Large halibut weighing over 100 kg contain very high levels of undesirable substances, according to a new survey. Halibut weighing less than 40 kg, however, which accounted for more than 80 per cent of the analysed fish, contained low levels of undesirable substances like mercury and organic contaminants. For 85 per cent of the halibut analysed, the levels of undesirable substances were below the EU maximum level in all parts of the fillet.