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ENCYCLOPEDIA OF MARINE MAMMALS 　 ／Second Edition，2009
Edited by William F. Perrin, Bernd Wursig and J. G.M. Thewissen
【Population Status and Trends】の項（920頁）から引用。
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II. Trends

An upward or downward trend in population size is obviously a
significant component in evaluating the status of a population; how-
ever, it is also one of the more difficult components to determine.
Population trends have been directly estimated for only a tiny proportion
of all cetacean populations. The primary problems are that
population size cannot be estimated very precisely and population
growth is typically slow.

Cetacean population size is usually estimated from line-transect SURVEYS.
Trends can also be based on an index of relative abundance. such
as the number of whales seen per hour on standard transects with consistent
survey methods. In either case, the precision of the estimates is
measured as a coefficient of variation (CV = standard deviation divided
by the mean); CVs of 20% are considered very good and CVs of 30-
50% are considered typical. Given their life-history constraints, cetacean
populations can grow at a maximum rate of about 10% per year, and,
for some slow-reproducing species [such as the killer whale (Orcinus
orca) and sperm whale], maximum growth rates may be as low as 2-
3%. There is no similar constraint on the rate at which populations can
decline; however, there have been few instances where long-term rates
of decline have been found to exceed 8% per year.

Statistical power is a measure of the probability of detecting a
significant change in a population if that population is truly growing
or declining. Power increases with the actual rate of change in
population size and with the sample size (both the amount of survey
effort and, more importantly, the length of the monitoring period).
As a rule of thumb for cetaceans, at least 10 annual surveys with
good precision (CV < 20%) are required to yield a high probability
(>80%) of detecting a 50% change in total population size. Taylor
et al. (2007) found that in the US, where marine mammal monitoring
is probably the best in the world, the majority of populations could
decline by 50% without reliable statistical evidence of a decline. The
situation is made even more difficult for endangered species [such
as the baiji (Lipotes vexillifer) and vaquita (Phocoena sinus)], which
become almost impossible to census as they become increasingly
rare. A recent large-scale survey for baiji failed to find even a single
individual (Turveyet al., 2007), and the species is now considered
"possible extinct" by IUCN. Similar to cetaceans, the detection of
population trends for dugongs and manatees is extremely difficult.

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