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D’Arros
Island and the D’Arros Research Station have agreed to
extend the loan of their Valeport ECM/CTD unit to MCSS for
a further season. This advanced oceanographic monitoring
instrument measures temperature, conductivity, and depth
as well as current flow, allowing a profile of the marine
environmental conditions to be constructed. These profiles
provide information about how water masses change over
time and for MCSS, they may help in unravelling the
reasons why whale sharks exhibit certain behaviour in
specific areas.
The unit was purchased by D’Arros Island to enable MCSS
to construct these profiles for the areas around D’Arros
as a part of the D’Arros Research station’s on-going
research programme. Initial profiles have already been
developed for the channel between D’Arros island and St.
Josephs atoll and they clearly show how the temperatures
and salinity change with tidal flow through this area.
In the context of whale shark behaviour, temperature and
salinity can both affect the distribution of plankton, on
which the whale sharks feed. In particular, where a body
of cold water meets a body of warmer water, thermal fronts
can develop which promote the aggregation of plankton
along the boundary between the two water masses. These
have been shown to be particularly attractive to the other
big plankton eating shark, the basking shark, in UK waters
where the sharks will cruise up and down the boundary
layer feeding on the rich plankton associated with it.
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Temperature
conductivity and salinity profilevs. depthbetween
D’arros and St Joseph Atoll measured by MCSS
using the Valeport ECM/CTD unit purchased by the
D’arros Research Station
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Thermal
fronts as such, have not yet been recorded in Seychelles
but the strong South Easterly trade winds do set up a
system of up-welling current along the Southern margin of
the Seychelles plateau. This encourages the development of
strong horizontal thermoclines during this season with a
layer of warm water overlaying significantly colder water.
How these horizontal thermoclines affect plankton
distribution around Seychelles is not yet known. It is
thought that there may be some localised thermal front
development off bays in the South of Mahe where warm water
is ‘trapped’ in a bay by the northerly flowing
currents bringing in colder water. It is hoped that the
Valeport ECM/CTD will provide some insight as to if this
does in fact happen and if so whether these features are
used by whale sharks in the same way as basking sharks use
them in temperate waters.
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