SPECIAL SECTION: ANGLING FOR DINOSAURS Comparison of Two Biometric Methods for Nonlethal Sex Determination of Alligator Gars Dusty L. McDonald* Texas Parks and Wildlife Department, Corpus Christi Management District, 9892 FM 3377, Mathis, Texas 78368, USA J. Warren Schlechte and Daniel J. Daugherty Texas Parks and Wildlife Department, Heart of the Hills Fisheries Science Center, 5103 Junction Highway, Mountain Home, Texas 78058, USA Abstract McDonald et al. (2013) proposed a morphometry-based method for nonlethal sex prediction of the Alligator Gar Atractosteus spatula. Their method correctly predicted the sex of 93% of the males and 72% of the females tested, but failed to predict the sex of 21% of the fish. The authors noted that the method was primarily developed using immature fish from a single Texas population and recommended further validation and refinement using fish of diverse sizes from a broad range of systems. We tested the McDonald et al. (2013) method using 149 fish (standard length, 707–1,920 mm) collected from seven Texas populations. Sex prediction based on the McDonald et al. (2013) method improved for both sexes (98% of males and 86% of females); however, sex still could not be predicted for 29% of the fish. As a result, we explored the use of a cross-validated, discriminant function analysis (DFA) using the morphomet- ric ratios developed by McDonald et al. (2013) to improve our ability to predict the sex of sampled fish. The DFA facilitated probability-based predictions of all fish sampled and had an overall accuracy of 84% for males and 89% for females. Logistic regression based on the success of the DFA suggested that fish >1,100 mm SL are easier to identify using the morphometric ratios (>90% accuracy), likely because changes in body shape are associated with sexual maturity. In addition to predicting the sex of all of the fish sampled, the DFA approach facilitated calculation of the probability of correct discrimination for each fish. Due to these advantages, along with its comparable accu- racy, we recommend the use of the DFA approach over the method proposed by McDonald et al. (2013) for predict- ing the sex of Alligator Gars. The ability to distinguish male and female fish has many applications, including the calculation of sex ratios and sex-specific estimates of growth, age, and size struc- ture. The development of nonlethal, rapid assessment tech- niques for identifying sex is preferred, as they can also be used in the field for general monitoring, the selection of broodfish for stocking programs, and trap-and-transfer reintroduction efforts. Management and conservation efforts for the Alligator Gar Atractosteus spatula could benefit from the development of an accurate, nonlethal, field-based technique to identify sex. Once abundant throughout much of the southeastern United States, Alli- gator Gars are now considered rare or extirpated over much of their historic range (Jelks et al. 2008). Recent interest in trophy angling for the species and improved understanding of the importance of top predators to aqua- tic ecosystem health have spurred efforts to sustainably manage existing stocks and rehabilitate or repatriate depressed and extirpated populations. McDonald et al. (2013) found that the snouts of female Alligator Gars are longer than those of similarly sized males, whereas the anal fin base lengths of males are *Corresponding author: dusty.mcdonald@tpwd.texas.gov Received February 8, 2017; accepted August 7, 2017 Transactions of the American Fisheries Society © 2018 American Fisheries Society ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1002/tafs.10046 1