 There are two major reproductive strategies of teleosts ; <unk> and <unk> . In the former , an individual breeds once after reaching maturity and then dies . This is because the physiological changes that come with reproduction eventually lead to death . Salmon of the genus Oncorhynchus are well known for this feature ; they hatch in fresh water and then migrate to the sea for up to four years before travelling back to their place of birth where they spawn and die . <unk> is also known to occur in some eels and smelts . The majority of teleost species , however , have <unk> , where mature individuals can breed multiple times during their lives . 
