MPAs -- Wrong null hypothesis?
jmcmanus at rsmas.miami.edu
Thu Mar 7 21:19:41 EST 2002
I have long been puzzled by the resistance to MPAs found among some
scientists. It seems to me to be intuitively clear that if you fish heavily,
stocks will decline, and if you stop fishing, and have not irreversibly
altered the habitat, stocks should increase. I have certainly seen no
reasonable evidence that this is not the case. I suggest that we have been
using the wrong null hypothesis. We have been treating the reserve as if it
were a "treatment", when in fact, it is the lack of, or removal of, a
treatment (fishing pressure).
The null hypothesis should be that a reduction in fishing pressure should
lead over time to a return toward (if not to) natural abundances of fish.
The alternative hypothesis, and the one that should carry the burden of
proof, is that reducing fishing pressure does NOT result in a return toward
natural levels of abundance. Where the alternative turns out to be true, one
should investigate why it was true. Was it because the habitat had not yet
returned to reasonable normalcy? Is the result likely to be because the
study period was too short? Or, is it truly because reducing fishing
pressure does not generally result in increases in fish abundance.
The labeling is mostly important because most scientists are deeply
concerned about reducing Type I error (the chance that you say something is
true that is not), and pay little heed to Type II error (the chance that you
say something is not true when it is). The error should relate to the
alternate hypothesis, and the null hypothesis should be accepted in the
absence of sufficient evidence that the alternative is true. Thus, the null
hypothesis is that which arises most naturally from simple common sense, and
the alternative is the challenge to that logic. If we construct tests of
high power, lots of sample units, long time frames, etc., then we minimize
Type II error, and the designation of hypotheses is not as important.
However, the fact is that exceedingly few studies of MPAs have been
conducted in such a way as to minimize Type II error. And, because of our
arrangement of hypotheses, we have encouraged stakeholders and managers to
wait around while we challenge a null hypothesis that heavy fishing is
generally completely irreversible and thus reserves are useless.
I also propose that we keep separate the question of the increase of fish
catch balancing out the loss of fishing grounds. Obviously, this depends
greatly on how much fishing ground is being lost, how much the fish
populations had been reduced around the reserve, as well as factors such as
the mobility of fish in and out of the reserve. This is thus a question of
design -- how big, what shape, etc., and not one of "are MPAs effective or
not". I know of many tens of small reserves, on the order of 1/4th to 2 sq.
km, that have been put in by coastal communities in heavily fished areas,
and have not heard of any that resulted in people complaining that the
reserve did not increase in fish abundance. Rather, there are many cases of
fishers fishing preferentially around the edges of these reserves,
indicating that abundances are indeed higher in the reserves. So, "do the
reserves gain more fish compared to heavily fished areas nearby of similar
habitat type?" -- in all the cases I know of, published or unpublished, the
answer is yes. "Did the reserve's output make up for the loss in fishing
ground?" -- that would depend entirely on local circumstances. And,
evaluating "making up for" should generally include weighing nonfishery
benefits into the equation.
I suggest that we stop worrying about "are reserves effective?" until strong
evidence shows that most of them (with real fishing exclusions) are not.
Instead, we should focus on how big, what shape, where, etc.
John W. McManus, PhD
Director, National Center for Caribbean Coral Reef Research (NCORE)
Rosenstiel School of Marine and Atmospheric Sciences (RSMAS)
University of Miami, 4600 Rickenbacker Causeway
Miami, Florida 33149.
jmcmanus at rsmas.miami.edu
Tel. (305) 361-4814
Fax (305) 361-4910
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