF based on species and aquaculture system differences (Ziegler et al. 2022; Diken 2023; Diken et al. 2022).

Based on the relationship between FCR and feed consumption reported by Ziegler et al. (2022), the higher FCR values of European seabass compared to meagre affected the CFCD budget value and CF expended value per kg harvested fish. da Silva Pires et al. (2022), in the aquaculture LCA, revealed the necessity to prefer strategies that reduce the environmental impact of feed ingredients obtained from different production systems and distances in feed production. This means that aquaculture diet production will have less global warming impact. Although aquaculture has a low CF, there is a need for feed safety, including the utilization of feed ingredients for sustainability (Hognes et al. 2011; D'Abramo 2021). The high CFCD value in the results of the current study reveals the necessity of sectoral innovations working on the production and certification of feed based on low-emission feed ingredient sources, the search for low-carbon emission feed ingredient sources and the creation of feed formulations without compromising feed quality and creating a price increase in the direction of reducing the CF of sustainable aquaculture (HatcheryFeedManagement 2021; EFA 2022; HatcheryInternationalPersonnel 2022).

When the conversion of electricity was made in the overall CF budget of the EPES (share of CFGM in the overall budget × item share in the CFGM), the total CF expended shares of electricity consumption were 29.49, 29.07, 28.20 and 29.59%, the total CF expended shares of labour were 1.96, 1.92, 1.89 and 1.98% and the total CF expended shares of diesel were 0.78, 0.77, 0.75 and 0.79%, respectively (Figures 1 and 4). The CF expended electricity values per kg harvested fish were 0.77, 0.82, 0.92 and 1.00 kg CO₂e kg⁻¹, respectively. When CFGM item conversions were made in the total CF budget of EPM, the average values of harvest-1, harvest-2, harvest-3 and harvest-final, the CF expended shares of electricity consumption were 30.51, 27.53, 27.09 and 28.14%, the CF expended shares of labour were 2.03, 1.85, 1.86 and 1.89% and the CF expended shares of diesel were 0.81, 0.73, 0.72 and 0.75, respectively. The CF expended electricity value per kg harvested fish was 0.66, 0.56, 0.53 and 0.64 kg CO₂e, respectively. The average share of 4.93% CFGM in concrete pond rainbow trout farming was considerably lower than EPES and EPM results given in Figure 1 (Diken 2023). This is due to the dependence of earthen pond farming on electricity use. The shares of labour, diesel and electricity items in the total CF expended on the CFGM of concrete pond rainbow trout farming were 2.30, 1.63 and 0.62%, respectively (Diken 2023).

The CFT shares were 0.26% for EPES and 0.25–0.27% for EPM, which were quite low due to the proximity of the earthen pond marine finfish farm to the hatchery and aquafeed factory (Figure 1). The CFT values per kg fish in partial harvests increased in EPES and decreased in EPM similar to the above situation but were around 0.01 kg CO₂e kg⁻¹ in both species (Figures 2 and 3).

The average values of total CF expended in EPES increased from 2.61 to 3.38 kg CO₂e kg⁻¹ (Figure 2). In the first earthen pond, which had the highest harvest-final average weight gain of European seabass, the total CF expended increase per kg fish was 28.39% compared to the first harvest. In the third and second earthen ponds, the total CF expended increase per kg fish according to the first harvest was 32.31 and 28.05%, respectively. The basic correlation relationship between fish weight and total CF expended was evaluated and the total CF expended for 1 kg fish weight was calculated. The total CF expended values based on the linear regression analysis of the total CF expended values per kg fish converted to per kg harvested weight of the first, second and third earthen ponds of the partial harvests with different average weight values considering the total CF expended values as 2.99 kg CO₂e (y = 0.0008x + 2.1865, R² = 0.9889), 3.32 kg CO₂e (y = 0.001x + 2.317, R² = 0.7662) and 2.94 (y = 0.0011x + 1.8356, R² = 0.8739) kg CO₂e, respectively. In the face of increasing weight gain of meagre, the average total CF expended values of the first three partial harvests had a decreasing trend from 2.16 to 1.96 kg CO₂e kg⁻¹ (Figure 3). The total CF expended value per kg harvested fish from the third earthen pond, which had the highest average weight increase compared to harvest-3, decreased by 11.85% compared to the first harvest. These values of other earthen ponds were decreased by 9.05 and 6.81%. According to the total CF expended values of the first three partial harvests, the total CF expended values per kg harvested weight based on the polynomial regression analysis of the first earthen pond and the linear regression analysis of the second and third ponds were 1.95 kg CO₂e (y = 0.0000005876x² − 0.0013290409x + 2.6887845382, R² = 1.0000000000), 2.05 kg CO₂e (y = −0.0002x + 2.252, R² = 0.9867) and 2.01 kg CO₂e (y = −0.0003x + 2.3079, R² = 0.8925), respectively. A similar relationship between fish size and decreased feed efficiency in rainbow trout, a cold-water species, was also found in meagre (Papatryphon et al. 2004). CFCD and total CF expended values decreased with the increase in fish weight due to temperature change in meagre.

