Conception, fetal growth, and calving seasonality of harbor porpoise (Phocoena phocoena) in the Salish Sea waters of Washington, USA, and southern British Columbia, Canada

We evaluated harbor porpoise (Phocoena phocoena (Linnaeus, 1758)) strandings in the Salish Sea to determine calving seasonality (1980–2015). A total of 443 strandings were analyzed, of which 134 were calves and 53 were neonates. Stranded calves were reported every month, but peaked in July, August, and September. Based on fetal size and an estimated fetal growth rate of 80 mm/month, mean (±SD) conception date (and range) was back-calculated to 11 October ± 30 days (16 August – 31 December) and was later than in most other studies. Using mean (±SD) length at birth (80 ± 5.8 cm), gestation was estimated to be approximately 10.8 months. Estimated birthing period was 16 July – 27 November, with a mean (±SD) birth date of 10 September (±30.7 days) and a birth length of 80.0 cm. Estimated pregnancy rate (0.28–0.29) is lower than reported in other areas and is likely an underestimate due to missed early embryos, poor postmortem condition of a large proportion of the stranded adult females, and potential biases related to the animals that strand and are available. This study of harbor porpoise reproduction and calving in the Salish Sea is the first assessment of calving seasonality for this species in the northeast Pacific Ocean.