Seasonal reproduction of female round stingrays (Urobatis halleri): Steroid hormone profiles and assessing reproductive state

This study characterizes the seasonal reproductive cycle of female round stingrays (Urobatis halleri) in an open coastal site at Seal Beach, CA and a protected estuary at the Seal Beach National Wildlife Refuge (SBNWR). Female round stingrays were sampled from August 2004 to July 2006, and assessed for reproductive parameters (GSI, maximum ova diameter, pregnancy status) and sex steroid (estradiol (E2), progesterone (P4) and testosterone (T)) concentrations. E2 and P4 increased at the time of ovulation (June and July) and remained elevated until parturition (October and November); recently partruded females were observed until November. Mature females were absent from Seal Beach in August and September, the same time period that abundance of mature females peaked in the SBNWR. This aggregation of predominantly mature females in the upper reaches of the SBNWR was seasonal, and was observed from April to September. To better understand the aggregation behavior, sex steroid hormones were assayed in SBNWR females. In July and August, E2 and P4 concentrations in females at the SBNWR were 1.5-fold and 2-fold higher, respectively, than concentrations in mature females at Seal Beach, and correlated with elevated water temperature in the estuary. Pregnancy was confirmed in aggregating females by increased levels of E2 and P4 and the presence of developing embryos. Our data suggest that coastal estuaries may play a crucial role in round stingray reproduction, perhaps by providing a thermal refuge for pregnant females during gestation.