[MARMAM] New paper on humpback whales

Phillip Clapham - NOAA Federal phillip.clapham at noaa.gov
Sat Jan 24 16:09:57 PST 2015

The following was just published online:

Clapham, P.J. & Zerbini, A.  2015.  Are social aggregation and temporary
immigration driving high rates of increase in some Southern Hemisphere
humpback whale populations?  *Marine Biology* doi 10.1007/s00227-015-2610-3.

*Abstract*  Humpback whales (*Megaptera novaeangliae*) in the Southern
Hemisphere were heavily exploited by commercial whaling.  Today, their
recovery is variable: humpbacks remains surprisingly scarce in some
formerly populous areas (e.g. New Zealand, Fiji), while in other regions
(such as eastern Australia) they appear to be rebounding at or even above
the maximum plausible rate of annual increase.  Here, we propose that this
phenomenon cannot be explained solely in demographic terms.  Through
simulation, we test the hypothesis that reported high rates of increase
represent a combination of true intrinsic growth rates and temporary
immigration, driven by a strong tendency to aggregate for mating.  We
introduce the idea that overexploitation diminished density at major
breeding grounds such that these were no longer viable; then, during
subsequent population recovery, a critical mass was attained in certain
areas which drew in whales that formerly bred elsewhere.  The simulations
show that, to maintain high increase rates, the contribution to that rate
by temporary immigration from a second, “source” population would have to
represent a larger and larger proportion of the source stock and would
require relatively large (but quite plausible) intrinsic rates of increase
for each population.  In the modeling scenarios, the demand for immigrants
would eventually exceed the supply and exhaust the source population, but
the simulations demonstrated that high increase rates can be sustained over
periods of more than 20 years.  This hypothesis, if correct, would not only
explain excessively high rates of increase in current “hotspots” such as
eastern Australia, but would also imply that formerly important areas (e.g.
Fiji) host few whales today not necessarily because of a failure to
recover, but because the species’ mating system leads the whales concerned
to migrate to higher-density breeding grounds elsewhere.  Overall, we
caution that assessments of depleted animal populations that do not
consider the social behavior of a species are missing a potentially vital
component of the picture.

Pdf reprints are available from me.

Phillip J. Clapham, Ph.D.
Leader, Cetacean Assessment and Ecology Program
National Marine Mammal Laboratory
Alaska Fisheries Science Center
7600 Sand Point Way NE
Seattle, WA 98115, USA

tel 206 526 4037
fax 206 526 6615
email phillip.clapham at noaa.gov
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