The typical prey consists of surface organisms such as small fish, squid, crustaceans and jellyfish. The storm petrel will also eat offal and oily food, often located by smell, and will follow ships. In the Atlantic, more than half the food items are zooplankton and the fish caught include small herring and sprats; whale carcasses are scavenged where available. During digestion, the plankton is quickly converted to an oily orange liquid that owes its colour to carotenoids. Larger prey items take longer to digest. The oil, rich in vitamin A, is produced by a large gland in the stomach. The Mediterranean subspecies' diet is mainly fish, particularly Mediterranean sand eels. Petrels also catch opossum shrimps from waters close to the colony. Bluefin tuna farms are exploited on the Maltese island of Filfla; birds from the large local colony feed on the unwashed food fed to the farmed tuna, a mixture of fish, squid and prawns which produces a sizeable oily slick. Small numbers of insects are caught near breeding colonies, and some plant material, including angiosperm seeds and sorrel, has been found in the stomach contents. A study on Leach's storm petrel, which consumes similar items, showed that the petrels were snipping pieces off plant leaves in flight, but it could not be confirmed that this was in the course of catching insects. Nasal glands remove excess salt from seawater consumed by the petrel as a concentrated solution excreted through the nostrils.