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In order to estimate
habitat amplification, we need to assess abundance.
The sponges are very
fragile so we need to device a way to assess numbers without
laying out transect lines, or measuring strips. |
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We designed a 2 meter long rod with
vertical laser pointers at each end, and a calibrated
flow meter on one side.
The first diver traveled for an estimated 100 meters
The second diver followed above and
behind counting all sponges which were at least 50%
inside the 2 meters wide transect.
The population in one 88 meter long
transect was 64 sponges or about a 1/3 of a sponge per
meter2.
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To measure the area of the sponge population on the reef, divers flew
propulsion
vehicles along and across the sponge bed.
Distances were calculated from elapsed time to travel known distances at a
set speed
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The area of the west reef is 0.17 hectares.
If it had a comparable sponge density 1/3 of a sponge per
meter2, it would contain approximately 635 sponges.
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This many sponges, typically ½ to 1m in size constitute
a major hard habitat amplification.
The density is not atypical. It is less
than the maximum densities in Jervis Inlet equivalent to
2.4 per meter2.
But significantly more than the maximum
of approximately 0.07 per meter2 previously
reported for Saanich Inlet (Leys et al. 2004).
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If we consider the surface area
determined for our typical sponge to be
representative (Austin 2007)
Then this surface area of 3.68 meter2 X
the average density of
0.36 sponges per meter2 =
1.3 meter2 or
a surface habitat amplification of 130%
In addition, the cavities inside and outside the
sponge provide potential refuges or lairs.
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And finally, the porous nature of
dead sponges provides additional
micro habitat.
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As noted earlier, living sponges provide available
habitat to only appropriately armoured species.
80% of the Senanus
sponges have dead bases.
These could stay intact
for decades.
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In a
preliminary survey of biota on one moderate size dead sponge we
found 27 species, including 10 higher taxa.
These results signify that the A. vastus here can be
considered a Foundation species. That is, it amplifies
habitat for many species when dead or when living but with a
dead base. |
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