Dissertation - HQ

Discussion 113
expression of different biological requirements and ecological strategies
of the different taxa.
Beyond taxonomic differences, the vertical spread and downward
shift seem to be widespread, and clearly show at community level
(Figure 5.5). They may represent a common strategy to increase selfrecruitment
because downward movement should increase retention 71,84 .
5.5.2 Influence on advection
When particles with the vertical distributions observed here were input
into a realistic flow field interacting with topography, their average
dispersion was not very different from that of vertically immobile
particles. This mild effect is surprising given the body of literature
indicating otherwise 24,71,84,197 . The effect would probably have been
more conspicuous if the advection experiment had been conducted on
a longer time scale and represented vertical shifts of greater amplitude.
Indeed, a large difference may exist between particles starting in the
neuston (eggs, very young larvae) and the pre-flexion larvae of the
model which were released at 25 m depth already. For example, a
numerical model showed that shallow ontogenetic vertical migration
had little influence on particles trajectories and connectivity patterns,
while deeper migration was influential 70 . In fact, in the configuration
used here, positively buoyant particles restricted to the surface layer
showed differences > 15 km compared to vertically migrating ones. In
addition, most post-flexion larvae captured here were still quite young
and, if the downward trend continues, the rate of divergence between
passive and migrating particles should increase in time. In this study,
the choice was made to avoid possibly misleading extrapolations by
restricting the experiment to the period around flexion, when field
information concerning the distributions was available.
However, even with this restricted span of vertical variability and
in a weakly stratified current, the effect of vertical migration was very
sensible for a few trajectories. They may seem anecdotal, but only one
on 10 5 larvae finally recruits 61 and it may well be that the important
cases are the exceptions, rather than the mean. The fact that no passive
particles were retained, while some vertically migrating ones were, could
make the difference between no recruitment and enough recruitment.
Even when retention was effective, the numerical experiment conducted
here suggests that it was not strong enough for larvae to selfrecruit
based on vertical migration alone. After eight days, even particles
initially retained were advected away from the island. In situations
where there is no strong backward flow at depth (as there could be
in estuaries and tidal channels 197,198 , or around particular topographic
structures 71,202 ) horizontal swimming is probably also required to selfrecruit.
In fact, it may be critical for a target as small as a Tetiaroa. Indeed,
the island in the model was 20 km in diameter while Tetiaroa is only
7 km wide. In Tetiaroa, retention through vertical migration would be
Vertical shifts in
distribution may
have more influence
The importance
of “exceptions”
Indirect influence on
horizontal swimming