Dissertation - HQ

116 Oceanography vs. behaviour
Vertical and
horizontal swimming
are indivisible
Interactions with
currents, prey,
and predators
Mesoscale features
disrupt mean flow . . .
. . . and interact
with larval swimming
abilities as they develop
situations. For example, chapter 5 demonstrated that ontogenetic vertical
migration occurs frequently in coral reef fish larvae, and may influence
retention. Actually, even the first model of the early-life history of fish
included vertical migration 21 . Since then, evidence has accumulated,
highlighting the great swimming and orientation capabilities of fish
larvae 25 and the large extent to which they can impact trajectories 70,143,199 .
Progress has also been made in small scale modelling of feeding and
how vertical position is adapted in consequence 223,224 . But the examples
of large scale integration of such models are still rare 78,80 .
Similarly to how vertical diffusion may have a greater impact on
horizontal displacement than horizontal diffusion (because of vertical
shear in the flow 195 ), vertical swimming may impact dispersal trajectories
more than horizontal swimming, because it matters even at very
low swimming speeds (as low as < 1 cm s -1 196 ). However, chapter 5 suggested
that these two components of swimming should be investigated
together, because vertical swimming may place larvae in weak flow
environments where the impact of horizontal displacement increases.
Furthermore, from a biological point of view, no such distinction exists
between “vertical” or “horizontal” swimming: larvae are only faced
with a continuum of possible displacements.
Two sets of biophysical interactions govern swimming behaviour.
First swimming interacts with advection by currents, the constraint
being to reach a suitable recruitment area by the end of the larval
phase. Second, swimming requires energy and there is often a trade-off
between feeding and being fed upon, because food rich areas are usually
also predators rich 16 .
Many mesoscale oceanographic features may contribute to the first
set of interactions. Vortices concentrate or eject particles depending on
whether they are anti-cyclonic or cyclonic 225 . Up- and down-welling
flows are respectively accompanied by offshore and inshore currents
at the surface, which deviate particles from the mean along-shore
flow 226,227 . Fronts, slicks, or other linear features concentrate particles
15,228 . Tidal and estuarine circulation are characterised by a strong
vertical shear 197,198 . All these processes affect particles’ trajectories, making
them diverge from the mean flow. An energetically efficient swimming
strategy would exploit the heterogeneities of the currents 199 , and
such behaviours are probably central because a small displacement
at some point can lead to strongly diverging trajectories. Thus, the
development of swimming abilities and of orientation in larvae governs
their interactions with the currents. While orientation at the end of the
larval phase has been observed for coral reef fishes 25 (see chapters 1
and 2), its development is unknown except for a few exceptions 190,229 .
Similarly, swimming speed and endurance have been studied at the
end of the larval phase (for coral reef fishes in particular 25 ) but their
ontogeny is less known. When ontogenetic data is available, it usually
describes the development of speed or endurance with size and not with
age 25,56,60,190,230 . For these relationships to be used in models, however,