[Coral-List] Parrotfish (and Urchin Introductions)

[David Fisk]

If it only was so simple to "put more effort and resources towards
reestablishing this keystone invertebrate herbivore", ie, in this
case, urchins. In itself, this most likely will not fix the issue for the
obvious reason that there needs to be natural controls on the urchin
population.

Without a check on numbers by urchin predators, the reefs will be
overgrazed like many in the Pacific where the carbonate base and fabric of
some reefs are diminishing because of large populations of urchins. Urchin
overgrazing results in algal free substrates but there is no new coral
recruitment happening either, and the remaining live corals are undercut
and eventually carried away by waves and storms. Eventually, increased
exposure of adjacent coastal areas to storm waves are one consequence of
this situation. I have seen reefs in the Pacific where it appears that up
to 30-40cm of limestone pavement has been eroded away by urchins, judging
by the age and size of the remaining few large live corals, which were
probably less than 50 years old.

There is plenty of evidence in the literature indicating that too little or
too much grazing pressure will lead to different but equally undesirable
outcomes. Furthermore, a single beneficial grazing level and density of
grazers (fish or invertebrate) that will enhance natural coral recruitment
will not necessarily be the same for all locations.

It might be worth trying a small trial study for urchin introductions, but
such an intervention would clearly have to have a longer term management
and monitoring component to head off further problems, bearing in mind the
known consequences of getting it wrong, as well as allowing for the risk of
some unknown detrimental factor coming into play.

Cheers, Dave Fisk