## 1 Introduction  
The high-pressure processing (HPP) technique for inactivating spoilage and pathogenic bacteria was already introduced during the 19 th century [1], but industrial applications were not launched until 1990 [2]. Thus, HPP is not a new method for processing and preserving food products. Additionally, the value of HPP has recently increased in response to consumers' increased interest in minimal processed food with healthy nutritional components. HPP is a nonthermal pasteurisation method for effectively inactivating vegetative pathogenic and spoilage microbes by hydrostatic pressure [3]. The pressure range in the process is between 100-1000 MPa depending on the food product [4, 5]. The challenge of HPP is to ensure food microbiological safety for each individual product. The efficiency of HPP treatment depends on several factors, such as properties of the food matrix, parameters of the treatment, and microbe to be inactivated [3]. Thus, HPP must be customised based on several factors. The main extrinsic factors influencing microbial inactivation in food products are the target pressure and the holding time [3]. Usually, a higher pressure and longer holding time increase inactivation of microbes [5, 6]. Certain parameters, such as a pressure between 400-600 MPa for 2-7 min at room temperature, have been observed to be the most effective at decreasing vegetative spoilage microbes and foodborne pathogens, in some cases four logarithmic times or more [4]. According to the European Food Safety Authority (EFSA) Panel on Biological Hazards [3], a disadvantage is the absence of an indicator to ensure that each HPP treatment is sufficiently effective under factory conditions. Therefore, further studies are required on the effectiveness of HPP to inactivate, e.g., different amounts of _Listeria_ in ready-to-eat (RTE) fish products to achieve general minimum processing parameters, thereby ensuring microbiological safety [3]. _Listeria_ bacteria are destroyed at 65 \({}^{\circ}\)C or higher, but can also contaminate food after heating. Fish and fish products, meats, cheeses, and raw vegetables have been reported to be the most likely foods in which _Listeria_ is present [7,8]. For example, in 2020 listeriosis was the fifth most common zoonosis in humans in the EU, and 1876 _Listeria monocytogenes_ invasive human cases were confirmed [8]. In the same year, _L. monocytogenes_ was found in 4.3% of RTE fish and 4.1% of RTE fishery products in the EU area [8]. According to a study by [PERSON] et al. [9], _L. monocytogenes_ contamination was detected in 6.5% of the cold-smoked salmon samples after a storage time trial. Based on a study by [PERSON] et al. [10], the main source of _Listeria_ contamination is slicing or skinning machines in the processing environment and they found _Listeria_ only in sliced RTE fish products. In addition, [PERSON] et al., [11] observed that most (80%) of the naturally _Listeria_-contaminated RTE cold-smoked salmon samples were sliced and vacuum-packed. In a study by [PERSON] et al. [12], cold-smoked salmon samples were high-pressure treated. Based on sensory, chemical, and microbiological analysis, pressure treatment at 250 MPa at 3 \({}^{\circ}\)C for 5 min and at 250 MPa at 25 \({}^{\circ}\)C for 10 min increased the shelf life of the salmon samples by two weeks compared to nontreated control samples [12]. The pressure level of 600 MPa for 5 min effectively lowered the amount of _L. monocytogenes_ (6.46 log\({}_{10}\) cfu/g, \(p<0.01\)) in mild-smoked rainbow trout fillets [13]. In addition, the treatment at 600 MPa for 1 min had a significant (\(p<0.01\)) decreasing effect on the amount of _L. monocytogenes_ in rainbow trout samples compared to nontreated control samples [13]. Despite the 600 MPa for 5 min treatment, not all _L. monocytogenes_ bacteria in samples were inactivated, and the amount of _Listeria_ increased significantly during the storage time [13].