Message #134
For years, scientists have debated if artificial reefs attract fish or
produce fish. Of course the answer is a little of both but we thought
everyone might like this e-mail from Bill Alevizon, a renowned artificial
reef researcher.
RDBG's Favorite quote from this e-mail, 'I believe that the available
evidence overwhelmingly supports this view [Production], and that the
"burden of proof" has shifted to those who still dogmatically maintain
that ARs simply rearrange fish biomass within an area, and result in no
real increase.'
Here's the e-mail:
An open letter from Bill Alevizon,
Research Associate in Coral Conservation
Osborn Laboratories of Marine Science
Regarding the issue of Attraction Vs. Production
The experiments upon which these (my) papers are based clearly demonstrate
the capacity of artificial reefs to increase local population sizes of
certain types of fishes under the "right" environmental conditions. The
earlier experiments (circa 1982-1985) simply documented the accumulation
of grunts and snappers around replicate sets of artificial reefs placed in
different types of habitats on the Little Bahamas Bank. Because some of
the reefs were
almost a mile from the nearest sizeable recruitment sources ((extensive
mangroves) it was not possible to establish that the recruitment sources
suffered no long-term population decrease concomitant with the observed
increases at the ARs.
This led to the second experiment in the Florida Keys (1989 ref), in which
we actually experimentally demonstrated that upon deployment of an
artificial reef, nearby recruitment sources at first suffered rapid
population loss due to emigration, but soon recovered to pre-disturbance
levels.
Now, of course we could not conclusively "prove" that the observed results
did not ultimately result in fewer snappers or grunts in Trinidad or
Bermuda, but the most parsimonious interpretation of these experiments is
that, for shelter-limited species in shelter-poor coastal habitats (e.g.,
sand plains, grass beds), artificial reefs function by drawing recruits
from nearby small shelter-spaces. These shelters in turn - because they
are now "under-populated" in terms of available shelter - quickly
re-recruit even smaller individuals from less-optimal shelters such as
sponges, small shells, etc. - fishes that will not "make it" much longer
without access to such larger shelter sites as they grow.
So, in a nutshell, here is my take on all of this. All my experience in
observing Caribbean reef fishes at many locations and contexts over the
last thirty years has convinced me that the kinds of fishes (snapper,
grunts) most abundantly recruited to artificial reefs in this region have
a pronounced innate drive to search out and establish residence on any
available unoccupied or sparsely occupied shelter space that will support
individuals of their size, which is of course continually increasing with
age. We therefore have continually-mobile populations of younger fishes,
searching out increasingly larger shelter spaces that will accommodate
their increasing size.
This leads to the results so widely observed and reported -not just by my
experiments but by many others. Artificial reefs in Caribbean waters are
quickly and heavily occupied by such fishes. I am convinced (and my
experiments have shown) that initial rapid population decreases incurred
from habitats near the site of a recently deployed artificial reef are
soon made up by recruitment from nearby, even less-optimal shelters, and
so on, until
we are left with the ultimate recruitment source - an overabundance of
very many small individuals living amongst seagrass blades, etc., most of
whom will eventually fall prey to larger fishes because there is simply
not enough naturally available shelter space to accommodate all these
individuals as they increase in size.
Relatively large artificial reefs, by providing shelter at the top end of
this chain, have ripple effects all the way down, actually increasing the
biomass of these fishes in the area of deployment without affecting
biomass of these same species in adjacent waters; in other words in this
context ARS do not "take" fish from other suitable habitat - rather, they
function by increasing survivorship within a spectrum of sixe-classes
within a population. I believe that the available evidence overwhelmingly
supports this view, and that the "burden of proof" has shifted to those
who still dogmatically maintain that ARs simply rearrange fish biomass
within an area, and result in no real increase.
Of course, my comments are based on the particular species and
environmental contexts specified - I have also seen that where natural
reef is plentiful, ARs are of little value and have very different
recruitment characteristics.
The bottom line is that ARs are a fishery management tool with the
capacity to increase OR simply redistribute fish populations, depending
upon the species and environmental context - there is no simple "either
or" answer.
End of E-mail
At Reef Ball, we have observed similar trends in many varied worldwide
habitats. One exception seems to be that extraordinarily large reefs (in
comparison with natural reefs) seem to hold higher than natural
populations of fish which might crash local resources which some suggest
may reduce fecundity and growth rates in the fish assemblages in immediate
proximity of these overpopulated reefs.
Thanks Bill for your candid yet insightful observations!
You can find more links and contact information to Bill's work on the Reef
Ball researcher pages (www.reefball.org/research.htm)
Have a great day.
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