The omega-3 content of a fish depends on the species. In herring there are about 2 g of omega-3 per 100 g. This makes herring a good natural source of omega-3 fatty acids. Tuna, salmon and mackerel are also rich in omega-3 fatty acids, although about 250 g of salmon is required for 2 g of omega-3. In other conventional types of fish, the omega-3 levels are rather low. For example, you need to consume 1.5 kg of trout to absorb 2 g of omega-3.