Steller sea lion pup plasma metabolites

Geographic differences in population growth trends are well-documented in
Steller sea lions (Eumetopias jubatus), a species of North Pacific
pinniped listed under the U.S. Endangered Species Act in 1990 following a
marked decline in population abundance that began during the 1970s. As
population growth is intrinsically linked to pup production and survival,
examining factors related to pup physiological condition provides useful
information to management authorities regarding potential drivers of
regional differences.  During dam foraging trips, pups
predictably transition among three fasting phases, distinguished by the
changes in the predominant metabolic byproduct. We used standardized
ranges of two plasma metabolites (blood urea nitrogen and
β–hydroxybutyrate) to assign pups to fasting categories (n=1528,
1990–2016, 12 subpopulations): Recently Fed–Phase I
(digestion/assimilation–expected hepatic/muscle glycogen usage), Phase II
(expected lipid utilization), transitioning between Phases II–III
(expected lipid utilization with increased protein reliance), or Phase III
(expected protein catabolism). As anticipated, the majority of pups were
classified as Recently Fed–Phase I (overall mean proportion=0.72)
and a few pups as Phase III (overall mean proportion=0.04). By
further comparing pups in Short (Recently Fed–Phase II) and Long
(all other pups) duration fasts, we identified three subpopulations with
significantly (p<0.03) greater proportions of pups dependent upon
endogenous sources of energy for extended periods, during a life stage of
somatic growth and development: the 1) central (0.27 ± 0.09) and 2)
western (0.36 ± 0.13) Aleutian Island (declining population trend) and 3)
southern Southeast Alaska (0.32 ± 0.06; increasing population trend)
subpopulations had greater Long fast proportions than the eastern Aleutian
Islands (0.10 ± 0.05; stabilized population).  Due to contrasting
population growth trends among these highlighted subpopulations over the
past 50+ years, both density-independent and density-dependent factors
likely influence the dam foraging trip duration, contributing to longer
fasting durations for pups at some rookeries.