Bottlenose dolphins eat several kinds of fish (including mullet, mackerel, herring, cod) and squid. The compostion of the diet depends very much on what is available in the area they live in and also on the season.
The amount of fish they eat depends on the fish species they are feeding on: mackerel and herring have a very high fat content and consequently have a high caloric value, whereas squid has a very low caloric value, so to get the same energy intake (calories) they will need to eat much more if they feed on squid than if they feed on mackerel or herring. On average an adult dolphin will eat 4-9% of its body weight in fish, so a 250 kg (550 lb) dolphin will eat 10-22.5 kg (22-50 lb) fish per day.