Progesterone concentrations after implantation and the observation of live births were used to investigate the reproductive performance and timing of reproductive failure in New Zealand fur seals (Arctocephalus forsteri). Progesterone concentrations in females that gave birth were relatively low during early active gestation (8.6 ± 0.9 ng/ml) and increased significantly toward the end of the 1st trimester (14.9 ± 0.9 ng/ml). In contrast, progesterone concentrations in females that did not give birth remained low. Estimated pregnancy rates and live births varied significantly between years. In 2000, the overall reproductive success of mature females was low (25.7%) and 42.3% of reproductive failure occurred at or before implantation. In 2001 and 2002, reproductive rates were higher (56.5% and 63.5%, respectively) and reproductive failure was greatest (70% and 89.5%, respectively) in mid- to late active gestation. Reproductive failure during active gestation appears to be the most significant stage in determining reproductive success; however, in years of low reproductive success, failure before or at implantation also contributed significantly to reduced reproductive rates. The finding that significant fetal mortality occurs in late gestation reinforces the caution that the use of pregnancy rates, without consideration for the stage of gestation at which measurements were taken, can positively bias estimates of reproductive rates.
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