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

1. High juvenile mortality rates are typical of many long-lived marine
vertebrate predators. Insufficient development in dive and forage ability
are considered key drivers 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. 20 post-weaning pups were equipped with (1) a new
generation satellite relay data tag, capable of remotely transmitting
fine-scale behavioural movements from accelerometers, and (2) 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 behavioural movements and encountered environmental conditions
of non-surviving 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 penalised
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,
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
behaviour 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.