Dissertation - HQ

152 Oceanography vs. behaviour
Figure 6.16 Comparison of five passive (top) and active (bottom) trajectories
of temperate larvae in the promontory case. The gaussian-like shape is the
promontory. When larvae are passive, the first two are entrained away from
the promontory, the next two are retained for a time but eventually transported
away along the coast, and the last one is flushed on shore and away from the
promontory by South-Eastward surface currents. When larvae are active, only
the first one is still unable to reach the promontory; all four others recruit there.
Vertical movements
facilitate horizontal
swimming
High retention with so little swimming is achieved by the exploitation
of the horizontal structure of the flow, as described above, but also of
vertical stratification. Indeed, currents are weaker at depth (12 cm s -1
at 100 m in the incoming flow, instead of 20 cm s -1 at the surface) and
an efficient way to avoid advection is to move down, from surface to
deeper layers. The distances in the vertical are much smaller than in the
horizontal and low swimming speeds are enough to reach large depths
(0.9 cm s -1 allows to move from surface to 100 m in a single three hours
time step). As shown in the right panels of Figure 6.17 for P. amboinensis,
optimal strategies feature such downward movement, in areas of strong
surface flow such as the tip of the promontory in particular. Then, at
depth, the horizontal swimming decisions allow to move from one
current regime (e.g. eastward jet) to the other (e.g. westward returning