Subject: Harbor Porpoise: Abstract

Mike Williamson (
Mon, 14 Dec 1998 07:42:08 -0500 (EST)

---------- Forwarded message ----------
Date: Sat, 12 Dec 1998 06:14:17 -0800
From: MARMAM Editors <marmamed@UVic.CA>
Reply-To: Marine Mammals Research and Conservation Discussion
Subject: New publication on harbor porpoise and effects of bycatch (fwd)

From: Solange Brault <>

The following paper has just appeared:

Harbor Porpoise and Fisheries: An Uncertainty Analysis of
Incidental Mortality


Hal Caswell
Solange Brault
Andrew J. Read
Tim D. Smith

Ecological Applications 8:1226-1238 (1998).


The harbor porpoise (Phocoena phocoena) in the western
North Atlantic is subject to mortality due to entanglement in
gillnets.  Such incidental mortality threatens a population if it
is too large relative to the potential population growth rate.
Critical values for incidental mortality have been established by
the International Whaling Commission and the U.S. Marine Mammal
Protection Act.  As in many situations in conservation biology,
use of these critical values depends on demographic calculations
that are based on uncertain data.  It is important to report not
only estimates of demographic parameters, but also the
uncertainty in those estimates.  Here, we use a Monte Carlo
approach to evaluate uncertainty in population size, incidental
mortality, and population growth rate of harbor porpoise.  To
describe survival, we used model life tables derived from other
mammals with similar life histories.  By randomly sampling the
space of model life tables and the distributions of estimated
fertility and age at first reproduction, we produced a
probability distribution that characterizes the uncertainty in
the potential population growth rate.  The median estimate for
the potential annual rate of increase lambda is approximately
1.10.  Combining this information with the uncertainty of
incidental mortality and of population size, we estimate the
probability that the rate of incidental mortality exceeds the
critical values established by the various management agencies;
this probability ranges from 0.46 to 0.94.  We conclude that
recent incidental mortality rates are a threat to harbor porpoise
populations.  The methods developed here are applicable to other
situations where demographic analyses must be based on uncertain

For reprints, contact

Hal Caswell
Biology Department MS-34
Woods Hole Oceanographic Institution
Woods Hole MA 02543

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