Mercury levels in commercial mid-trophic level fishes along the Portuguese coast – Relationships with trophic niche and oxidative damage

•Commercial fish as bioindicators of mercury levels in mid-trophic levels.•Mercury levels in tissues were much lower than the limits set by European Union law.•Mercury levels showed no relationships with oxidative damage.•Trachurus spp. differed from S. pilchardus and Scomber spp. in trophic niche.•Oxidative stress biomarkers showed differences between sampling areas and seasons.