 Of the major groups of teleosts , the Elopomorpha , Clupeomorpha and Percomorpha ( perches , tunas and many others ) all have a worldwide distribution and are mainly marine ; the Ostariophysi and Osteoglossomorpha are worldwide but mainly freshwater , the latter mainly in the tropics ; the Atherinomorpha ( guppies , etc . ) have a worldwide distribution , both fresh and salt , but are surface @-@ dwellers . In contrast , the Esociformes ( pikes ) are limited to freshwater in the Northern Hemisphere , while the Salmoniformes ( salmon , trout ) are found in both Northern and Southern temperate zones in freshwater , some species migrating to and from the sea . The Paracanthopterygii ( cods , etc . ) are Northern hemisphere fish , with both salt and freshwater species . 
