## 2 Methodology  
### Life-Cycle Assessment  
An LCA is a standardised method that offers a holistic approach to assess the environmental impact related to the released emissions and energy used throughout the life-cycle of a product, process, or system [80]. According to ISO guidelines (ISO 14040) [81], an LCA has four fundamental phases: goal and scope, inventory analysis, impact assessment, and interpretation.  
The first phase of an LCA represents the definition of its goal, scope, functional unit, system boundary, and the identification of data and impact categories. The Functional Unit (FU) is used to compare the investigated power systems on the same basis, and for this assessment, the FU is the lifetime of a ship, i.e., 20 years. The investigated emissions can be divided into three groups. The first group represents emissions from the Well-to-Tank (WTT) phase, which includes raw material extraction, fuel production, and distribution to the ship. The second group are emissions released during the ship's operation (the Tank-to-Wake (TTW) phase). The third group of emissions belongs to the Manufacturing (M) phase, related to the manufacturing process of the major elements of a power system (engine, fuel cell, battery, etc.).  
Emissions from the WTT and M phases were obtained using the LCA software GREET 2022 [82], whose database contains many products (fuels) and stationary and transportation processes related to their life-cycles. The TTW phase refers to the ship's operation, i.e., fishing activities. During that time, fuel is used onboard the ship for its propulsion, gear operation, and other activities (hotelling, refrigeration of fish, etc.). Fuel combustion resultsin tailpipe emissions, \(E_{TTW}\) (kg), which are calculated by multiplying emissions factors, \(EF\) (kg gas/kg fuel), by fuel consumption, \(FC\) (kg), according to the following equation:  
\[E_{TTW}=EF\cdot FC\;, \tag{1}\]  
where \(E\)Fs are obtained from [22, 83, 84, 85] and are presented in Table 3.  
Total life-cycle emissions, \(E_{i}\) (kg), are calculated by summing the emissions of a particular gas \(i\) (e.g., CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\)O, SO\({}_{X}\), NO\({}_{X}\), PM) from each LCA phase, namely the M, WTT, and TTW phase:  
\[E_{i}=E_{WTT,i}+E_{TTW,\;i}+E_{M,i}\;. \tag{2}\]  
The third phase of an LCA is life-cycle impact analysis, which was carried by means of GREET 2022.  
Global Warming Potential, \(GWP\), (kg CO\({}_{2}\)-eq), was calculated by using CO\({}_{2}\)-equivalent (CO\({}_{2}\)-eq) factors over 100 years (CO\({}_{2}\): 1; CH\({}_{4}\): 36; N\({}_{2}\)O: 298), as in the following equation [86]:  
\[GWP=1\cdot E_{CO_{2}}+36\cdot E_{CH_{4}}+298\cdot E_{N_{2}O}\;. \tag{3}\]  
The contribution of a particular gas to acidification was evaluated by calculating Acidification Potential, \(AP\) (kg SO\({}_{2}\)-eq), obtained by multiplying the emissions of an acidifying gas by SO\({}_{2}\)-equivalence factors [86]:  
\[AP=1\cdot E_{SO_{X}}+0.7\cdot E_{NO_{X}} \tag{4}\]  
Aerosol Formation Potential, \(AFP\) (PM2.5-eq), was obtained by multiplying emission quantities by PM2.5 equivalence factors (PM10: 0.5; SO\({}_{X}\): 0.54; NO\({}_{X}\): 0.88) [86]:  
\[AFP=0.5\cdot E_{PMI0}+0.54\cdot E_{SO_{X}}+0.88\cdot E_{NO_{X}} \tag{5}\]