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Biomonitoring of coral reef
health using Butterflyfishes

(Family: Chaetodontidae)

Ryan Gounder © 11/6/99


"Coral feeding Butterflyfishes respond to declines in coral quality or abundance by
behavioral and spatial adjustments that can be easily and rapidly observed." (Crosby &
Reese 1996)


A fundamental question that one asks when using Chaetodontidae, or for that
reason any species as biomonitors is: why use an organism when
sophisticated machines are available to detect very small quantities of
pollution in short space and time? One might think that biomonitoring has
become redundant because of these machines, but the opposite is true,
chemical monitoring tells us what is there, but not its effects -
especially long term effects on ecosystems therefore biomonitoring has
become a preferred method of assessing ecosystems (Spellerberg 1993).

The conditions required for healthy coral reef growth as well as factors
causing damage to reefs are well understood (Reese 1981). Usually acute
environmental impacts on coral reefs can be easily assessed (Brown 1988).
However it is not as simple when it comes to chronic ‘sublethal’ factors.
Using conventional methodologies the overall ‘health’ of a coral reef can
be determined against a baseline study (Brown 1988). But it is of no-use
because the damage has already been done, that is why indicators of
‘sublethal’ are urgently required so stress can be detected early, and
counter-measures put forth (Brown 1988, Reese 1981, Hourigan et al 1988).
In this essay the use of Butterflyfishes as indicators of reef health will
be discussed.

What are Butterflyfishes?

Butterflyfishes are consipicious, largly dirunal fishes ranging typically
from 10 - 20 cm in length (Sale 1991). There are 114 species of
Butterflyfishes world-wide (Crosby & Reese 1996), thirty five found in Fiji
(Seeto 1999). The name Chaetodontidae is derived from Greek "khaite",
meaning hair and "odont-" meaning tooth (Allaby 1991). They are, perhaps
with some disagreement, the most beautiful fishes on coral reefs. Watching
a Butterflyfish on the reef is an awesome acrobatic like performance! Their
sheer colourfullness, beauty and apparent will to let people come
physically close to them make this group of fishes rather special (personal
experiences and observations).

The body shape helps Butterflyfishes escape from predators (Seeto 1999,
Allen at al 1998). When Butterflyfishes are viewed from the side, the
impression one gets is that they are comparatively large, but as the fish
move to front view they almost disappear because their bodies are so
compressed (personal observations and experience). Its significance is that
Butterflyfishes are able to live relatively long lives partly because of
lack of predators (Seeto 1999), making them even better indicators of reef

Furthermore Butterflyfishes are either home-ranging or territorial (Reese
1989). What this means is that the Butterflyfishes live out their lives in
the same area of coral reefs unless there is any stress. Butterflyfishes
have a life span up to 10 - 12 years depending on species and have been
observed on the same territories for seven to eight years (Reese 1991). Its
significance is that since the Butterflyfishes are always relatively in the
same area they will "feel" the stresses and respond accordingly (Crosby &
Reese 1996,

Why use Butterflyfishes as Biomonitors?

There are many species of Butterflyfishes that have co-evolved with corals
and are obligate corallivores (Reese 1981, 1991 Harmelin-Vivien &
Bouchon-Navaro 1983). The metabolic or energy demand of these species are
so "intimately linked" to the health of the corals that these species have
excellent potential as indicators of changes on coral reefs (Crosby & Reese

Crosby and Reese (1996) have described four important reasons why
Chaetodontidae are potentially good indicators:

Scientific names of corals and fishes are not required to be known by
the data collectors. Suggested species for Fiji are in Appendix 1.

Information collection can be stepwise, example, the first step could
be counting the Butterflyfishes along the transect and the next step
could be counting the corals. This method allows the matching of time,
recourses and personal available.

Butterflyfishes are best used where there is gradual, chronic
(sublethal) disturbances which would be difficult to measure by
alternative methods, example, collection of tissue and water samples
for analysis. However this method is not appropriate for catastrophic
disturbances, example, oil spills, storms, etc.

Lastly, it is environmentally friendly, relatively cheap,
nondestructive and non-consumptive method that does not require
technical scientific training thus can be used by volunteers, local
communities and individuals.

Practical Application of Butterflyfishes as Indicator Species

Crosby and Reese (1996) propose a 8 step process in the application of this
method. Firstly the problem has to be assessed, the question asked clearly
identified and the goals of the monitoring program stated. The second is
field observations, since the Butterflyfishes are directly observed - it is
necessary to have scuba diving equipment.

The third step involves the establishing of transect lines. Transect lines
are placed in sections of approximately 30m. They are placed purposefully
in areas of high coral cover since change in living coral cover and
behavior of Butterflyfishes are of interest.

The data is recorded on underwater paper. Usually data sheets are
photocopied on underwater paper to make recording of data easier. However
sometimes the underwater paper has to be run through the copy-machine twice
(See appendix 2) to get a good result (Reese 1999).

Step four involves counting and recording the numbers of each species of
Butterflyfish within five meters of the transect lines (See Appendix 2).
This method is known as the Belt Transect Census Method and is reviewed by
Brock (1982).

Step five involves the estimation of coral cover along the transect lines.
Data is again recorded on underwater data sheets (see Appendix 3). In the
sixth step the territory boundary is marked and the chasing behavior
measured (see Fig 2 and appendix 4).

The seventh step is the measuring of the feeding behavior and the size of
the territory (see appendix 5 & 6). The size of the territory is estimated
by measuring the territories to the left and right of the transect line and
then the area in square meters is calculated (see appendix 6). The eight
step is like a clean-up operation, all the equipment (transect lines,
colour-tagged nails, etc) are removed. However in order to return to the
same site, accurate coordinates must be recorded.

The final step is the analysis of the data. A comprehensive data analysis
description with examples can be found in Crosby and Reese (1996). However
correct data analysis depends on an understanding of the design of the
monitoring program (Crosby and Reese 1996).

However in a nutshell under stressful conditions some of the behavioral
changes that is expected: The amount of chases will increase because the
Butterflyfishes will try to find more food in their neighbors territories,
the territory sizes will change and also the feeding rates will change
(Crosby and Reese 1996).


The one major conclusion that can be made from this study is that the use
of Butterflyfishes as indicators of coral reef health may be an excellent
tool for the assesment of reefs in the South Pacific Region. This is
because the use of Butterflyfishes for this purpose is cheap and requires
relatively less resources than other methods.

APPENDIX 1 (compiled from: Reese 1991, Crosby & Reese 1996)

Possible Indicator Species of Butterflyfishes from Fiji region

Species marked with an asterisk (*) are particularly promising indicators.

* Chaetodon baronessa occurs from Cocos-Keeling Island in the Indian Ocean,

to Fiji, north to Japan, and south to the GBR. It is replaced by
C. Triangulum in the rest of the Indian Ocean. Where these
species are abundant they are excellent choice because of their
relatively small territory size. Feeds on Acropora. Feeding mode:

Chaetodon plebeius occurs from the Andaman Sea in the eastern Indian Ocean,
extending eastward to Fiji, north to Japan and north to the Great Barrier
Reef. Apparently it is rare or does not occur in Micronesia and


* Chaetodon trifascialis (formally: Megaprotodon strigangulus) occurs from

Africa eastward to Tahiti

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