Matkin et at: Abundance and residency patterns of two sympatric populations of Orcmus orca in the northern Gulf of Alaska 
147 
■ ATI 
GOA 
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Year 
Figure 2 
The number of individuals photographically identified from the ATI and 
Gulf of Alaska (GOA) transient killer whale ( Orcinus orca) populations in 
each annual April-September period, 1984-2010. 
5 transmitter (Wildlife Computers, 
Redmond, WA), is housed in an epoxy 
casing with dimensions of 65x30x22 
mm. The tag is held externally on 
the dorsal fin of the whale by two 
4-mm-diameter medical-grade tita- 
nium darts that were affixed to the 
bottom of the tag, for a total mass 
of 49 g. The darts were designed to 
penetrate 6.5 cm into the connective 
tissue in the dorsal fin and remain 
embedded with a series of backward- 
facing barbs which acted as anchors 
for the darts. The LIMPET tags were 
projected onto the whales by using a 
crossbow with 150-lb draw weight, 
and the tag was held on the end of an 
arrow in a special rubber boot. 
This type of satellite tag transmits 
ultra-high frequency (UHF) radio 
signals to Argos receivers onboard 
weather satellites in sun-synchro- 
nous polar orbits. To conserve power, 
transmissions are limited by a sub- 
mersion sensor to times when the 
whale is at the surface. Locations 
were calculated by the Argos sys- 
tem by the method of least squares 
( http://www.argos-svstem.org . accessed October 2007), 
and we determined the plausibility of each location us- 
ing the Douglas Argos filter, vers. 7.03 (Douglas 2 ). We 
retained locations with high location accuracies (LC2 
and LC3), as well as consecutive points separated by 
less than 3 km. All other locations were removed if the 
rate of movement between consecutive locations exceeded 
25 km/h or the angle formed by the previous and sub- 
sequent locations indicated extreme return-movements. 
The angle of each triad of points and the distance be- 
tween the shortest leg of the triad was assessed by the 
filter and compared with the dimensionless rate coeffi- 
cient (Ratecoef) that was set to 25. Location data were 
imported into Google Earth (Google, Mountain View, 
CA) for basic visual inspection and into ArcMap 9.3.1 
(ESRI, Redlands, CA) for further analysis. Distance 
traveled was calculated for each tagged animal, as well 
as a calculation of oceanic home range developed by 
subtracting the land area from the total area in the 
Minimum Convex Polygon, which was the polygon that 
described the perimeter of all filtered satellite locations 
received during the period of attachment. 
Results 
During the 27 years of this study we averaged 106 
(range=29-249) vessel days per year with at least 59 
2 Douglas, D. 2007. The Douglas Argos-Filter. [Available at 
http://alaska.usgs.gov/science/biologv/spatial/manual.html . 
accessed 1 October 2007.] 
days logged in all years except for 1987 (29 days). During 
these surveys we recorded a total of 203 encounters 
with members of the ATI transient population and 91 
encounters with members of the GOA transient popula- 
tion (Fig. 1). Over 27 annual (May-September) periods, 
a total of 88 individual whales were documented. There 
were three times as many GOA individuals (66) as ATI 
individuals (22), but the average number of individu- 
als identified in each summer interval was similar for 
both populations (GOA: median=8, range 0-18; ATI: 
median=7, range 4-22; Fig. 2). This finding reflected a 
higher resighting rate for individual ATI whales; indi- 
viduals were seen in a median of nine different annual 
intervals (range 3-25) compared with a median of just 
two intervals (range 1-16) for GOA whales (Fig. 3). It 
is notable that 7 of 22 total ATI whales were identified 
in more than 20 annual intervals, whereas only 1 of 66 
GOA whales was identified in more than 10 intervals 
(Fig. 3). The number of ATI individuals seen each year 
clearly declined across the study period from around 20 
individuals in the 1980s to fewer than 10 individuals 
in the 2000s, whereas the number of GOA individuals 
remained at a more consistent but low number with 
a median 8 individuals identified per year (Fig. 2). 
However, to formally assess changes in abundance, we 
adjusted our sighting data for capture probabilities using 
mark-recapture models. 
The mark-recapture model with emigration and re- 
immigration provided a better fit to the photo-identifi- 
cation data than the standard CJS model, for both ATI 
and GOA individuals. For GOA whales, there were 49 
discrepancies between 1079 observed and predicted data 
