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How
the type of atoll affect the fish diversity
As
mentioned in the general presentation of the atolls,
there are more than 900 different species of fish living
at depths of 0 to 100m in Polynesia: more than 500 of these species inhabit the Tuamotu atolls.
The species are not distributed at random among the
atolls, however: their distribution depends on several
factors.
There
are two possible ways of looking at the diversity of the
fish species: in terms of the total number of species
present around a given atoll (diversity), or in terms of
the number of species present per unit area of the atoll
in question (species density). These two values are
linked, but their consequences differ as regards the way
in which the fish
communities function.
The
number of species which can be encountered at a given
atoll naturally depends on :
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The
larger the atoll, the larger the number of fish
species living there will be. fig: Variation
of the diversity (total number of species in an atoll)
depending on the size of the atoll lagoon. |
Several
explanations have been suggested for this fact. The
first explanation is that the number of species may
depend on the number of biotopes (or ecological
niches) available, which means that a
large island will have a larger number of
biotopes to offer than a small one. The second
explanation is an ecological one. Most of the reef
and lagoon fish have a pelagic larval life. Although
the larvae are able to swim in a given direction,
they are also transported by the sea currents, and
the likelihood that they may colonise an island
depends mainly on the size of the island.
Correlation
between diversity and species density
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It
is worth noting (fig4)
that the total number of species present on an
atoll is closely correlated
with the number of species per unit surface area
of the atoll (which is also know as the species
density). In other words, the larger the number of
species living around a given atoll, the more
richly the biotopes on that atoll will be endowed
with species. |
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This
affects the way in which the communities function, and
especially the way in which the resources available are
used: the more species there are, the better the use
made of the primary production will be. This means that
in otherwise equivalent biotopes, the production rates
of the fish communities are likely to be greater in
general at the larger
atolls than at the smaller ones.
Factors
which affect diversity and species density
The
pattern of diversity and density of the fish species
inhabiting an atoll lagoon is not a random one. The main
factors at work here include: the exposure to the wind,
the depth, the type of substrate and the biotope.
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The
parts of an atoll lagoon which are the most highly
exposed to the wind generally harbour a wider range
of species than the more sheltered parts. This may
be due to the waters being colonised by larvae (which
are swept along by the wind) as well as to the fact
that the winds play an important role in the
distribution of nutrients.
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The
depth is another important factor: the maximum
diversity is observed at depths of only a few metres,
whereas the deepest and shallowest parts harbour
only a few species of fish.
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The
biotope is probably one of the most decisive factors
on which the diversity and the composition of the
fish communities encountered depend. The reefs by
the passes and the pinnacles are the biotopes
associated with the largest number of species,
followed by the windy reefs by the motus; whereas
soft bottoms and grassy and weedy areas are poorly
endowed with fish species.
-
The
roughness of the substrate, the proportion of hard
floors and the coral covering are three other
factors which have positive effects on the diversity
of the fish species.
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The
densities of the species observed in the Tuamotu atoll
lagoons are relatively low in comparison with those
observed in the lagoons of the high-rising atolls or in
the West Pacific, probably due to the fact that there is
a smaller pool of species available. Consequently, as we
will see below, the potential production rate is lower
and the fish populations are more sensitive to fishing
activities and disturbances of various kinds (to
know more).
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Composition
of the pool of species
The
composition of the pool of species (see
the checklist) largely determine the proportions
observed between the various families of fish inhabiting
an atoll lagoon (Tab2).
Most of the families which feature prominently on the
checklist are also those most frequently observed in
situ (allowing for any bias due to the fact that
underwater divers’ observations are known to be a
mthod which under-estimates the numbers of cryptic
species). This means that the specific bio-geographical
region to which the fish belong will greatly affect the
distribution of the species and hence, the way in which
the communities
function.
Tableau
2 :
comparison between the total number of known species in
the Tuamotu atolls (at depths of 0 to 100m) and the mean
specific composition of the fish communities present in
a cross-section of an atoll lagoon.
Similarity
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The
specific composition of the fish communities
varies from one atoll to another. Greater
similarity can be seen to exist in this respect,
however, between the large atolls than between the
smaller ones. In addition, the more widely open
lagoons tend to be inhabited by similar
communities (Fig5). |
References Kulbicki
M;, Galzin R., Harmelin-Vivien M., Mou Tham G., Andréfouët
S. 2000 Les communautés de poissons lagonaires dans les
atolls des Tuamotu, principaux résultats du programme
TYPATOLL (1995-1996). Nouméa, IRD, Doc.Sci. Tech. II3:
26-125
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