143 
Contrasting abundance and residency patterns 
of two sympatric populations 
of transient killer whales ( Orcinus orca ) 
in the northern Gulf of Alaska 
Abstract — Two sympatric popula- 
tions of “transient” (mammal-eating) 
killer whales were photo-identified 
over 27 years (1984-2010) in Prince 
William Sound and Kenai Fjords, 
coastal waters of the northern Gulf 
of Alaska (GOA). A total of 88 indi- 
viduals were identified during 203 
encounters with “ATI” transients (22 
individuals) and 91 encounters with 
“GOA” transients (66 individuals). 
The median number of individuals 
identified annually was similar for 
both populations (AT1=7; GOA = 8), 
but mark-recapture estimates showed 
the ATI whales to have much higher 
fidelity to the study area, whereas the 
GOA whales had a higher exchange of 
individuals. Apparent survival esti- 
mates were generally high for both 
populations, but there was a signifi- 
cant reduction in the survival of ATI 
transients after the Exxon Valdez oil 
spill in 1989, with an abrupt decline 
in estimated abundance from a high 
of 22 in 1989 to a low of seven whales 
at the end of 2010. There was no 
detectable decline in GOA population 
abundance or survival over the same 
period, but abundance ranged from 
just 6 to 18 whales annually. Resight- 
ing data from adjacent coastal waters 
and movement tracks from satellite 
tags further indicated that the GOA 
whales are part of a larger popula- 
tion with a more extensive range, 
whereas ATI whales are resident to 
the study area. 
Manuscript submitted 1 June 2011. 
Manuscript accepted 18 October 2011. 
Fish. Bull. 110:143-155 (2012). 
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. 
Craig O. Matkin (contact author ) 1 
John W. Durban 2 3 
Eva L. Saulitis' 
Russel D. Andrews 4 
Killer whales (Orcinus orca ) in the 
eastern North Pacific can be geneti- 
cally and acoustically separated 
into three nonassociating lineages: 
“resident,” “transient,” and “offshore” 
(Ford and Ellis, 1999; Matkin et ah, 
1999; Barrett-Lennard, 2000; Yurk 
et al., 2002). Of these lineages, Morin 
et al. (2010) found the transients to 
be the most genetically divergent 
and indicated that they should be 
considered a separate species. Only 
the transient form has been observed 
consuming marine mammals in this 
region and observations indicate 
that they feed on marine mammals 
exclusively (Ford et al., 1998; Sauli- 
tis et al., 2000; Herman et al., 2005; 
Matkin et al., 2007a, 2007b; Barrett- 
Lennard et al., 2011). The potential 
for these whales to affect trajectories 
of prey populations has led to con- 
siderable debate over the role of pre- 
dation by transient killer whales in 
the decline of coastal pinnipeds and 
sea otters in western Alaska (e.g., 
Estes et al., 1998, 2009; Springer 
et al., 2003, 2008; DeMaster et al., 
2006; Wade et al., 2007, 2009). In 
Janice M. Straley 5 
Dena R. Matkin 1 
Graeme M. Ellis 6 
addition to data on feeding habits, 
evaluation of their top-down impact 
requires data on abundance and res- 
idency patterns of these transient 
killer whales within specific marine 
systems, particularly with respect 
to the abundance and trends of their 
primary prey. 
The coastal waters of Prince Wil- 
liam Sound and the Kenai Fjords 
in the northern Gulf of Alaska are 
unique in being regularly used by 
two sympatric populations of tran- 
sient killer whales (Matkin et al., 
1999). Members of both the Gulf 
of Alaska-Aleutian Islands-Bering 
Sea transient stock and the ATI 
transient stock (Allen and Angliss, 
2010) have been photographically 
identified over the past 27 summer 
seasons (Matkin et al., 1999, 2008). 
Individuals from both populations 
regularly use the same region but 
have never been recorded swimming 
together and do not associate (Mat- 
kin et al., 1999), and they can be 
separated by behavior (Matkin et al., 
1999; Saulitis et al., 2000), by acous- 
tics (Yurk et al., 2002, 2010; Saulitis 
Email address for contact author cmatkm@acsalaska.net 
1 North Gulf Oceanic Society 
3430 Mam St., Suite B1 
Homer, AK 99603 
2 National Marine Mammal Laboratory 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, WA 98115 
3 Protected Resources Division 
Southwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
3333 N. Torrey Pines Ct. 
La Jolla, CA 92037 
4 School of Fisheries and Ocean Sciences 
University of Alaska Fairbanks 
and Alaska SeaLife Center 
301 Railway Ave. 
Seward, AK 99664 
5 University of Alaska Southeast 
Sitka Campus 
1332 Seward Ave 
Sitka, AK 99835 
6 Department of Fisheries and Oceans 
Pacific Biological Station 
3190 Hammond Bay Rd. 
Nanaimo, British Columbia, V9R 5K6 Canada 
