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How
the type of atoll affect densities and biomasses
The densities and the biomasses
of the fish populations observed in the atoll lagoons
are lower than those recorded around the high-rising
Polynesian atolls, or a fortiori, those in the West
Pacific, for several reasons. One
of the main factors probably involved here is the
bio-geographical position of the Tuamotu islands and the lack of terrigenous
inputs on the atolls.
Lower
densities
and biomasses
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There
also exists a fairly high correlation between
the diversity and the species density, and to a
lesser extent, their biomass, at both the
regional (fig6a,b)
and local levels (fig7a,b). |
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Sp.D. vs Density
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Sp.D. vs
Biomasse
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Sp.D. vs Density
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Sp.D. vs Biomass
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This
means that an island where the fish populations
show a low level of diversity and hence a low
species density will tend to produce lower
biomasses than islands with a higher level of
diversity.
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-
The
effects of terrigenous inputs on the densities and
biomasses are not easy to establish. Comparisons
between the high islands and the atolls have
indicated, however, that the density of the fish
populations is greater around the high islands. Care
should be taken, however, to ensure that
comparisons of this kind are made on similar
biotopes. On the islands which receive terrigenous
inputs, the relationships between the diversity, the
density and the biomass of the fish populations and
the terrigenous inputs (measured in terms of the
distance from the coast) are often rather complex.
The figure
is an example based on the data of the lagoon in the
SW of New-Caledonia.
The
density and the biomass at a single atoll
The
density and the biomass also vary at a single atoll. The
factors affecting these parameters are the same as those
on which the species density depends: large-scale
factors such as bio-geographical factors, the size of
the atoll and its degree of aperture and more local
factors such as the biotope, the exposure to the wind
and the depth of the waters.
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the
larger an atoll is, the more species it can
sustain and the larger the densities and the
biomasses will therefore be (Figures). |
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Surface vs Density
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Surface vs Biomass
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If
we compare two atolls o of a similar size, it will be
that with the largest number of passes or hoas whose
fish populations will show the greatest diversity and
therefore have the greatest densities and biomasses.
The
densities and biomasses generally reach a maximum in the
vicinity of the passes, around the coral pinnacles and
on the reefs fringing the motus. These parameters have
minimum values on soft
bottoms.
The
roughness of the bottom covering is probably one of the
main parameters liable to account for differences in the
density and the biomass of the fish populations
inhabiting a given reef, but very few data on this point
focusing specifically on the atolls are available at
present. Along with the roughness, the coral covering
and the algal covering play opposite roles: the presence
of coral is associated with high densities and
biomasses, whereas the presence of algae tends to have
negative effects.
The
densities and biomasses have minimum values in
windy regions.
The
depth can also play a role: the highest values are
mostly to be found at depths of 2 to 10 m, whereas much
lower values have been recorded in the deepest parts of
the lagoons and on the reef flats than on the sloping
shores.
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Factors
which affect density and biomass : |
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Propitious:
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Unpropitious:
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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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