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detection by one or both observers at different distances from the line. One
approach was provided by Manly et al
.
(1996), who used it on data from aerial
surveys of polar bears in the Arctic. This approach is easily modified to allow
the probabilities of detection to depend on covariates. Other approaches with
double observers were discussed by Buckland et al
.
(2001, 2004).
EXAMPLE 4.3 Sperm Whale Sampling
In the Gulf of California, Mexico, the density and abundance of sperm
whales
Physeter macrocephalus
were estimated in spring 2005 by setting
up line transect surveys (Díaz-Gamboa, 2009). The observation platform
was 6.6 m high from a vessel at 8.3 knots speed with two main observers
covering 180° visual range to the front using 7 × 50 binoculars equipped
with reticle and compass and an independent observer covering 360°
of visual range to report sightings undetected by the main observers.
Observations were made at Beaufort 3 sea state or less to reduce bias.
The reticle and sighting angle degree were reported on each sighting
to determine the perpendicular distance from an algorithm that con-
verted radial to linear distance at the proper latitude. The geographic
position and transect course were registered every minute. The study
area was divided into northern, central, and southern strata (Figure 4.5).
32
30
N
28
C
26
S
24
N
Pacific Ocean
22
-116
-114
-112
-110
-108 106
FIGURE 4.5
Sighting effort for sperm whale sampling from the Gulf of California, Mexico, using line tran-
sect sampling. Transects were grouped in three strata: northern (N), central (C), and southern
(S) Gulf of California.
