Variation in body condition during the post-moult foraging trip of southern elephant seals and its consequences on diving behaviour.

Gaëtan Richard,Frédéric Bailleul,Jade Vacquié-Garcia,Christophe Guinet,Alexandre Génin,John P Y Arnould,Baptiste Picard,Joffrey Jouma'A

doi:10.1242/jeb.088542

Abstract

Mature female southern elephant seals (Mirounga leonina) come ashore only in October to breed and in January to moult, spending the rest of the year foraging at sea. Mature females may lose as much as 50% of their body mass, mostly in lipid stores, during the breeding season due to fasting and lactation. When departing to sea, post-breeding females are negatively buoyant, and the relative change in body condition (i.e. density) during the foraging trip has previously been assessed by monitoring the descent rate during drift dives. However, relatively few drift dives are performed, resulting in low resolution of the temporal reconstruction of body condition change. In this study, six post-breeding females were equipped with time-depth recorders and accelerometers to investigate whether changes in active swimming effort and speed could be used as an alternative method of monitoring density variations throughout the foraging trip. In addition, we assessed the consequences of density change on the swimming efforts of individuals while diving and investigated the effects on dive duration. Both descent swimming speed and ascent swimming effort were found to be strongly correlated to descent rate during drift dives, enabling the fine-scale monitoring of seal density change over the whole trip. Negatively buoyant seals minimized swimming effort during descents, gliding down at slower speeds, and reduced their ascent swimming effort to maintain a nearly constant swimming speed as their buoyancy increased. One per cent of seal density variation over time was found to induce a 20% variation in swimming effort during dives with direct consequences on dive duration.

Highlights

The investigation of the temporal change in foraging success of top predators provides essential information on their ecology, as well as on the distribution of their prey
No differences could be detected in the daily drift rate calculated from time–depth recorders (TDRs)-only data versus TDR–accelerometer data, indicating that drift rates were precisely estimated from TDR-only data
The results of the present study clearly demonstrate that monitoring descent swimming speed and ascent swimming effort using accelerometers combined with TDRs can provide information on changes in body density in female southern elephant seal (SES) during their post-breeding foraging trip

Summary

Introduction

The investigation of the temporal change in foraging success of top predators provides essential information on their ecology, as well as on the distribution of their prey. Head-mounted accelerometers have been shown to provide reliable information on the occurrence of prey capture events in seals during extended foraging trips (Naito et al, 2010; Suzuki et al, 2009; Viviant et al, 2010, Gallon et al, 2013). This approach, does not provide information on the size and energy content (i.e. quality) of either the prey or the associated foraging costs, such that the energy gain by the individual cannot be assessed. To assess prey quality, there is a need to estimate fine temporal changes in the condition of the seal, as well as the foraging costs

Methods

Results

Conclusion