Female reproductive success and calf survival in a North Sea coastal bottlenose dolphin (Tursiops truncatus) population.

Kevin P Robinson,Texa M C Sim,Graham J Pierce,Ross M Culloch,Sonja M Eisfeld,Miranda Filan,Isabel Cordoba Aguilar,Genevieve Williams,Gary N Haskins,Thomas S Bean

doi:10.1371/journal.pone.0185000

Abstract

Between-female variation in reproductive output provides a strong measure of individual fitness and a quantifiable measure of the health of a population which may be highly informative to management. In the present study, we examined reproductive traits in female bottlenose dolphins from the east coast of Scotland using longitudinal sightings data collected over twenty years. From a total of 102 females identified between 1997 and 2016, 74 mothers produced a collective total of 193 calves. Females gave birth from 6 to 13 years of age with a mean age of 8. Calves were produced during all study months, May to October inclusive, but showed a seasonal birth pulse corresponding to the regional peak in summer water temperatures. Approximately 83% (n = 116) of the calves of established fate were successfully raised to year 2–3. Of the known mortalities, ~45% were first-born calves. Calf survival rates were also lower in multiparous females who had previously lost calves. A mean inter-birth interval (IBI) of 3.80 years (n = 110) and mean fecundity of 0.16 was estimated for the population. Calf loss resulted in shortened IBIs, whilst longer IBIs were observed in females assumed to be approaching reproductive senescence. Maternal age and size, breeding experience, dominance, individual associations, group size and other social factors, were all concluded to influence reproductive success (RS) in this population. Some females are likely more important than others for the future viability of the population. Consequently, a better knowledge of the demographic groups containing those females showing higher reproductive success would be highly desirable for conservation efforts aimed at their protection.

Highlights

In long-lived mammals, such as dolphins, that produce just one infant at a time, age at first birth and the spacing between births are the primary determinants for fecundity [1]
The estimated interbirth interval (IBI) of 3.8 years was comparable to that recorded in other Tursiops populations (e.g. [20, 22,23,24,25]), Fruet et al [19] reported shorter IBIs (3.3 years, mode of 2) for animals from the southwest Atlantic
Low IBIs between 2 and 2.25 years were observed in three females with surviving calves in the present study

Summary

Introduction

In long-lived mammals, such as dolphins, that produce just one infant at a time, age at first birth and the spacing between births are the primary determinants for fecundity [1]. Animals from the MF range widely beyond the designated Special Area of Conservation (SAC) established for their protection in the inner Moray Firth in 2005 [5,6,7,8,9,10], making robust estimations of individual birth rates and calf survival difficult to resolve for this population. Integrated datasets from multiple research sites have been used to provide a best estimate of ~200 animals using the northeast coastline [11]. Both males and females are seen to range widely along the east coast [12], but females are found to be more site-faithful within the SAC and adjacent outer MF regions [7,13]. During the summer and fall, large numbers of nursing females with calves use this region, with ~90% of the known population having been recaptured over the course of a 20-year mark-recapture study [Robinson unpublished data, 14]

Methods

Results

Conclusion