Alder, J., Campbell, B., Karpouzi, V., Kaschner, K. & Pauly, D. Forage fish: from ecosystems to markets. Annu. Rev. Environ. Resour. 33, 153–166 (2008). Bahuaud, D. et al. Effects of -15°C super-chilling on quality of Atlantic salmon ( Salmo salar) pre-rigor fillets: cathepsin activity, muscle histology, texture and liquid leakage. Food Chem. 111, 329–339. https://doi.org/10.1016/j.foodchem.2008.03.075 (2008). Ytrestøyl, T., Aas, T. S. & Åsgård, T. Utilisation of feed resources in production of Atlantic salmon ( Salmo salar) in Norway. Aquaculture 448, 365–374 (2015). Asche, F. & Tveterås, S. On the relationship between aquaculture and reduction fisheries. J. Agric. Econ. 55, 245–265 (2004). Magdic, D., & Dobricevic, N. (2007). Statistical evaluation of dynamic changes of ‘Idared’ apples colour during storage. Agriculturae Conspectus Scientificus, 72, 311–316.

Erikson, U. & Misimi, E. Atlantic salmon skin and fillet color changes effected by perimortem handling stress, rigor mortis, and ice storage. J. Food Sci. 73, 50–59. https://doi.org/10.1111/j.1750-3841.2007.00617.x (2008). Sun, D. (2000). Inspecting pizza topping percentage and distribution by a computer vision method. Journal of Food Engineering, 44, 245–249. Cottrell, R. S. et al. Time to rethink trophic levels in aquaculture policy. Rev. Aquac. https://doi.org/10.1111/raq.12535 (2021). Digre, H. et al. Rested and stressed farmed Atlantic cod ( Gadus morhua) chilled in ice or slurry and effects on quality. J. Food Sci. 76, 89–100. https://doi.org/10.1111/j.1750-3841.2010.01956.x (2011).There was an effect of storage day ( p< 0.001) but not of treatment ( p = 0.747) on drip loss in raw fillets (Fig. 2c). Drip loss increased steadily to 1.4–1.5% for all three groups. The product yield after smoking for all fillets was 94% and there was no effect of treatment observed ( p = 0.737). Drip loss of smoked fillets increased to 6.5% after smoking for all groups and plateaued throughout storage. There was also no difference in salt contents at the end of storage ( p = 0.733; RSW: 3.2 ± 0.4%, ice: 3.1 ± 0.7%, control: 3.0 ± 0.2%). The large increase in drip loss before and after dry salting and cold-smoking was mainly due to the diffusion and evaporation of water from the muscle to the surface. Since the drip loss of fillets from all treatments were similar, the observed increase in drip loss was only affected by storage duration ( p< 0.001). Therefore, the choice of storage regime on whole fish seems unlikely to affect the drip loss of cold-smoked fillets. Comparison of salt uptake indicated that iced fish had a consistent salt content at 0.1%, while RSW held fish almost doubled from 0.1 to 0.2% during 4 days of storage, and further gained to 0.3% after RSW removal (Fig. 2d, storage days: p< 0.001, treatment: p = 0.05). The effect of storage duration was also significantly influenced by the treatment method (interaction: p< 0.001). The results consonates with other studies where chinook salmon had a salt content of 1.1% after 7 days in RSW and 0.1% on ice 7. Himelbloom et al. 11 further showed that salt content of pink salmon stored in chilled seawater (CSW) increased to 0.5% during 10 days of storage, while iced salmon maintained at 0.1%. In RSW systems, the addition of salt in water causes the structure of pure water to be disrupted as salt dissociates into Na + and Cl − ions, increasing the ion–dipole interaction between salt and water. The salt gained on day 7 observed in the present study after the removal of RSW possibly signifies that the retained Na + and Cl − ions from seawater that was absorbed continued to diffuse into the fish muscle and binded with the muscle proteins. As such, the drip lost after removal of fish from RSW probably contains mainly water, likely from the free water located outside the myofibrillar network that can be easily lost from the tissue. Nevertheless, the application of salt uptake in Atlantic salmon has been considered relatively unimportant due to its large size and subcutaneous fat layer which hinders salt migration 22. Hence salt uptake during RSW storage is not considered a problem as it is also dependent on other factors like species, lipid and salt content, temperature, physiological state and storage duration 10. Espe, M. et al. Interactions between ice storage time, collagen composition, gaping and textural properties in farmed salmon muscle harvested at different times of the year. Aquaculture 240, 489–504. https://doi.org/10.1016/j.aquaculture.2004.04.023 (2004). Rosenlund, G., Torstensen, B. E., Stubhaug, I., Usman, N. & Sissener, N. H. Atlantic salmon require long-chain n-3 fatty acids for optimal growth throughout the seawater period. J. Nutr. Sci. 5, e19 (2016). Sanden, M., Stubhaug, I., Berntssen, M. H. G., Lie, Ø. & Torstensen, B. E. Atlantic salmon ( Salmo salar L.) as a net producer of long-chain marine ω-3 fatty acids. J. Agric. Food Chem. 59, 12697–12706 (2011).

