Dissertation - HQ

Oriented swimming and passive advection 153
Figure 6.17 Detail of ten optimal trajectories of P. amboinensis in the island case
(top) and five in the promontory case (bottom). Panels on the left present swimming
speeds (black arrows) along 2D views of the trajectories. Most swimming
occurs early on and places larvae in retentive areas afterward. In the island case
physical retention is weaker so more swimming is necessary to stay in the lee of
the topography. Right panels hold 3D representations of the trajectories which
highlight that they exploit the stratification of the current and the topography.
For example, at the tip of the cape, where a powerful surface jet occurs, the
optimal strategy is to move down, where the flow is weaker. Eventually all
trajectories reach areas of reduced surface flow, behind the promontory or the
island. Once retained there, little swimming (hence little energy expenditure) is
necessary to finally recruit. The depth range represented is 0-110 m (the bottom
is not represented at locations where it is > 110 m deep).
flow). Overall, optimal strategies exploit the heterogeneities of the flow
through the interaction of vertical and horizontal movements.
Finally, comparing optimal swimming decisions to potentially available
decisions for P. amboinensis (Figure 6.18) highlights that larvae
seldom swim at their maximum swimming speed. In fact, most optimal
decisions are to “not swim”, which makes sense because swimming is
energetically costly. When they do swim, the mean speed of larvae is
about 2 cm s -1 . The situation for the temperate larva, not presented here,
is similar. Overall, swimming, and particularly swimming at speeds
close to the maximum, is only common early in the larval phase, even
though swimming abilities are weak at this stage. Therefore, the model
suggests that it is more energetically efficient for larvae to swim early on,
even at very low speeds, to reach retentive areas and finally self-recruit,
The importance of
swimming early