Multivariate Discrimination of Colorado Plateau Gila spp.: The “Art of Seeing Well” Revisited

Abstract

Fishery managers have long been troubled by phenotypic variation within and among Colorado Plateau Gila. The problem is twofold. From an historical perspective, there was reticence to investigate fishes long considered as “undesirable.” In a taxonomic sense, there is confusion over within- and among-species variability. We document the former, then clarify the latter by applying discriminant analysis to meristic and morphometric data collected from museum specimens. We test three hypotheses: roundtail chub G. robusta humpback chub G. cypha and bonytail G. elegans are morphologically indistinguishable; juveniles are assignable to species based upon adult characters; and putative hybrids are morphologically intermediate between parental forms. Through the use of meristic characters in a nonparametric discriminant analysis, over 95% of all adults were segregated to species. By using morphometric characters, 97% could be allocated to species. Gila robusta was easily separated from G. cypha and G. elegans. The latter were most difficult of all species-pairs to discriminate, yet field characters still segregated them at better than 95%. A discriminant function, based upon five morphometric characters, will allocate unknowns to species. Juvenile G. robusta were easily discriminated (>97%), but juveniles of G. cypha and G. elegans were often misidentified as G. robusta. Putative hybrids were generally assigned to one or the other parental form; thus, hybrid intermediacy could not be rejected. However, paucity of hybrids weakened the test. We conclude that factors most important in segregating these species are character selection, adequate analyses, and “the art of seeing well.”