Parrotfish and sand production (part 2)

Stoplight Parrotfish (Sparisoma virde)

The sight is common on the reef: a parrotfish swims by and a thin stream of coral dust and sand particles is ejected from underneath.  Coral sand is made here in the belly of the beast.   

Here’s a picture I took last week in Bonaire.  You can see the stream of sand coming from below.  

(BTW - do you know how hard it was to get this shot?  I took a LOT of photos, trying to capture this magical moment.)  

Parrotfish caught in the act of creating more sand.

Our question is simple:  How much sand is created by an average parrotfish in the course of a year.  One set of sources says it’s 200 pounds, another says it’s 2000.  One of those can’t be right. 

This kind of discrepancy comes up all the time, and it’s the reason you don’t want to single-source anything, even simple facts like this.  (In an earlier SearchResearch episode wefound widely varying numbers for how far a specific bird flies.)  

So where do these numbers come from?

200 pounds / fish / year?  The source for most searcher’s “200 pounds” claim seems to be from the Wikipedia article on Parrotfish which points to the article by Thurman (see below for reference list).   The Wikipedia entry looks like this: 

Unfortunately, the link to the Thurman article specified in Wikipedia is dead (it was last checked in 2009) and the target site no longer supports this reference.  I spent some time searching for this chapter, and while I can find lots of references to it, I can’t find the book chapter itself. 

Even worse, many of the references to this chapter I found are clearly copies of the citation and link from Wikipedia.  (Even at such well-respected sites as the Encyclopedia of Life whose parrotfish page-- http://eol.org/pages/5214/details—would be considered authoritative—merely quote the Thurman reference and give the same link to a dead page.) Sigh.  

So went to my local library’s website and inter-library loaned the darn thing.  Amazingly enough, I got the book after only 3 days!  And there, on page 310 is the assertion:  “By eating coral (and corraline red algae) a single parrot fish may produce up to 90 kg  (190 lbs) of sand per year.” 

Case closed?  Not yet.  Unfortunately, THEY don’t give any reference for that bit of data.  Where did THEY get that information from??   

2,000 pounds / fish / year?  The other data SearchResearchers found was the "1 ton / year" number.  The source for most searchers’ “2,000 pounds” seems to originate from the Ichthyology department’s page about parrotfish at the Florida Museum of Natural History (FMNH). http://www.flmnh.ufl.edu/fish/Gallery/Descript/Sparrotfish/SParrotfish.html 

On that page they say “Parrotfish may produce as much as one ton of coral sand per acre of reef each year.” 

But read that carefully:  This means “all of the parrotfish on the reef in 1 acre” are producing around 2,000 pounds of sand over the entire acre.  That’s the contribution by all of the parrotfish working (so to speak) together. 

Sadly, they don’t give a reference for their source material either.  And when I search for the exact phrase as seen on the FMNH site,

     [“Parrotfish may produce as much as one ton of coral sand per acre of reef each year”]

it’s clear that all of the references are using the FMNH page as their source.  (Typically, much of the text in the article is quoted directly, including that phrase. This kind of not-thought-out copying always makes me suspicious.  )

However, by being diligent and following various sources, I also found the 2,000 pound reference as coming from the book “Fishes of the Bahamas” by (Bšhlke & Chaplin, 1993).  The citations would look like this:  

Bšhlke, J.E., and C.G. Chaplin 1993. Fishes of the Bahamas and Adjacent Tropical Waters. University of Texas Press, Austin, 464-482. 

So I go to Google Books and find…  this citation is slightly wrong.  The first author’s surname is Böhlke, not Bšhlke.  (Some character encoding failed and people weren't careful about their cites.)

And, once again, this book falls into the “scanned on Google Books, but snippet-view only” (which doesn’t help).  It’s not scanned on Amazon.com or any of the other sources.  So…

… back to the library.  I ordered this book and got it along with the Thurman book.  Thing is, “Fishes of the Bahamas” is a giant tome:  771 pages of fish data.  Not much plot, but still pretty interesting to read (if you’re a diver). 

