In most farmed fish species, sexual maturation is a problem because sexually mature fish stop growing and get sick more easily. In the case of salmon farming, fish are sent for slaughter as soon as they show signs of sexual maturation.

Researchers from the Norwegian Institute of Marine Research developed a “super sperm” that only results in male salmon. “In 2012, we treated male salmon fry with estrogen to make them develop eggs. When sexually mature, they produce both eggs and sperm,” said Tom Hansen, scientist at the Institute of Marine Research. The sexually mature hermaphrodite with a male genotype (XY) was used to produce self-fertilized offspring. A quarter of the eggs resulted in what scientists call “super males” (YY), fish that only get male offspring. 

This is the first time super males are documented in Atlantic salmon, a phenomenon known in other species. “These results show that supermale Atlantic salmon are viable and fertile and can be used as a research tool to study important aspects of sexual maturation, such as to further explore the sex-dependent parental genetic contribution to age at puberty in Atlantic salmon,” researchers said.

Download the study here.