When the CF expended results of the present study and the general evaluation in this report and Figure 6 are taken into consideration, it is revealed that the meagre species and its earthen pond aquaculture culture system are at least as sustainable as Atlantic salmon and rainbow trout in terms of climate change emission values. While the world CF values of aquaculture are similar to those of world poultry farming, they are relatively lower than those of livestock pig farming and considerably lower than those of sheep and cattle farming (Nemry et al. 2001; Nijman et al. 2012; Boyd 2013; MH 2017; MacLeod et al. 2020; FEAP 2022). Aquaculture species have lower emissions than ruminant monogastric species because they cannot produce CH₄ through enteric fermentation, excrete ammonia directly, have lower FCR, require less energy for locomotion and are cold-blooded (MacLeod et al. 2020, 2021). The differences in every stage of production in each of the three different aquaculture systems for striped catfish, Nile tilapia and Indian major carp species in Asia have resulted in differences between greenhouse gas emission values. These are raw materials used, energy used in the mills, transport methods for moving the feed to the farm, farming methods, survival of fish to harvest and feed conversion ratios (Robb et al. 2017). Reducing energy in feed production, improving feed utilization rate, increasing feed ingredient diversity, selecting species with edible portion weight depending on feed change rates and management practices that increase production efficiency will enable CF values to be reduced along with energy savings in aquaculture (Flos & Reig 2017). It has been reported in many studies that the CF of aquaculture per product will vary largely depending on the feed and feed ingredient components, transport, investment-induced system differences, energy value and project capacity (Henriksson et al. 2015; Diken et al. 2022; Diken 2023). The study results support the results of these reports. Especially in this study, very low transport values showed a distribution to the operating and capital (CFGM and CFMEC) ratios.

Compared to EPES and EPM, the study results given in Figure 6 are within the scope of single-harvest analysis. The partial harvest analysis of European seabass and meagre, which were harvested intermittently due to the increase in fish weight and duration of cultivation, allowed the results to be interpreted accurately. The necessity of partial harvesting in EPES and EPM aquaculture is related to the market demand for fish length and weight and to the balancing of the carrying capacity of earthen ponds due to increased biomass. In a study where a similar methodology was applied, Diken et al. (2022) and Diken (2023) evaluated the CF of rainbow trout harvested at once. In these studies, rainbow trout are harvested once at a size of around 250–300 g. However, in the present study, European seabass and meagre are harvested at different sizes up to approximately 1,500 g. Therefore, the CF values of each harvest were analysed, and the CF values of the partial harvest were given. According to the results obtained from this study, the CF analysis of partial harvesting, which shows significant differences, should be taken into consideration in future studies (Figures 2 and 3).

The aquaculture industry's carbon emission reduction targets in building public awareness and corporate positioning to tackle climate change should include planning for innovation in feed, transport and operations specific to salmon farming (Hogan 2021). In 2021, a private company with approximately 2 million tonnes of aquafeed production aims to save 2 billion kg of CO₂ per year by reducing the CF of seafood farming by 30% until 2030 (Cargill 2022). As stated in this report, meagre with a high FCR-dependent portion weight is an important aquaculture species in reducing CF values and ensuring global food security in the face of increasing world population and climate change. Also, according to a report by Zoli et al. (2023), the meagre should be considered as a species that will contribute to the development of sustainable management strategies of aquaculture in the Mediterranean Basin. The results express the necessity of determining the CF values based on species and species-specific farming systems, the share of feed in the CF budget and transportation evaluations in this study, together with the innovation plans in feed, transportation and operations specific to the salmon farm reported by Hogan (2021), support the necessity of creating public awareness and institutional positioning within the scope of the aquaculture industry targets to reduce carbon emissions to combat climate change.

