Rainbow Trout Vent Anatomy: a Non-Lethal, Experimental Method to Identify Sex

Classic stock assessment calls for the estimation of abundance, size structure, and sex-ratio of a given population. Determining sex, however, when fishes are not in spawning condition (ripe/gravid) can be challenging without destructive sampling and the examination of internal reproductive organs. For stocks that have low abundance or are protected by conservation laws, destructive sampling may be undesirable or prohibited. In September 2010, during preliminary stock assessment prior to the permitting of a small hydroelectric project in Southeast Alaska, two forms of rainbow trout (Oncorhynchus mykiss) vent anatomy were noted : “keyhole shaped” and “ellipsoid”. Most captured RBT (49 out of 51) were placed in one of the two categories; observations on the remaining three were inconclusive. Trout with a keyhole-shaped vent (n=21) are possible females and RBT with ellipsoid-shaped vents (n=28) are possible males but no effort was made to verify this through destructive sampling.  Other techniques, including ultrasound and genetic sampling have been developed, however, visual examination by trained observers may represent a low-cost alternative to these techniques.