## 4 Results and Discussion
### Estuary and Gulf of St. Lawrence Mussel Watch in the North Atlantic Context  
The Hg concentrations in mussel soft tissues ranged from 0.06 to 0.51 ug g\({}^{-1}\) dw (n = 504), with a grand mean of 0.17 \(\pm\) 0.08 ug g\({}^{-1}\) dw (\(\pm\) 1 standard deviation), and a median of 0.16 ug g\({}^{-1}\) dw. In 1977-1979, the Hg concentrations were very similar with a mean of 0.16 \(\pm\) 0.05 ug g\({}^{-1}\) dw, and a median of 0.15 ug g\({}^{-1}\) dw (n = 442) [33]. Very few Mussel Watch programs have been carried out uninterrupted for longer than a few years with the notable exception of the US and French monitoring programs that have been running yearly since the 1980s [34; 35]. The median concentration in the St. Lawrence system is slightly higher than the most recent available medians: 0.11 ug g\({}^{-1}\) dw (2005-2012, n = 298) for the US Mussel Watch and 0.12 ug g\({}^{-1}\) dw (2000-2004, n = 303) for the French Mussel Watch programs [34; 35]. It is similar to the annual median concentrations of the French coasts  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Period** & **h/L** & **I/L** & **L vs M** & **AGR** \\ \hline \multicolumn{5}{c}{_South shore LSLE_} \\
1977–1979 & 0.52 \(\pm\) 0.03 & 0.40 \(\pm\) 0.07 & L = 2.62 M\({}^{0.28}\) & 0.57 \\
2016–2019 & 0.49 \(\pm\) 0.03 & 0.42 \(\pm\) 0.04 & L = 2.92 M\({}^{0.25}\) & – \\ \multicolumn{5}{c}{_Gaspe Peninsula_} \\
1977–1979 & 0.50 \(\pm\) 0.05 & 0.42 \(\pm\) 0.03 & L = 2.85 M\({}^{0.25}\) & – \\
2016–2019 & 0.49 \(\pm\) 0.05 & 0.41 \(\pm\) 0.04 & L = 2.97 M\({}^{0.26}\) & – \\ \multicolumn{5}{c}{_North shore LSLE_} \\
1977–1979 & 0.50 \(\pm\) 0.05 & 0.42 \(\pm\) 0.03 & L = 2.85 M\({}^{0.25}\) & \\
2016–2019 & 0.51 \(\pm\) 0.03 & 0.42 \(\pm\) 0.07 & L = 2.88 M\({}^{0.29}\) & 0.52–0.60 \\ \multicolumn{5}{c}{_Moyenne Côte Nord_} \\
1977–1979 & 0.52 \(\pm\) 0.04 & 0.42 \(\pm\) 0.05 & L = 2.78 M\({}^{0.28}\) & 0.57–0.65 \\
2016–2019 & 0.52 \(\pm\) 0.04 & 0.42 \(\pm\) 0.03 & L = 2.89 M\({}^{0.28}\) & – \\ \multicolumn{5}{c}{_Baie des Chalers_} \\
1977–1979 & 0.52 \(\pm\) 0.04 & 0.42 \(\pm\) 0.05 & L = 2.67 M\({}^{0.26}\) & 0.56–0.67 \\
2016–2019 & 0.56 \(\pm\) 0.04 & 0.43 \(\pm\) 0.04 & L = 2.60 M\({}^{0.28}\) & – \\ \multicolumn{5}{c}{_Northumberland/Cape Breton_} \\
1977–1979 & 0.57 \(\pm\) 0.06 & 0.42 \(\pm\) 0.05 & L = 2.72 M\({}^{0.23}\) & 0.40–0.57 \\
2016–2019 & 0.57 \(\pm\) 0.06 & 0.42 \(\pm\) 0.04 & L = 2.54 M\({}^{0.28}\) & – \\ \multicolumn{5}{c}{_Estuary and Gulf of St Lawrence_} \\
1977–1979 & 0.53 \(\pm\) 0.06 & 0.43 \(\pm\) 0.06 & L = 2.80 M\({}^{0.26}\) & 0.40–0.57 \\
2016–2019 & 0.52 \(\pm\) 0.07 & 0.43 \(\pm\) 0.04 & L = 2.83 M\({}^{0.27}\) & – \\ \hline \end{tabular}
\end{table}
Table 1: Allometric relationships in the mussel shell shape and their changes between 1977–1979 and 2016–2019 periods. L: length (cm), l: width (cm); h: height (cm); M: mass (g). All the relationships were statistically significant with a \(p<0.001\). AGR: Absolute growth rate (cm y\({}^{-1}\)) from ref. [17]. 1977–1979 statistics were calculated on 1222 individual mussels vs. 504 in 2016–2019.  
Figure 2: Length-mass relationship of _Mytilus edulis_ shell from six hydro-ecological regions of the St. Lawrence Estuary and Gulf (Figure 1). Mussels sampled in 1977–1979 (n = 1222) and in 2016–2019 (n = 504) periods.  
Mussel Watch for the period 1980-94 (0.15 \(\mu\)g g\({}^{-1}\) dw, n = 937) [36; 37]. In summary, the current median Hg concentrations of the St. Lawrence Mussel Watch is similar to the long-term representative surveys carried out along the Atlantic shores in the last 40 years.