Assessing oocyte development and maturation in the threatened Delta Smelt, Hypomesus transpacificus

Abstract

Delta Smelt is a pelagic annual fish species endemic to the San Francisco Estuary, which is protected under U.S. Federal and California State Endangered Acts. In this study we established criteria for scoring Delta Smelt maturity based on the histological features in ovary, as well as measurements of the female sex hormone 17β-estradiol (E2) from liver tissue using radioimmunoassay. We then analyzed the reproductive status of maturing females caught from November 2011 through April 2012 (2011 year-class) by histological examination, E2 quantification, and several common fish metrics including size of oocytes, gonadosomatic index, hepatosomatic index, and condition factor. Histological scoring revealed that Delta Smelt oocytes matured in February through April 2012. The presence of postovulatory follicles with immature oocytes (Stage 1 to 3) further suggests that Delta Smelt are iteroparous, capable of reproducing multiple times during the spawning season. Hepatic E2 concentration was significantly correlated with gonadosomatic index, hepatosomatic index, size of oocytes, and maturation stage. Histological examination of ovaries combined with measures of hepatic E2 and size of oocytes provide a powerful approach to evaluate reproductive performance and maturation timing for the imperiled Delta Smelt.