The presence of these markers allows farmers to easily sort their fish and stock their broodstock tanks with a specific ratio of males to females. When these external markers are not present, the process of stocking broodstock tanks is more difficult and time consuming. To determine the sex of fish lacking external markers, the most common technique is to gently apply pressure to the abdomen of the fish in an attempt to release oocytes or sperm. Another frequently used technique is a gonadal biopsy which involves inserting a cannula into the cloaca of the fish and applying suction on the tube to extract tissue from the gonads for examination. Both techniques are reliable, especially when the fish are in season, and the only costs are for the anesthesia and cannula.
Cannulation requires skill and experience to avoid injury to the fish.
The project required the stocking of hundreds of snakehead, Channa striatus , into broodstock tanks at a specific sex ratio. Since the snakehead does not exhibit external morphological markers which can be used to distinguish males from females, HFC requested assistance from the University of Hawaii Animal Science Department, Center for Tropical and Subtropical Aquaculture Disease Management Project, and Sea Grant Extension Service to evaluate ultrasound technology as a means to resolve the problem of determining sex in this species. HFC staff and extension personnel used this method as well as the more traditional method of cannulation to determine if either method was superior in identifying the sex of the snakehead. Snakehead were anesthetized using 300 ppm of 2-phenoxyethanol. When the probe was placed on the abdomen of a female fish, oocytes appeared as a mass of light grey dots just below the pelvic fins.