Nonlethal Sex Determination in Brook Trout

Practical methods of sex-determination in stream fishes are rare, but needed by researchers and managers. Ideal techniques are nonlethal, quick, and affordable yet allow sex determination throughout the year and at all stages on ontogeny. Although brook trout are known to be genetically (XX/XY karyotype) and morphologically distinct, there are no established practical methods to determine sex outside of spawning season. Three candidate methods are being developed and evaluated for nonlethal sex determination in brook trout using morphology and genetics. Brook trout were collected during the spawning season when sex could be determined via manual expression of gametes. Genetic samples and standardized photographs were collected for each fish. Morphological analysis of the photographs revealed substantial variation in morphology between male and female brook trout. We were able to correctly identify the sex in roughly 80% of individuals just by looking at the head shape in photographs of even the smaller, but mature fish. Additional research to identify sex-specific genetic markers for brook trout and allow nonlethal sex determination at any stage of ontogeny is ongoing.