Use of Veterinary Ultrasound to Identify Sex and Assess Female Maturity of Pacific Halibut in Nonspawning Condition

Abstract

Ultrasound was used for sex identification and female maturity assessment of Pacific halibut Hippoglossus stenolepis ranging from 64- to 144-cm fork length collected during their nonspawning period (boreal summer) in the central and eastern Gulf of Alaska. Ultrasonic sex identification employed inspection of gross morphology and gonadal morphometrics, the latter involving determination of gonad length (maximum posterior gonad extent [MPGE]) and testing for difference between sexes. Morphometric analysis was refined further for females by removing ovaries, macroscopically determining maturity status, and comparing MPGE and proportional ovarian length (POL) among maturity stages. Ultrasonically determined gross morphology was sufficient to achieve accurate sex identification in even the smallest individuals examined. Ovaries were rounded anteriorly with no apparent internal structure, whereas testes were sickle-shaped with characteristic internal structure formed by the junctures between testicular lobes. Maximum posterior gonad extent was significantly greater for females but did not provide accurate sex determination because values overlapped considerably between sexes; 80% of all fish (n = 147) exhibited MPGE within the range of overlap. Values of both MPGE and POL overlapped considerably between mature and immature females, but threshold values were identified for both metrics above which all individuals were mature. Only for POL was a lower threshold apparent, below which all individuals were immature. Considering MPGE and POL simultaneously suggested a possible means for identifying first-time spawners and may warrant additional investigation.

Received January 13, 2011; accepted June 2, 2011

ACKNOWLEDGMENTS

The authors thank the captains and crews of the fishing vessels Van Isle, Waterfall, and Kema Sue, as well as Bruce Biffard, Tracee Geernaert, and Andy Vatter, for providing vital at-sea support. Reid Tyson provided training in ultrasonic techniques, and Claude Dykstra, Eric Soderlund, and Evangeline White provided logistical support without which vessel operations would not have been possible. The quality of the manuscript was greatly improved by thoughtful commentary provided by three anonymous reviewers.