Himelbloom, B. H., Crapo, C., Brown, E. K., Babbitt, J. & Reppond, K. Pink salmon ( Oncorhynchus Gorbuscha) quality during ice and chilled seawater storage. J. Food Qual. 17, 197–210. https://doi.org/10.1111/j.1745-4557.1994.tb00143.x (1994). Briones, V., & Aguilera, J. M. (2005). Image analysis of changes in surface color of chocolate. Food Research International, 38, 87–94. Roth, B. et al. Crowding, pumping and stunning of Atlantic salmon, the subsequent effect on pH and rigor mortis. Aquaculture 326–329, 178–180. https://doi.org/10.1016/j.aquaculture.2011.11.005 (2012).NRC Nutrient Requirements of Fish and Shrimp (National Academies Press, 2011); https://doi.org/10.17226/13039 Froehlich, H. E., Jacobsen, N. S., Essington, T. E., Clavelle, T. & Halpern, B. S. Avoiding the ecological limits of forage fish for fed aquaculture. Nat. Sustain. 1, 298–303 (2018).

There was no effect of treatment on the colour of raw fillets (L*: p = 0.478; a* p = 0.823; b*: p = 0.106). The RSW fish were slightly lighter and less yellowish in colour than iced. A common problem associated with RSW stored fish is the bleaching of fillets which may hinder its market value 10. Bleaching was not seen in the fillets, as adjacent to the findings of Erikson et al. 12 who found that continuous storage of fish in RSW did not lead to lighter fillets. By visual observation, gill colour from the RSW fish seemed to be grayer and less reddish in colour which could be a more obvious evidence of bleaching, as also reported in RSW-stored cod for 4 days 29 and RSW-stored ocean pearch for several days 30. This was further verified by the present study when the lightness observed on day 7 for the RSW fish was significatly higher ( p = 0.001; ice: 36.2 ± 3.2; RSW: 41.1 ± 3.4; control: 39.4 ± 3.2), while redness lower ( p = 0.001; ice: 18.1 ± 3.2; RSW: 13.2 ± 2.5; control: 16.0 ± 4.4) than both the iced and control fish. Nevertheless, quality of fillets from RSW stored fish are still considered highly acceptable with its shelf life surpassing traditionally iced fish 31. On 13th of May 2019, 77 Atlantic salmon ( S. salar) were obtained from a fish slaughtering facility from the west coast of Norway. Fish were pumped, electrically stunned, automatically slaughtered, gutted and thoroughly bled (temperature: 3.9 ± 1.1 °C, weight: 4.5 ± 0.9 kg, condition factor: 1.4 ± 0.2). An 800-L polyethylene fish chilling tank containing RSW (− 0.60 to − 0.88 °C) was also obtained from the same facility.

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Cardinal, M. et al. Sensory characteristics of cold-smoked Atlantic salmon ( Salmo salar) from European market and relationships with chemical, physical and microbiological measurements. Food Res. Int. 37, 181–193. https://doi.org/10.1016/j.foodres.2003.12.006 (2004). Duun, A. S. Superchilling of muscle food. Storage stability and quality aspects of salmon (Salmo salar), cod (Gadus morhua) and pork. Norwegian University of Science and Technology (2008). Marty-Mahé, P., Loisel, P., Fauconneau, B., & Haffray, P. (2004). Quality traits of brown trout (Salmo trutta) cutlets described by automated color image analysis. Aquaculture, 232, 225–240. Kang, S. P., East, A. R., & Trujillo, F. J. (2008). Colour vision system evaluation of bicolour fruit: A case study with [`]B74¢ mango. Postharvest Biology and Technology, 49, 77–85.