In this book I found the sentence:  “On a Bermuda study reef it was recently estimated that one ton per acre per year of this material [“calcareous material,” i.e., coral sand] passes through the intestinal tracts of reef fishes and is redeposited as fine sand, and that the scarids [ parrotfish ] are primarily responsible.” [pg 465]

And the reference given here is…   (finally, a real reference!)

Bardach, John E. "Transport of calcareous fragments by reef fishes." Science 133.3446 (1961): 98-99.

By going to Google Scholar and searching for that author + title I was able to get to the original article:  http://www.sciencemag.org/content/133/3446/98.short

Now we’re getting somewhere.  In that paper (published originally in the well-respected journal, Science) the author writes:  “ It was found that browsing omnivorous reef fishes which rely, in part, on a plant diet ingested and redeposited at least 2300 kg of such material on a 1-hectare study reef near Bermuda.”  

Let’s do some Google conversions to check this number. 

Conversion:

     [ 2000 pounds per acre in kg per ha ] 

means  2241.7 kg per ha (a hectare is 10,000 square meters) 

Converson: 

     [ 2300 kg per ha in pounds per acre ] 

means  2052.0 lbs per acre (an acre is pretty close to 1 football field).  

I wanted to get the full text of the paper, but that’s $20 for a “short-term use” (1-day) license.  As you can imagine, I’m not about to pay for that. 

But we still don’t know how much sand/fish/year that represents.  To figure this out we need to know How many parrotfish live in on an acre of reef?  

Hoping to figure out this sub-puzzle, I started doing searches like this, using the scientific name of the Stoplight parrotfish: 

     ["Sparisoma viride" sand OR sediment production ]

This is reasonably productive (esp. when done in Google Scholar) and leads to several papers on the topic.  The most cited paper is:

Bruggemann, J. H., et al. "Bioerosion and sediment ingestion by the Caribbean parrotfish Scarus vetula and Sparisoma viride: implications of fish size, feeding mode and habitat use." Marine ecology progress series. Oldendorf 134.1 (1996): 59-71.

I got a PDF of the paper by doing this search:

[ filetype:pdf  “Bioerosion and sediment ingestion by the Caribbean parrotfish Scarus vetula and Sparisoma viride" ]
 

That led me to the PDF file, which I printed out and carefully read. 

This is when I realized that this is a much more complex problem than just looking up the number of parrotfish / square meter on the reef. 

By reading this (and a few other papers) I learned that the density of parrotfish on the reef varies hugely by depth.  (Shallow water has a lot; water deeper than 50 feet has fewer.)  They also sleep at night, and feed only between 7AM and 6PM (and, btw, they mate daily, between 7AM and 9AM… but I digress).  I also found out that the quantity of coral ingested when they bite varies a good deal by the size of the fish, but it varies even MORE depending on the kind of parrotfish.

Among other things, we HAVE to be careful not to mix up our parrotfish species (e.g, Hawaiian parrotfish with Caribbean parrotfish, or even parrotfish that live side-by-side in the waters of Bonaire—as the paper points out “…the erosion rates per bite by Sparisoma viride is more than an order of magnitude higher than in Scarius vetula.”  That is, the Stoplight parrotfish (Sparisoma viride) has a bite size that’s ten times larger than that of Scarius vetula (Queen parrotfish). 

But we’re getting somewhere. 

If you read this paper carefully, you’ll eventually find Figure 5 “Scarus vetula and Sparisoma viride Erosion rates per bite (upper graphs) and per day (lower graphs) as a function of fish size.” 

Figure 5 from Bruggemann, et al. (Yellow highlighting mine)

If you look at the graph in the lower right corner, that’s a chart of “mean daily erosion rate (g ind ^-1 d ^-1)” – that is, “grams per individual fish per day.”  Although the “grams / day” vary greatly by fish size, the average is right around 300 grams / fish / day.   Basic math shows us:

300 grams * 365 days = 109,500 grams / year

Or, 109.5 kg / year (204 pounds), which is close enough (given all of our averaging) to our earlier estimate of 95 kg / year.  (And, incidentally, nothing close to 2,000 pounds / fish / year.) 

If this is correct, then to get 2000 lbs of sand contributed per acre by all of the parrotfish, we’re going to need around 10 Stoplight parrotfish / acre.  By my direct observation on the reefs of Bonaire, that seems low, but let’s see if we can’t find some numbers in the literature. 

