[Coral-List] Artificial reefs

John McManus jmcmanus at rsmas.miami.edu
Sat Mar 6 18:59:51 EST 2004

Hi all,

I believe that artificial reefs are potentially valuable tools for coral
reef conservation. However, most are probably not. Based on some
fascinating emails I have received, I think it important to explain my
reasoning. Unfortunately, the explanation is long, and so read on only
if you have a strong interest in all this.

Within the first several years or so, none of the newly-settled fish on
an artificial reef can possibly have a positive impact in terms of
alleviating fishing pressure. They will be too young. So, here are two
of the issues with respect to fishing.

1. How do you manage the reef in such a way that people do not use it to
increase fishing pressure on area stocks within the first five or so
years until the settled fish can reach the age of first reproduction?
Generally, the structure will have to be in an area in which fishing is
strictly and successfully prohibited for several years.

2. How do you demonstrate that after that time, the majority of fish
caught in the artificial reefs are those that settled there as
juveniles, especially given that there is usually incontrovertible
evidence (as in question 1) that the artificial reefs do indeed attract
fish from natural habitats? And, consequently, how many fish per unit
area of artificial reef do you then allow people to remove by fishing
(basically some very small fraction of the ones that settled there
several years before and managed to survive all those years despite the
high mortality imposed by the in-migrant fish)?

I've seen the impressive thousands of juveniles that often settle on
artificial reef. However, I'm also aware that only a few of those ever
survive to the age of first maturity. As Polovina pointed out very
elegantly more than a decade ago, an area is overfished only once its
natural stocks have been reduced by over 40 - 60% or more. Why then
would anyone think that there is a need to provide more habitat space in
order to get more fish? In most cases, if you want more fish, reduce
fishing. Do not make it cheaper for people to catch fish, as this
usually leads to fish stock depletion beyond the point where continued
overfishing leads to zero net profit. 

Let's suppose you have an artificial reef of 10 by 10 meters, and that
after 5 years, there is one 5-year-old fish per sq. meter. (ignore the
1, 2, 3 and 4-year-olds, you'll need them in the following years). Let's
assume that that species has just exceeded its age of first reproduction
at five years, and is ready to start harvesting. There are now 10 x 10
or 100 fish to be harvested in that year. You cannot even keep one
fisher's family fed with the fish that grew up in that artificial reef.
The truth is, however, that the artificial reef will probably be
supplying far more than 100 fish per year. It will be getting them from
surrounding areas -- that is, unless the decline in fish stocks have
already made that source negligible.

Show me a situation where an artificial reef will not lead to
overfishing. Show me the fishing restrictions and the evidence that they
will be followed. Then show me that parts of it will not lift up during
large storms and destroy natural habitat. Show me that it won't pollute.
Show me that you have not diverted public funds from conservation
efforts, and that the expected returns justify the investment. Show me
that these and other harmful effects have been accounted for and I will
be public ally supportive.

I think artificial reefs, including those enhanced with electric
current, could increasingly be important ways of helping reefs to
recover from losses of corals (perhaps replacing them with
bleaching-resistant corals), and possibly fish. However, they should
meet the above criteria. Very few do.




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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