Diving behaviors of juvenile northern and southern elephant seals

Study System and Animal Handling In 2018, 24 juvenile northern elephant seals (15 females and 9 males) from the population at Año Nuevo Reserve, CA, USA (Figure 1A; 37°5’ N, 122°16’ W) were equipped with an archival time-depth recorder (MK9, Wildlife Computers, measures time, depth, light) to record their very first trip to sea. The tags of only four individuals (4 females; mean weaning mass ± SD = 133 ± 10 kg; mean straight length from nose to tail tip ± SD = 139 ± 7 cm) were recovered when they returned to land after 229 ± 16 days at sea. Animals were sedated with an initial injection of tiletamine hydrochloride and zolazepam hydrochloride (Telazol), administered intramuscularly. Immobilization was maintained with intravenous injections of Ketamine when needed. Using quick-setting epoxy (Loctite®, Epoxy General Purpose), the MK9 tag was affixed to the fur on the center of the back. In 2014, 20 juvenile southern elephant seals (10 females and 10 males) from Kerguelen Islands, sub-Antarctic French territories (49°20′S, 70°20′E) were equipped with a custom-designed Argos relay satellite tag (SPLASH10-F-2961-DSA tag, Wildlife Computers, hereafter DSA tag) and a smart position transmitting tag (SPOT 293A, Wildlife Computers) that measured diving depth to record their very first trip to sea. The satellite tags of nine individuals (7 females and 2 males; mean weaning mass ± SD = 108 ± 16 kg; mean straight length from nose to tail tip ± SD = 134 ± 9 cm) were recovered when they returned to land after 175 ± 25 days at sea. Animals were captured with a canvas head-bag and anesthetized using a 1:1 combination of tiletamine hydrochloride and zolazepam hydrochloride (Zoletil 100) injected intravenously. Using quick-setting epoxy (Araldite® AW 2101), the DSA tag was attached to the fur on top of the head and the SPOT tag to the center of the back. Data Collection The MK9 tags attached to northern elephant seals sampled pressure every 4 s during the entire trip to sea. The DSA tags sampled pressure every 1 s, but only by recording one complete dive (max depth > 15 m and duration > 60 s) every ~2.25 h. Data Preprocessing Time-depth records of southern elephant seals were downsampled to 4 s to match the sampling frequency of the northern elephant seals and facilitate comparison. Dive identification and zero offset correction of depth were performed using the IKNOS Toolbox with Matlab 9.1 (The MathWorks, Natick, MA, USA). Dives were identified as excursions from the surface reaching a maximum depth of at least 10 meters and lasting a minimum duration of 30 seconds. For northern elephant seals, location data were obtained from the light-level data using the Wildlife Computers GPE3 algorithm, with the exception of seal ID 2018074 in which the light levels were corrupted. Using the aniMotum R package, we fitted a correlated random walk with a maximum travel rate of 3 m.s-1 on our location data to re-estimate the animal's position every hour in order to get a regular time step and handle gaps. The time of day was calculated using the function sunriset() from the R package maptools, which estimates night-time and day-time based on date-time and location from the NOAA Solar Calculator.