The use of ultrasonography as a tool to determine gender of the paddlefish was investigated. Paddlefish were captured by net in the Chippewa River, Wisconsin, as part of a project using telemetry to monitor migration patterns of the fish. The objective of the project was to find a non-invasive technique to identify female fish for selection of these fish for surgical implantation of a telemetry transmitter.
Fourteen paddlefish, body weights 8.6-31.5 kg, were imaged in dorsal recumbency with a 5 MHz linear transducer and portable ultrasound system (Aloka 500). Organs that were well characterized in the fish were heart, liver, stomach, spiral valve, gonad and spleen. The egg mass in the gravid female was located in the ventral coelom and easily identified. The egg mass was very sound attenuating, resulting in a characteristic hyperechoic nearfield interface with rapid decrease in signal intensity becoming hypoechoic. The testes were located dorsolaterally in the mid to caudal coelom, were hyperechoic and "L" shaped in transverse images. The gender was verified in six fish by laparotomy. Successful location and identification of gonads was correlated to size of the paddlefish (i.e., larger, sexually mature fish were easily identified).
Ultrasound guided biopsy and fine needle aspirates (FNA) were performed in a second group of 36 dead paddlefish. These fish were voluntarily submitted by sportfishing participants for ultrasound and postmortem examination. The gonad was identified by ultrasonography and gender was identified by the investigators. The biopsy and FNA samples were independently evaluated by a pathologist. The post-mortem exam confirmed the gender of the paddlefish. FNA cytology was not a reliable technique for sex identification. The biopsy technique correctly identified 19 females, 12 males and 5 samples were non-diagnostic.
This study confirmed that ultrasonography is a useful tool to determine gender of the paddlefish in field situations. It is a rapid and non-invasive procedure. It is sensitive and specific for gravid female identification. A brief learning curve should be expected when developing an expertise with this technique.