[Coral-List] bioerosion and Bumphead Parrotfish

[Douglas Fenner]

     One thing that seems to be missing from the bioerosion discussion so far is the role of large parrotfish in the Indo-Pacific.  In a 2003 article, Bellwood, Hoey & Choat report that of the 3000 or so coral reef fish species, only the parrotfish family produce significant bioerosion, and among the parrotfish in the Indo-Pacific, most of that is due to 2 species, the majority of which on the outer, clear-water Great Barrier Reef is due to just one species, Bolbometopon muricatum.  These huge parrots travel in schools of 30-50, and each individual is estimated to remove 2.33 m3 or 5.69 tonnes of carbonate a year, which with the population of these fish on the GBR crest amounts to 27.9kg/m2/yr.  They say that annual calcification on coral reefs is usually estimated to be on the order of 3-10 kg/m2/yr, but may approach 35kg/m2/yr.  They say that their figure for B. muricatum is the highest erosion rate ever recorded on coral reefs, and alone balances local accretion.  Remember, this was on outer northern GBR, some of the least human-disturbed reefs in the world.  They went on to look at a total of 44 sites across the range of this species, from Indonesia to Tahiti, and found that the population of this species goes down rapidly with increasing human impact and quickly approaches zero.  These huge fish sleep in holes too small to fit in, and are easy prey for night time spear fishing (with waterproof flashlights, which may be the critical piece of technology).  Howard Choat has shown me a photo of a skiff filled with them from one night's spearfishing of a small group of spearfishermen in the Solomon Is.  Dulvy and Polunin report that at one point in Fiji, they comprised a majority of the fish in the fish markets, but now they are rare in the markets and locally extinct in parts of Fiji.  Gerry Davis and Chuck Birkeland tell me that big schools used to be common in Guam, but were spearfished out in the '70s, and now they are rare there.
    It seems to me that if people are concerned about bioerosion, it would be far easier to monitor the populations of B. muricatum than microborers or even macroborers.  They're huge, out in the open, and in most places the job of recording them is very easy- just write 0, 0, 0, 0 on your slate.  Probably most monitoring reports don't say what their populations are, because they didn't record any.  Well, none is a real number, folks, and in this case it's important to report.  It's a measure of what we've lost.
    Further, this raises an interesting question.  Some people have been hinting that reef accretion might be equal to reef health.  If so, then perhaps we should encourage the spearfishing of this species, and not shed any tears if it goes locally extinct.  That will reduce bioerosion, and that's a good thing, right?
    I think people are on the right track when they say that bioerosion as well as accretion are important.  And likely neglected.  I do seriously advocate monitoring populations of B. muricatum, and think it would be good to do for macroborers and microborers, if a practical method can be developed for monitoring programs.  But I also point out that coral reefs are not just geological structures (and I'm not saying anyone has said they are just geological!), but they are also ecosystems.  One of the points that Bellwood et al are making is that while Indo-Pacific reefs, particularly near the diversity center, are incredibly species-rich, some guilds have very few species, and in this case the guild of bioeroding fish falls to just one species, which has been removed from much of its range, with unknown but perhaps major consequences for these ecosystems.  B. muricatum is also a coral predator, with about half it's bites on live coral.  So its removal may have effects on coral populations as well.  Once again, humans are performing an unplanned experiment on natural ecosystems, with unknown consequences.  Not smart.  I personally think we need to start thinking seriously about protecting large reef species in places where they have been depleted, which is nearly everywhere people are.    And not just inside MPA's, which are usually too small to protect this species which ranges over a large area, and other large fish like sharks, which also move over a large area.  Friedlander and DeMartini have reported that big predatory fish (sharks and Giant Trevally) are amazingly abundant in the Northwest Hawaiian Islands, which are some of the most pristine reefs in the world.  In something like 155 transects around the Main Hawaiian Islands (where the people are), just 1 shark was recorded.  They point out that removing a huge biomass of predatory fish certainly has an effect on the food web.  I personally think the big reef fish (including Humphead Wrasse and some big Grouper species) need to be protected throughout whole archipelagos or reef systems, until they recover.
 
   -Doug Fenner

The opinions expressed here are those of the author alone, and not that of any institution.

Bellwood, D. R., Hoey, A. S., and Choat, J. H.  2003.  Limited functional redundancy in high diversity systems: resilience and ecosystem function on coral reefs.  Ecology Letters 6: 281-285.

Dulvy, N. K. and Polunin, N. V. C.  2004.  Using informal knowledge to infer human-induced rarity of a conspicuous reef fish.  Animal Conservation 7: 365-374.


Friedlander, A. M. and E. E. DeMartini.  2002.  Contrasts in density, size, and biomass of reef fishes between the northwestern and the main Hawaiian islands: the effects of fishing down apex predators.  Marine Ecology Progress Series 230: 253-264.

Sadovy, Y., Kulbicki, M., Labrosse, P., Letourneur, Y., Lokani, P., Donaldson, T. J.  2003.  The humphead wrasse, Cheilinus undulatus: synopsis of a threatened and poorly known giant coral reef fish.  Reviews in Fish Biology and Fisheries 13: 327-364.