297 
In cold blood: evidence of Pacific sleeper shark 
{Somniosus pacificus ) predation on Steller sea 
lions {Eumetopias jubatus ) in the Gulf of Alaska 
Email address for contact author: markus.horning@oregonstate.edu 
Abstract— Temperature data re- 
ceived post mortem in 2008-13 
from 15 of 36 juvenile Steller sea 
lions (Eumetopias jubatus) that 
had been surgically implanted 
in 2005-11 with dual life history 
transmitters (LHX tags) indicat- 
ed that all 15 animals died by 
predation. In 3 of those 15 cases, 
at least 1 of the 2 LHX tags was 
ingested by a cold-blooded pred- 
ator, and those tags recorded, 
immediately after the sea lion’s 
death, temperatures that cor- 
responded to deepwater values. 
These tags were regurgitated or 
passed 5-11 days later by preda- 
tors. Once they sensed light and 
air, the tags commenced trans- 
missions as they floated at the 
ocean surface, reporting tem- 
peratures that corresponded to 
regional sea-surface estimates. 
The circumstances related to the 
tag in a fourth case were ambig- 
uous. In the remaining 11 cases, 
tags sensed light and air imme- 
diately after the sea lion’s death 
and reported temperatures that 
corresponded to estimates of re- 
gional sea-surface temperatures. 
In these 11 cases, circumstances 
did not allow for inferences on 
the species of predator. Among 
reported poikilotherm predators 
of Steller sea lions, only the Pa- 
cific sleeper shark ( Somniosus 
pacificus) is known to have body 
core temperatures that are near 
ambient. The data from this 
study indicate that Pacific sleep- 
er sharks need to be considered 
as a possible source of mortality 
of juvenile Steller sea lions in 
the region of the Gulf of Alaska. 
Manuscript submitted 19 June 2013. 
Manuscript accepted 21 August 2014. 
Fish. Bull. 112:297-310 (2014). 
doi:10.7755/FB.l 12.4.6 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Markus Horning (contact author ) 12 
Jo-Ann E. Mellish 3 - 4 
1 Department of Fisheries and Wildlife and 
Marine Mammal Institute 
College of Agricultural Sciences 
Oregon State University 
2030 SE Marine Science Drive 
Newport, Oregon 97365 
2 Coastal Oregon Marine Experiment Station 
Oregon State University 
2030 SE Marine Science Drive 
Newport, Oregon 97365 
The western distinct population seg- 
ment (WDPS) of the Steller sea lion 
(. Eumetopias jubatus) (west of 144°W) 
has declined to approximately 20% of 
the levels encountered before 1975 
(National Research Council, 2003), 
and declines were at or above 5% 
per year in some areas. Recent data 
indicate trends of slightly increasing 
population overall and continuing 
steep declines for the western Aleu- 
tian Islands from 2000 to 2012 (Fritz 
et ah, 2013). Limited trend data indi- 
cate stable or slightly increasing lev- 
els for the Prince William Sound-Ke- 
nai Fjords area of the Gulf of Alaska 
(Fritz et al., 2013). 
Multiple factors have been posited 
as driving past and present popula- 
tion trends of the Steller sea lion 
(National Research Council, 2003). 
York (1994) hypothesized reduced ju- 
venile survival in the Gulf of Alaska 
region during the height of the de- 
cline. Holmes et al. (2007) hypoth- 
esized that juvenile survival rates 
for the period of 1997-2004 in the 
same region had recovered but that 
birth rate had steadily declined. Spe- 
cific proximate drivers of the hypoth- 
esized reduced juvenile survival or 
reduced birth rates remain unknown. 
Suggested causes include nutritional 
stress through competition with com- 
3 Alaska Sea Life Center 
P.O. Box 1329 
Seward, Alaska 99664 
4 School of Fisheries and Ocean Sciences 
University of Alaska Fairbanks 
P.O. Box 757220 
Fairbanks, Alaska 99755-7220 
mercial fisheries (National Research 
Council, 2003), the “inadequate 
food/ocean climate hypothesis” (e.g., 
Trites et al., 2007), and the “preda- 
tion hypothesis” (e.g., Williams et al., 
2004). However, to date no direct test 
of any of these hypotheses has been 
published in peer-reviewed literature 
for any region of the WDPS. 
Direct observations (Ford et al., 
1998; Maniscalco et al., 2007), physi- 
cal evidence (Heise et al., 2003), and 
telemetered data (Horning and Mell- 
ish, 2009, 2012) reveal the occurrence 
of predation on Steller sea lions. 
From a review of research conducted 
through the 1990s, the National Re- 
search Council (2003) concluded that 
the recovery of the WDPS was more 
likely limited by predation than by 
resource-driven effects. Reported 
predators of Steller sea lions include 
not only the transient ecotype of the 
killer whale ( Orcinus orca) (Ford et 
al., 1998) but also the white shark 
(CarcharocLon carcharias), the salm- 
on shark (Lamna ditropis), and the 
Pacific sleeper shark ( Somniosus 
pacificus) (Loughlin and York, 2000). 
However, the accurate detection and 
quantification of sea lion depreda- 
tion at sea has been labeled as “em- 
pirically intractable” (Williams et al., 
2004). 
