## 2 Materials and Methods  
Sliced cold-smoked rainbow trout (_Oncorhynchus mykiss_) fillets and whole warm-smoked rainbow trout (_Oncorhynchus mykiss_) fillets were obtained from a local fish factory (Escamar Seafood Ltd., Kuopio, Finland). The fish products had been salted two days and smoked one day before vacuum-packing in the factory. Cold-smoked rainbow trout fillets were salted by the injection salting method, containing a salt solution and lactate solution (E326, E262). Warm-smoked rainbow trout fillets were also salted by the injection salting, containing only salt solution. The final NaCl content in the cold-smoked rainbow trout product was 2.5%, and in the warm-smoked rainbow trout product, 1.8%. Cold-smoked rainbow trout fillets were smoked 11 h and warm-smoked rainbow trout fillets were smoked 3 h with natural alder wood smoke. The smoking processes were performed using the parameters specified by the factory. On the analysis day, the fish samples were cut with sterile knives to a weight of 25.0 \(\pm\) 0.5 g and repacked in vacuum plastic bags. Before HPP treatment, an _L. monocytogenes_ suspension was inoculated into the samples. In addition, control samples without added _L. monocytogenes_ and without HPP treatment were analysed. All samples were stored at 4 \(\pm\) 2 \({}^{\circ}\)C, also during the cutting and weighing. The _Listeria monocytogenes_ ATCC 7644 strain was cultured aerobically on Tryptone Soy (TS) agar plates (LabM, Lancashire, Greater Manchester, UK). For the test, a colony was transferred from a plate to the Tryptone Soy (TS) broth solution (LabM, Lancashire, Greater Manchester, UK) and cultured at 37 \({}^{\circ}\)C overnight. On the next day the bacterial culture was pelleted, washed, and suspended in physiological saline (0.9% NaCl). The turbidity of the microbial suspension was measured with a spectrophotometer (UV-1600 PC, VWR, Leuven, Belgium) at a wavelength of 625 nm and the absorbance was set to 0.1-0.15. The suspension was further diluted 100-fold in a dilution solution (0.9% NaCl, 0.1% peptone). The microbial suspension concentration used in the test was 1 \(\times\) 10\({}^{6}\) cfu/mL and 250 \(\upmu\)L was pipetted into the fish samples. Inoculated samples were kept chilled (4 \(\pm\) 2 \({}^{\circ}\)C) until the HPP treatment on the next day. The vacuum-packed fish samples were processed by HPP in a local food factory (Torpiha Ltd., Suonenjoki, Finland). The treatments were carried out in a hydrostatic press (Uhde-350-60, ThyssenKrupp Ag, Essen, Germany), using water as the pressurising fluid. The samples were pressure-treated at three different pressures, either at 200, 400, or 600 MPa, at 4 \(\pm\) 1 \({}^{\circ}\)C for 3 min at each pressure. All treatments of samples were repeated three separate times on different days. Bacterial enumerations of the inoculated samples and control samples were analysed 1, 14, and 28 days after the HPP treatment. The sample was transferred into a sterile Stomacher bag and 225 mL of dilution solution (0.9% NaCl, 0.1% peptone) was added. The sample was stomached for 60 s at the speed 'normal' (Laboratory Blender Stomacher 400, Seward, GWB, England) and 10-fold serially diluted. Next, samples (100 \(\upmu\)L) were placed onto plates of _Listeria_-selective agar, acc. Ottaviani and Agosti (ISO) (VWR Chemicals BDH, Leuven, Belgium), in duplicate using the spread plate technique. The plates were incubated for 48 h-72 h at 37 \({}^{\circ}\)C and the colonies were counted. The detection limit of the method was 100 cfu/g, which is also the official limit of _L. monocytogenes_ according to Regulation (EC) No. 2073/2005 on commercial RTE food products, stating a product must not exceed the limit 100 cfu/g during its shelf life [19]. The data were not normally distributed, thus a nonparametric Kruskal-Wallis H test with Bonferroni correction was applied, and \(p\)-values less than 0.05 were considered statistically significant. Statistical analysis was performed by IBM SPSS Statistics version 27 software.