Final Technical Report: - Southwest Fisheries Science Center - NOAA

isobath, and Beaufort sea state. Distance to the 2,000 m isobath was added to the list of
predictors because sighting plots suggested that this variable could potentially improve model
performance for some species (e.g., sperm whale, Physeter macrocephalus, and Baird’s beaked
whale, Berardius bairdii) that are generally found only in slope or deep waters. This variable
was coded to indicate whether the location was deeper (-) or shallower (+) than the 2,000 m
isobath. Beaufort sea state affects the probability of detecting animals (Barlow et al. 2001), and
the average observed sea state value on each segment was included as a continuous predictor
variable in our models in order to account for sighting conditions.
In addition to the variables used for the remotely-sensed models, the combined models
included three potential predictors derived from data collected in situ: sea surface salinity, the
natural logarithm of surface chlorophyll concentration, and mixed layer depth, measured as the
depth at which the water temperature was 0.5C less than at the surface. Remotely sensed
measures of SST and CV(SST) were used in the combined models because the remotely-sensed
CV(SST) was found to be more effective at characterizing frontal regions than our in situ
CV(SST) measures (Becker 2007), and SST measures performed similarly. The in situ data were
derived in one of two ways. Salinity was sampled continuously along the transect and segmentspecific
estimates were obtained by averaging values within 5 km of the mid-point of each
transect segment included in the analysis. Chlorophyll and mixed layer depth were measured
much less frequently, and a linear interpolation between nearby stations did not accurately
capture values at the edges of the study area or when samples were sparse, causing 'bull’s eye'
effects in estimated cetacean density. Therefore, the data were first contoured (see Section 3.2)
to provide a 2-D surface of estimated chlorophyll and mixed-layer depth values, and segment
mid-point values were extracted from the contour grid using the Surfer 8.0 (Golden Software,
Inc) Residuals feature.
Density Estimation
Segment-specific density estimates were derived by incorporating the predicted values
for encounter rate and group size into the standard line-transect equation (Buckland et al. 2001)
as described by Becker (2007) and in Section 3.3.1. We relied on published values of detection
probability (f(0) and g(0)) for each species as estimated from the same survey data used for
model development (Barlow 2003). Published values for many species were stratified by group
size and, for purposes of estimating densities, we incorporated weighted f(0) and g(0) values
based on the number of small and large groups observed during the surveys (Becker 2007, Table
4). All final model predictions were made using the average observed Beaufort sea state for
conditions 0-5 during the SWFSC cruises. This is appropriate because it corresponds to the
conditions for which the line-transect parameters f(0) and g(0) were estimated (Barlow 2003).
For Dall’s porpoise and small beaked whales, published f(0) and g(0) values were available only
for Beaufort conditions of 0-2. Model predictions for this species and guild were made using the
average observed Beaufort sea state for conditions 0-2.
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