Life history of harbor porpoises (Phocoena phocoena) in Scottish (UK) waters

Abstract

Life history parameters were determined for stranded and bycaught harbor porpoises (Phocoena phocoena) from Scottish (UK) waters (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). Fetal growth rate was 84.4 mm/mo and mean size at birth was 76.4 cm (range 65-88 cm). Males and females had a similar range of body lengths (65-170 cm and 66-173 cm, respectively), although asymptotic lengths were higher in females than males (approximately 158 cm and 147 cm, respectively). Nonpregnant females were significantly lighter, in relation to their length than males. Maximum estimated age was 20 yr for both sexes. Age at sexual maturity (ASM) was estimated as 4.35 yr in females and 5.

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

“…Lag phase duration (t 0 ) was then estimated utilizing the equation: t 0 = 7.25m birth 0.19 , where m birth is mean birth mass in grams (Calder 1982). Other studies (Bӧrjesson and Read 2003;Learmonth et al 2014) calculated mean birth mass by using the mean mass overlap of the smallest neonates with the largest fetuses; however, in the present study most fetuses were not weighed so the mean of the 10 smallest neonates was used to estimate mean birth mass. Probable dates of birth for fetuses were estimated from calculated conception dates and gestation period, assuming growth rate is the same for all fetuses.…”

“…In the Salish Sea, conception (11 October) peaks approximately three, two, and one and one-half months later than the Bay of Fundy/Gulf of Maine (6 July), the Kattegat, Skagerrak and North Seas (25 July), and Baltic Seas (18 August), respectively (Börjesson and Read 2003) and two months later than harbor porpoise in Scottish Waters (4 August) (Learmonth et al 2014). Peak conception dates have not been determined for this species in California; however, a 5-cm fetus was observed in August in a study of 332 harbor porpoises from central California waters (Hohn and Brownell 1990), indicating conception in that animal occurred no later than late July or early August.…”

“…Following steps taken in other studies, samples from the period of conception were excluded from this calculation to avoid missing the existence of early embryos (Read 1990;Read and Hohn 1995;Learmonth et al 2014). To determine the amount of bias that could result from missing early embryos, the calculation was repeated using mature females from the whole year (i.e., not excluding the conception period females).…”

“…The conception date was calculated by subtracting the estimated fetal age (defined as t days) from the date on which the animal was first reported stranded (Julian date) (Bӧrjesson and Read 2003;Learmonth et al 2014). Therefore, fetal age at birth (i.e., length of gestation) is defined as (L t /u) * 30.5 + t 0 , where, u is the fetal growth rate (equal to the slope of the linear regression of fetal length [mm] on month), L t is the mean length at birth (mm), 30.5 is the mean number of days in a month, and t 0 is the estimated duration of the lag phase.…”

“…Information on the reproductive biology of harbor porpoises is known primarily from stranded specimens. In the western North D r a f t 5 Atlantic, reproduction is considered seasonal with yearly peak parturition occurring in mid-May in the Bay of Fundy, late spring-early summer in the Gulf of Maine (Read and Hohn 1995;Börjesson and Read 2003), and in the eastern North Atlantic in May through July (Learmonth et al 2014). However, in California, females appear to be on a two-year calving cycle (Read 1999).…”