## 2 Results and Discussion
### Determination of Heavy Metals and Mycotoxins in Salmon Side Streams  
The concentrations of As, Hg, Cd, and Pb in salmon muscle, heads, viscera, skin, and tailfins are shown in Table 5. Mean concentration ranges, expressed as \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{g}\) of wet weight (ww), were 0.4186-0.6922, 0.0095-0.0408, 0.0004-0.0104, and 0.0071-0.0859 for As, Hg, Cd, and Pb, respectively. For all salmon side streams, the most abundant element was As, whereas the lowest concentration was observed for Cd. There is a lack of information in the literature on heavy metal contents in salmon discards. For instance, one study reported liver Hg accumulations in four wild species of Pacific salmon [31]. The results (0.120-0.192 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{g}\), ww) were higher than those found in the present study for viscera samples, which include more organs than the liver. The contents of As, Hg, Cd, and Pb in several fish side streams of sea bass, sea bream, and meager have also been \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sequence\_Modification**} & **Sequence in** & **Identity of Sequences with** \\  & **BIOPEP-UWM Database** & **Antioxidant Potential** \\ \hline DVDVRVGGEMKAG & GGE & ID 8114 peptide from sardinella \\  & & by-products \\ GEGGSSAAKPSG & KP & ID 8217 \\ GVLLNTAVSGAKDP & KD & ID 8134 peptide from dried bonito \\ HAKKAELFELRVK & EL & ID 7888 \\ QPPPRGPPPPP & GPP & ID 8987 \\ AGGGGPEPGGAGTPGAAAAP & GAA & ID 8983 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._  
Figure 4: Chromatogram of total ion counts of salmon viscera protein extracts, obtained through conventional stirring and pressurized liquid extraction (PLE). described [18; 19; 23; 25]. The arsenic levels in the viscera (1.867-2.587 \(\upmu\)g/g, ww) of these fish species were higher than those in the salmon viscera. The data available on toxic elements in fish usually refer to edible muscle due to the potential health risk for consumers. In this sense, levels of Cd and Pb in 21 samples of smoked salmon from a Polish market were determined [32]. The results were on the order of 0.0040-0.0196 \(\upmu\)g/g (ww) for Cd and 0.0109-0.1559 \(\upmu\)g/g (ww) for Pb, both of which are considered safe for consumers. In addition, As, Hg, Cd, and Pb contents in fresh salmon muscle were evaluated [33; 34]. It should be noted that the limits for heavy metals in fish side streams are not currently regulated. Therefore, the safety assessment could be based on the limit values established for edible muscles of fish (\(\upmu\)g/g): 13.5 for As, 0.5 for Hg, 0.05 for Cd, and 0.30 for Pb [23; 25; 35]. According to this, the toxic elements analyzed in all salmon side streams in this study are below the limits set by authorities and could be considered safe for consumers in terms of As, Hg, Cd, and Pb content. [PERSON] et al. [33] showed a trend towards a decrease in As and Hg content in farmed Atlantic salmon, which was related to the decline in the use of fish meal and fish oil in commercial fish feed. However, the replacement of marine ingredients by others of plant origin can lead to the presence of contaminants such as mycotoxins in both aquafeeds and fish tissues. In this way, [PERSON] et al. [36] conducted a toxicokinetic study of deoxynivalenol (DON) and ochratoxin A (OTA) mycotoxins in farmed salmon fed with contaminated feeds for 8 weeks. The authors observed an even distribution in the liver, kidney, brain, skin, and muscle for DON, as well as a distribution mainly in the liver and kidney for OTA. According to this, the possible occurrence of mycotoxins in the muscle, head, viscera, skin, and tailfin of farmed salmon was investigated in the present study. Through a simultaneous multi-mycotoxin evaluation using a non-targeted screening approach, no mycotoxins or related metabolites were identified in salmon side streams. These results are in agreement with those found by [PERSON] et al.