Final Technical Report: - Southwest Fisheries Science Center - NOAA

Consistent with Barlow and Forney (2007), we created four northern/southern strata: waters off
Oregon and Washington (north of 42°N), northern California (south of 42°N and north of Point
Reyes at 38°N), central California (south of Point Reyes and north of Point Conception at
34.5°N), and southern California (south of Point Conception). These regions were further
stratified into western and eastern regions at the 2,000 m isobath. Therefore, we were able to
evaluate spatial predictions on a yearly basis as well as for all years combined. In addition,
inspection of predicted species density maps overlaid with survey sighting locations provided a
means for qualitatively comparing the models’ predictions.
Density Interpolation
The segment-specific predictions from the model were interpolated to the entire study
area using Surfer 8.0 (Golden Software, Inc). For the California Current models, interpolation
grids were created at a resolution of 25 km, using inverse distance weighting to the power of 2.
This weighting method gives points closer to each grid node greater influence than those farther
away. All data within a search radius of 2 degrees latitude (222 km) were used for interpolation,
because transect spacing ranged from 150 to 230 km during the five different survey years, and
contouring results were more robust when data from more than one transect line were included.
Grids were created for each of the individual survey years (1991, 1993, 1996, 2001, and
2005) for the California Current Ecosystem. Subsequently, the individual grid cells were
averaged across all years to calculate mean species density and its variance. To eliminate
occasional over-specification ('bull’s eye' effects) in the final average prediction grid, a 5x5 pixel
moving average filter with equal weights was applied to the entire grid. The complete gridding
process provided smoothed multi-year average cetacean densities, taking into account both the
varying oceanographic conditions and different levels of sampling coverage achieved during the
SWFSC cetacean surveys. Standard errors and upper and lower lognormal 90% confidence
limits were calculated from the grid cell averages and variances using standard formulae.
Following selection of the final models, we performed an abundance cross-check to
further validate model predictions. We compared the final model overall study area density
predictions to the Barlow (2003) estimates derived using line-transect analyses to examine
potential bias. Although the estimates provided by Barlow (2003) also have uncertainty
associated with them, they provide a benchmark against which our model predictions can be
evaluated. If the model-based estimate was substantially different from the line-transect
estimate, we re-examined the model and performed additional analyses as necessary. In sum,
evaluation factors used to select and validate our final models included expert opinion, temporal
and spatial density ratios (including novel dataset predictions), density plots reflecting both
yearly and averaged predictions, and abundance cross checks.
26