Fish Type
Fish owls take their prey from rivers, streams, swamps and lakes rather than the sea. All the fish the owls consume therefore are freshwater varieties. Many sources of information simply give "fish" as an element of the owls' diet, suggesting they feed indiscriminately on a range of different kinds.
Asian Owls' Diet
Blakiston's Fish Owl takes pike, trout, salmon, catfish, sculpin, lampreys, grayling and charr. A 2004 study of Tawny Fish Owls by the Raptor Research Foundation notes they also prey on farmed fish such as rainbow trout and ayu. M. Fogden, author of a chapter on fish owls in the 1992 book "Owls of the World," suggests Malaysian and Brown Fish Owls eat a similar diet, and only the owls' wide distribution prevents them competing for food.
African Owls' Diet
According to the website Biodiversity Explorer, the Pel's Fishing Owl will take catfish, bream, pike and squeakers. The Vermiculated and Rufous Fishing Owls may have similar feeding habits, but as Fogden notes, very little is known about these elusive birds.
Fish Size
Fogden notes Asian fish owls feed mainly on larger fish such as salmon or trout. The wildlife website ARKive says the Pel's Fishing Owl regularly catches fish weighing 3 to 7 ounces, but may take fish as heavy as 4 1/2 pounds.
Shellfish and Crustaceans
Fogden notes that two Asian species, Blakiston's Fish Owl and the Tawny Fish Owl, are known to eat crayfish. Many owls, including the Rufous owl, also eat freshwater crabs. ARKive includes freshwater mussels in the diet of the Pel's Fishing Owl.
Other Prey
Although fish forms a significant part of the birds' diets, fish owls do not seem to be fussy about taking other prey too, such as small mammals, birds, snakes and insects. Fogden notes that a Brown Fish Owl was seen scavenging from the body of a dead crocodile. ARKive says the Pel's Fishing Owl will even catch small live crocodiles to eat.