Dissertation - HQ

134 Oceanography vs. behaviour
Initial mortality impacts
self-recruitment rate
from the island, for a longer time, and are still capable of swimming
back to the island to recruit.
The absolute value of the optimal self-recruitment rate is probably
not realistic here. Indeed, the parameters used in the model are size
order values and do not represent a precise field situation. In particular,
while the space-averaged mortality rate is deduced from the literature
(and equals 22% per day), the rest of the probabilities have to be inferred.
Nevertheless, we can notice that the recruitment rate is two orders of
magnitude lower in Acanthuridae (∼10 -4 ) than in Pomacentridae (∼10 -2 ).
Note that the difference is probably compensated at the juvenile stage
because juvenile mortality is size dependant 48,50,170 and Acanthuridae
recruit at larger sizes than Pomacentridae. Once again, the difference
in recruitment rate is probably related to the greater duration of the
pelagic phase in Acanthuridae, which exposes them longer to predation.
However, this may also be related to their incapacity to swim during the
early part of dispersal. Indeed, all Acanthuridae trajectories beginning
in the lee of the island are entrained through the predator-rich zones
(zones 1 and 2) by the current and this results in high mortality. As
sketched in Figure 6.7, this is not true for Pomacentridae, which use
their rudimentary swimming abilities combined with predominant
currents to avoid these high predation zones, hence diminishing their
early mortality rate.
Figure 6.7 Schematic comparison of the beginning of trajectories starting on the
downstream side of the island for Acanthuridae (left) and Pomacentridae (right).
The shading in the three zones is proportional to the amount of predators and
plankton. Acanthuridae cannot swim at the beginning of the pelagic phase and
are only driven by the current field. This keeps them mostly in the predator-rich
areas and many die (crosses). By contrast, Pomacentridae can swim and flee
these areas.
Sensitivity to the strength of the island mass effect
Stronger island mass
effect leads to lower
recruitment rate
As we just showed, predation and feeding drive the decisions and
trajectories of larvae. Due to their contrasting early life history, the two
types of larvae considered here may be influenced by the island mass
effect in different ways. The sensitivity of recruitment rate to values
of f, the island mass factor (i.e. to the concentration of predators and
plankton around the island) is tested. Once again, the absolute values of