High mortality rates in a juvenile free‐ranging marine predator and links to dive and forage ability

Abstract

1. High juvenile mortality rates are typical of many long-lived marine vertebrate predators. Insufficient development in dive and forage ability is considered a key driver of this. However, direct links to survival outcome are sparse, particularly in free-ranging marine animals that may not return to land.2. In this study, we conduct exploratory investigations toward early mortality in juvenile southern elephant seals Mirounga leonina. Twenty postweaning pups were equipped with (a) a new-generation satellite relay data tag, capable of remotely transmitting fine-scale behavioral movements from accelerometers, and (b) a location transmitting only tag (so that mortality events could be distinguished from device failures). Individuals were followed during their first trip at sea (until mortality or return to land). Two analyses were conducted. First, the behavioral movements and encountered environmental conditions of nonsurviving pups were individually compared to temporally concurrent observations from grouped survivors. Second, common causes of mortality were investigated using Cox's proportional hazard regression and penalized shrinkage techniques.3. Nine individuals died (two females and seven males) and 11 survived (eight females and three males). All but one individual died before the return phase of their first trip at sea, and all but one were negatively buoyant. Causes of death were variable, although common factors included increased horizontal travel speeds and distances, decreased development in dive and forage ability, and habitat type visited (lower sea surface temperatures and decreased total [eddy] kinetic energy).4. For long-lived marine vertebrate predators, such as the southern elephant seal, the first few months of life following independence represent a critical period, when small deviations in behavior from the norm appear sufficient to increase mortality risk. Survival rates may subsequently be particularly vulnerable to changes in climate and environment, which will have concomitant consequences on the demography and dynamics of populations. | 411COX et al.

Inter‐ and intrasex habitat partitioning in the highly dimorphic southern elephant seal

“…What is known, is that foraging areas change as seals age (Authier et al., 2012; Chaigne et al., 2012; Field et al., 2005). Typically juvenile seals of both sexes forage in oceanic waters near their natal islands with no clear differences between males and females with respect to habitat, geographic location or foraging depths (McConnell et al., 1992), although they do show a bias to the east of their natal islands (Cox et al., 2020; McConnell et al., 2002; Orgeret et al., 2019). Post year one, their foraging ranges increase and they forage at deeper dive depths, reflecting enhanced diving abilities of the seals as they grow and mature (Hindell et al., 1999; Irvine et al., 2000).…”

Inferring Variation in Southern Elephant Seal At-Sea Mortality by Modelling Tag Failure

“…Mortality is unlikely to be constant across the foraging trip, and a higher rate may potentially be associated with the return to haul-out locations, for two reasons. First, it's possible that seals which have not foraged successfully die on the energetically expensive return journey (Cox et al, 2020). Alternatively, seals may be exposed to high rates of predation as they approach haul-out locations (Isles Kerguelen) (Van Den Hoff and Morrice, 2008;Tixier et al, 2018;Cox et al, 2020).…”

“…A 20% difference in annual survival rate between these two foraging habitats would lead to this observed change in the proportion of male southern elephant seals using sub-Antarctic (Kerguelen Plateau), versus Antarctic shelf foraging habitat. The simultaneous failure of both tags for 9 of 20 doubletagged yearling elephant seals on the Kerguelen Plateau may further indicate a higher rate of predation events (Orgeret et al, 2019;Cox et al, 2020). The Kerguelen Plateau is home to elephant seal predators such as sleeper sharks and killer whales (Orcinus orca) (Van Den Hoff and Morrice, 2008;Tixier et al, 2018;Cox et al, 2020).…”

“…The simultaneous failure of both tags for 9 of 20 doubletagged yearling elephant seals on the Kerguelen Plateau may further indicate a higher rate of predation events (Orgeret et al, 2019;Cox et al, 2020). The Kerguelen Plateau is home to elephant seal predators such as sleeper sharks and killer whales (Orcinus orca) (Van Den Hoff and Morrice, 2008;Tixier et al, 2018;Cox et al, 2020). Therefore, the cluster of failures ( Figure 1B) and higher failure rate of ice-free foraging individuals (Table 2) may indicate higher mortality due to predation and/or lower foraging opportunities on the Kerguelen Plateau.…”

Environmental drivers of population-level variation in the migratory and diving ontogeny of an Arctic top predator

“…Young air-breathing marine vertebrates also need to develop the physiology to forage during breathhold diving. The diving capabilities of young animals are limited compared to adults [8,9] and undergo rapid development during the first months of life [10][11][12][13][14]. The foraging efficiency of young animals is lower, in part due to a higher mass-specific metabolic rate and lower oxygen storage capacity [15].…”