There was no difference in textural quality among the smoked products neither in storage day ( p = 0.120) nor treatment per se ( p = 0.635), while the compression force was significantly affected by the fillet height ( p< 0.001). In terms of gaping, the control fish had the highest gaping score on the last sampling day (ice: 1.0 ± 0.7; RSW: 1.0 ± 1.0; control: 2.0 ± 1.1). Therefore, the effect of treatment method on both texture and gaping seemed to apply only to the unprocessed fillets in this experiment. Hansen et al. 35 stated that textural deterioration occurs in cold-smoked salmon due to autolytic deterioration of tissue which develops rancid and oxidised off-flavours. However, the decline in textural properties for all smoked salmon groups was not apparent in our study, and is in agreement with earlier studies which found that shear force of cold-smoked salmon was stable during cold storage 36, 37. Enzyme and microbial activity Tocher, D. R. Omega-3 long-chain polyunsaturated fatty acids and aquaculture in perspective. Aquaculture 449, 94–107 (2015). Misimi, E., Mathiassen, J. R., & Erikson, U. (2007). Computer vision-based sorting of atlantic salmon (salmo salar) fillets according to their color lever. Journal of Food Science, 71, s30–s35. This study presented several quality parameters examined on salmon stored in RSW throughout the whole supply chain. In comparison to traditional chilling methods, whole fish stored in RSW had an overall increase in water and salt uptake, with better WHC before filleting. After filleting, better gaping scores, softer texture, lower cathepsin B + L activity and higher microbiological growth were observed. Although the raw fillets from RSW fish had a softer texture, this was likely unaffected by the enzymatic process of cathepsins B + L causing postmortem degradation. The microbiological analysis on raw fillets suggested that RSW fish had a shorter shelf life, but this is not representative of commercial practices due to the experimental scale. Drip loss and colour of both raw and smoked fillets from the 2 treatments were comparable, and storage duration was the main determinant affecting these parameters. These results indicate that RSW-stored fish is a viable method in minimizing the need for ice storage and land transportation, thereby introducing economical benefits and contributing to a positive impact on the environment. The idea of shifting fish slaughter from land to sea further introduces several advantages including reduced transportation costs, reduced fish diseases and mortality, increased slaughtering capacity and improved fish welfare. Therefore, the cutting-edge concept of slaughter vessels can provide great potential to increase its competitive advantage in the salmon industry. Industries seeking to understand more about the quality changes during storage of fish in RSW tanks, how this differs from the traditional chilling method on ice and how this affects fillet quality after primary and secondary processing to cold-smoked fillets can consider the results of this study during the formularization and streamlining of their processes. Jeffriess, B. A Review of Tuna Growth Performance in Ranching and Farming Operations (ASBTIA, 2015).

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Smith, I., & Furness, T. (2006). Improving traceability in food processing and distribution. Abington, Cambridge, England: Woodhead. Contradictory results have been reported when fish were stored in RSW and on ice possibly due to the difference between laboratory and industrial based experimental scale 43. The initial microbiological activity observed right after slaughter further suggested that the fish might have blood remnants remaining in the flesh after being bled in the bleeding tanks at the land-based slaughtering facility. The addition of seawater and seawater ice into the tank was also manually done as compared to mechanical procedures in commercial settings. Hence the rate of spoilage for the RSW fish was more pronounced in this study which affirms how easy contamination can occur. However, these problems should not arise in commercial settings due to more stringent rules with regards to filtering, ozonating and chlorinating of process water. Industries also use CO 2-based RSW tanks which improves energy efficiency, suppresses bacterial growth, and enhances sensory attributes of fish 8. Good hygiene has been posed as a possible challenge posed by closed RSW tanks as spoilage of fish may affect the entire catch 44. Therefore, a well-designed RSW system is important and proper considerations must be implemented including good insulation of tanks, evenly distributed and controlled temperatures, constant supply of clean seawater and adequate cleaning and disinfection of the factory after every harvest 10. This ensures good recirculation of water, lessens microbial spoilage, maintains good quality and offers the flexibility for fishing vessels to travel to further distances. Ozbay, G., Spencer, K. & Gill, T. A. Investigation of protein denaturation and pigment fading in farmed steelhead ( Onchorhychus mykiss) fillets during frozen storage. J. Food Process. Preserv. 30, 208–230. https://doi.org/10.1111/j.1745-4549.2006.00060.x (2006).