Digestion
& Passage Times
Identifying bones found in scats
(feces) is the most widely used technique to determine what sea lions
are eating in Alaska. However, the importance of some species recovered
in scats such as herring and sandlance may be underestimated if their
bones are small, fragile and digested before they pass out of the animal.
Other species such as pollock and cod could be overestimated if their
big, robust bones pass in high numbers.
Dr. Dom Tollit and colleagues fed salmon, herring, sandlance and pollock
to sea lions housed at the Vancouver Aquarium Marine Science Centre.
Their goal was to see how long it took bones of different species to
pass through the digestive tract, and whether swimming and resting affected
passage rates. They also wanted to determine how many identifiable fish
bones ended up in the scat, as well as derive numerical correction factors
to estimate the numbers of fish actually consumed by the sea lions.
Key fish bones identified as coming
from salmon, pollock, herring or sandlance were distributed over an
average of 3.2 scats (range 1-6) following a single meal. Pollock remains
were scattered over significantly more scats than the other three species.
More prey were noted when all recognizable bones (such as vertebrae,
jaw bones and teeth) were identified, compared to using the traditional
method of only identifying otoliths (ear bones). However, both techniques
showed that pollock was more likely to be recovered than herring (by
three-fold) or sandlance (by eight-fold).
Active (i.e., swimming) animals passed greater numbers of bones than
animals resting on land, but the overall effect of activity on the estimated
number of prey consumed was not significant. Defecation time following
a meal was variable. The first bones from a meal were recovered 2-56
hours after eating, and the last bones from the same meal took 28-148
hours to pass. Time to pass 95% of the recovered bones varied by a factor
of two among prey species, and was highest for pollock due to retention
of bones beyond 65 h in the stomach and intestinal tract.
Between-species comparison of cumulative
mean prey recovery
percentage since time of ingestion for (a) active and (b) inactive bouts
(all key structures, regurgitated meals excluded). Open and filled symbols
denote different animals.
Results from this captive experiment suggest that bones recovered from
scats in the wild are unlikely to represent a single meal consumed shortly
before a sea lion returns to land. Instead, a single scat picked up
at a rookery or haulout likely represents a composite of successful
feeding bouts over an entire feeding trip that may span a number of
days.
14 October 2003
Related pages:
Diet Determination
Further details can be obtained from the recently published paper:
Consortium | Steller
sea lion biology | Steller
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