Another paper I found from my previous query is

Choat, J. Howard, et al. "An age-based demographic analysis of the Caribbean stoplight parrotfish Sparisoma viride." Marine Ecology Progress Series 246 (2003): 265-277.

> http://eprints.jcu.edu.au/6807/1/6807_Choat_et_al_2003.pdf

He “conservatively estimates” 4 individuals / 300 square meter plot.

     [ 1 acre in square meters ]

is 4046.86 square meters in an acre.  If we divide 4046.86 / 300, we should get an estimate of the number of Stoplight parrotfish / acre (according to Choat).  This number is 13.5, a little higher than we estimated before, but not a crazy number. 

Search Lessons:  First, this one took me a long time.  I’m guessing I spent about 10 hours researching this, and I learned a huge amount along the way.  (Not just about the sex life of parrotfishes, but also about why it’s so difficult to estimate the total number of fish on the reef.) 

But I figured out that 1 Stoplight parrotfish generates around 0.6 pounds of sand / day (on average, mid-depth of the reef). 

I did this by going back to the original papers (using Scholar and Google Books) to dig out the charts and original data. 

In some cases, it’s hard to get to the original source document.  As you saw, I had to use my local library to get to the original paper by Thurman.  But once I had it, the rest of the story unfolded easily.  (Although slowly—I had to look up a lot of words.  Acanthurids and scarids and fork length were all new terms for me). 

This is a nice example of sensemaking: Pulling together information from multiple resources and cross-checking them against each other. 

In truth, I read much more than I’ve reported on here.  I didn’t want to tell you that I actually read about 10 papers, some of which were duds.  They seemed good from the abstract, but the reality was that the paper encoded the data in an unhelpful way or that it was about parrotfish on some other continent, or the wrong kind of fish. 

AND... a beautiful 2 minute video of parrotfish feeding from the "Blue Planet" BBC series (discovered by regular reader Ramón... thanks!).   Note the part at  56 seconds in--a lovely shot of sand being delivered to the reef.

____________________________
Reference List

Bruggerman JH, van Kessel AM, van Rooij JM, Breeman AM (1996) Bioerosion and

sediment ingestion by the Caribbean parrotfish Scarus vetula and Sparisoma viridae: implications of fish size, feeding mode and habitat use. Mar Ecol Prog Ser 134:59-71.
http://www.int-res.com/articles/meps/134/m134p059.pdf

Choat, J. Howard, et al. "An age-based demographic analysis of the Caribbean stoplight parrotfish Sparisoma viride." Marine Ecology Progress Series 246 (2003): 265-277.

                  http://eprints.jcu.edu.au/6807/1/6807_Choat_et_al_2003.pdf

Gygi RA (1975) Sparisoma viridae(Bonnaterre), the stoplight parrotfish, a major

sediment producer on coral reefs of Bermuda. Ecol Geol Helv 68:327-359

Thurman, H.V; Webber, H.H. (1984). "Chapter 12, Benthos on the Continental Shelf". Marine Biology. Charles E. Merrill Publishing. pp. 303–313. 

              http://www.geology.iupui.edu/academics/CLASSES/G130/reefs/MB.htm

van Rooij, Jules M., et al. "Resource and habitat sharing by the stoplightparrotfish, Sparisoma viride, a Caribbean reef herbivore." Environmental Biology of Fishes 47.1 (1996): 81-91.

               


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For comparison purposes of parrotfish feeding and sediment production on the Great Barrier Reef:

Bellwood D.R. (1995a) Direct estimate of bioerosion by two parrotfish species,

Chlorurus gibbus and C. sordidus, on the Great Barrier Reef, Australia. Mar Biol 121:419-429

Bellwood D.R. (1995b) Carbonate transport and within reef patterns of bioerosion and

sediment release in parrotfishes (family Scaridae) on the Great Barrier Reef. Mar Ecol Prog Ser 117:127-136.

Bellwood D.R. (1996) Production and reworking of sediment by parrtofishes (family

Scaridae) on the Great Barrier Reef, Australia. Mar Biol 125:795-800.