|Abstract or Summary
- This research was designed to evaluate the Fatty Acid Signature (FAS) technique
as a non-lethal alternative to more traditional, and sometimes destructive, methods of
studying the diet composition of piscivorous birds. Specifically we tested the technique
with Caspian terns (Hydroprogne caspia) which currently nest in large numbers in the
Columbia River estuary and are known to consume juvenile salmonids (Oncorhynchus
spp.) listed under the U.S. Endangered Species Act.
From captive feeding trials conducted with Caspian tern chicks, we determined
that FASs of the birds reflected differences in their diets. After 20 days of being fed
consistently mixed or monotypic diets of two fish types, chicks displayed different
adipose tissue FASs between all 4 diet treatments. When diets were changed, adipose
tissue FASs reflected the shift in diet treatments within two weeks. Fatty acid (FA)-
specific calibration coefficients (FA level in the consumer divided by FA level in the
food) were calculated for Caspian terns fed monotypic diets for 34 days; some calibration
coefficients varied in association with diet and age of the terns, and also differed between
terns and common murres (Uria aalge), whose calibration coefficients were measured in
a separate study. Variation in FA-specific calibration coefficients may be problematic for
obtaining accurate estimates of diet composition in piscivorous birds using the
Quantitative Fatty Acid Signature (QFASA) technique. We advocate sensitivity analysis
to test whether the QFASA models are robust to the magnitude of variation in calibration
coefficients detected in this study.
FASs differed among the 3 major fish prey types observed in diets of Caspian
terns nesting in the Columbia River estuary during the 2003 breeding season: juvenile
salmonids, surf smelt (Hypomesus pretiosus), and northern anchovy (Engraulis mordax).
We detected differences in FASs of nesting Caspian terns between early and late in the
nesting season of 2003; these differences were associated with a shift in diet compostion
from a diet dominated by juvenile salmonids to a diet dominated by northern anchovy.
The FASs of several species of juvenile salmonids, however, exhibited little inter-specific
variation, especially between species raised in hatcheries, which comprise the majority of
smolts consumed by Caspian terns in the estuary. We found levels of highly-unsaturated
FAs (HUFAs) to be higher in wild steelhead smolts than in hatchery-reared steelhead
smolts, but HUFA levels in terns did not reflect the changing prevalence of wild
steelhead in their diets. This is likely due to contribution of HUFAs to the diets of terns
from marine forage fishes. Thus, HUFAs do not appear to be useful indicators of wild
steelhead in the diets of these birds.
If the QFASA technique can be validated, it has the potential to provide general
information on diet composition for piscivorous birds foraging on broadly different prey
types over extended periods. However, due to similarities in FASs of key prey types
consumed by Caspian terns in the Columbia River estuary, namely salmonids, obtaining
the precise estimates of diet composition and consumption of different species of
salmonids that are requested by resource managers does not seem feasible using the
QFASA technique alone. Consequently, more traditional methods of diet composition
analysis for Caspian terns (bill load identification, stomach contents analysis) can not be
replaced by the FAS analysis technique.