Final Technical Report: - Southwest Fisheries Science Center - NOAA

for the individual ecosystems. These analyses also suggest that habitat preference is
similar for short-beaked common dolphins, which are characterized as a habitat specialist
in the ETP (Reilly and Fiedler 1994), in both cool-temperate and tropical ecosystems. In
contrast, striped dolphins, which are characterized as a habitat generalist in the ETP
(Reilly and Fiedler 1994), appear to have different habitat preferences. These results
suggest a general hypothesis that species habitat selectivity, which is related to the
breadth of a species niche, in low productivity ecosystems may determine whether their
habitat preferences are the same across multiple ecosystems.
4.4 Variance Estimation
One advantage of predictive density models, compared to simple stratified linetransect
analyses, is the ability to estimate variance at a finer spatial resolution. This
provides useful information on areas where abundance estimates are likely to vary the
most (or least). Geographic contour plots showing annual model predictions, multi-year
average densities, standard errors, and lognormal 90% confidence intervals are shown in
Appendix A for cetaceans in the CCE and in Appendix B for cetaceans in the ETP.
The greatest source of variability was attributable to the strong inter-annual
variability in oceanographic conditions (See Section 4.1). In contrast, the specific
methods used to build the models were a small source of variability among model
predictions. For example, plots of predicted average ETP striped dolphin density and the
associated estimates of standard error and lognormal 90% confidence intervals derived
from the complex vs. simple encounter rate (53.4 vs. 22.5 effective degrees of freedom)
and group size (17.9 vs. 12.6 effective degrees of freedom) models are nearly
indistinguishable (Fig. 19 and 20, respectively).
In the CCE region, uncertainty was generally greater off Oregon and Washington,
where fewer surveys were conducted (1991 and 1993 surveys were only conducted off
California). Variance was also greater for species with a large range in group size, e.g.
short-beaked common dolphins, and smaller for large whale species and Dall's porpoises,
which occur in smaller groups (Fig. 21). Similar patterns of variance were evident for the
ETP, where estimates of uncertainty were greatest in areas where survey effort was least
(for example, around the margins of the study areas), and for species having the greatest
range in encounter rate and group size (for example, spotted, striped, eastern spinner, and
whitebelly spinner dolphins).
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