Assessing oocyte development and maturation in the threatened Delta Smelt, Hypomesus transpacificus
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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.
KeywordsDelta Smelt Reproduction Maturity stage Estradiol Oocyte size
We would like to thank Randal Baxter and his crews at the California Department of Fish and Wildlife (CDFW) for providing the Delta Smelt and Sebastian Serrato, Gary Wu, Diana Le, Andrew Peterson, Christopher Tai, Rosalyn Lam, Stephanie Buasan, and Georgia Y. Ramos at the Aquatic Health Program at UC Davis for assistance on sampling and fish processing. Funding support for this study was provided in part by the CDFW, National Marine Fisheries Service, US Fish & Wildlife Service of the Ecosystem Restoration Program Grant# E118304 (program manager, Carol Atkins at CDFW) and by US Bureau of Reclamation Grant# G12AC20079 and G15AS00018. Some of the photo images of Delta Smelt oocytes were provided by Ms. Meredith Nagal and Dr. Joan Lindberg at Fish Culture and Conservation Laboratory at UC Davis.
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