Morphometric variation and sexual dimorphism of endangered longfin smelt Spirinchus thaleichthys in the San Francisco Estuary

Longfin smelt Spirinchus thaleichthys is a migratory forage fish that lives in coastal and estuarine habitats of the northeast Pacific Ocean. Longfin smelt in the San Francisco Estuary (California, USA) represent the southernmost breeding population that is both genetically distinct and federally endangered. Despite extensive research on the spawning and population biology of this species, little is known about sexual and geographic morphological patterns, which are essential for understanding reproductive biology and population dynamics. We hypothesized morphological differences between sexes and spawning regions, which are vital for reproductive and ecological adaptations. We used geometric morphometric techniques and traditional morphological measurements to analyze images of 225 longfin smelt collected in 2021 and 2022. Results revealed significant morphological differences between males and females, with females exhibiting larger pre-anal, pre-pectoral, and pre-dorsal fin lengths, while males exhibited larger caudal peduncle depth, dorsal fin, anal fins, and body height. Males also exhibited more convex bodies, more downturned mouths, and greater plumpness around the anal fin than females. Overall, geographic variation was weak and should be interpreted as exploratory rather than indicative of population divergence. These findings confirm significant sexual dimorphism in longfin smelt, providing a valuable, non-invasive means of identifying the sex of individuals. Furthermore, the results raise important hypotheses about the causes and consequences of this sexual, and potential spatial, morphological variation.