While CF expended per Mcal energy deposited in fillet gained during feeding and CF expended per kg of protein deposited in fillet gained during feeding values of the partial harvests increased with weight increase in EPES, they decreased with weight increase of the first three harvests in EPM (Tables 6 and 7). The average value of CF expended per Mcal energy deposited in fillet during feeding of European seabass, which has a higher fillet ratio compared to meagre, was 2.93–3.26 kg CO₂e kg⁻¹, which was lower than the average value of 3.20–3.76 kg CO₂e kg⁻¹ of meagre. The average value of 2.46 kg CO₂e kg⁻¹ CF expended per Mcal energy deposited in fillet during feeding of rainbow trout harvested at portion length is similar to the value of European seabass harvest-1, which is closest to the portion weight of rainbow trout (Diken 2023). This is due to the similar fillet ratios of rainbow trout and European seabass (Diken et al. 2022). This value was higher in meagre with a low fillet ratio (Table 7). A similar interpretation can be made for CF expended per kg of protein deposited in fillet gained during feeding. The mean values of 25.43 and 24.60 kg CO₂e kg⁻¹ harvest-1 for European seabass and meagre, respectively, given in Table 6 were higher than the mean value of 17.24 kg CO₂e kg⁻¹ of concrete pond aquaculture (Diken 2023).

CF values of compound diet and compound diet consumption are important criteria in CF expended assessments and determination of CF expended value. It is important to standardize the assessments according to the production amount for more accurate interpretations. While there is an increase in the CFCD percentage shares of the total CF expended budget of the first three harvests of the meagre given in Figure 1, the kg CO₂e kg⁻¹ value standardized according to the number of fish produced was decreased. Monitoring the CF of compound diet and compound diet consumption in aquaculture tracking and monitoring programmes used in aquaculture will enable the planning of the harvest periods of aquaculture species in the period with a low CF. In this respect, the lowest CF label values can be created in terms of the product. For this reason, it is extremely important to create CF values and consumer awareness in the fight against climate change in terms of global food security.

Species and cultivation system differences should be taken into consideration in establishing food security against global climate change. Species with tolerance to water temperature and temperature increase, which is the physicochemical evaluation element of the most important bioecological criterion of species differences, may also be specified with high resistance to global climate change. The results of this study may be important in this respect. Compared to European seabass, meagre reaches the same harvest weight in a shorter time. At the same time, when the first three harvests were taken into consideration, the CF expended value of the meagre, which is a species that increases in weight and grows rapidly due to the increase in water temperature, decreased. Therefore, meagre, which has both a shorter harvest period and a lower CF expended value, is an extremely important species in terms of global food security in the fight against climate change, at least as much as trout and salmon farming. Comparative evaluations of partial harvests of European seabass and meagre reveal the importance of fast-growing species in terms of food security in the fight against global climate change. Sectors in search of substitute food and feed ingredient sources should consider the indirect impacts of these resources on the global ecosystem in addition to product-based CF values. For example, the effects of ecosystem responses that are approaching the breaking point because of deforestation and/or increased chemical use in the face of agricultural product demands on global climate change should not be ignored. As a source of food and feed ingredients, emphasizing the preferences of climate-friendly marine-derived plant products, which are factors in reducing CO₂ emissions, will make a significant contribution to the global ecosystem. Due to the high protein, PUFA values, essential mineral, and trace element values of aquaculture products reported by Tacon (2023), we recommend the production of remarkable and permanent policies in the fight against climate change in terms of global food security for the Mediterranean Basin, which has the most important fish culture species richness of aquaculture. In future studies, we recommend that in CF expended analysis, which is a climate change assessment criterion, partial harvest analysis should be considered in harvests over portion size.

In this study, animals were not used.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.