Dissertation - HQ

136 Oceanography vs. behaviour
6.3.5 Discussion
Species with
demersal eggs are more
abundant near shore
Important effect
of predation
Vertical migration
may be less flagrant
The differences in behaviour of our two theoretical larval types, implied
by their reproductive strategies, have important consequences on their
optimal pelagic trajectories. The differences observed qualitatively fit the
few observations of fish larvae densities around tropical islands 130,259 .
Indeed, species with demersal eggs are more abundant in the vicinity of
the island, on the downwind side. This is interpreted as retention. On
the contrary, species with pelagic eggs are found mainly on the windward
side of the island. In that case, they are supposed to come from
an upstream reef and not to be retained there. In our case, recruitment
on another reef is not possible. Nevertheless, Pomacentridae larvae (demersal
eggs) are retained closer to their natal island than Acanthuridae
(pelagic eggs). This underlines the fact that some behavioural variables
may be crucial in determining dispersal trajectories and recruitment
rate.
Of the two environmental variables which interact with behaviour
here (distribution of prey and of predators), the influence of predation
seems to be the most important. It is the prime determinant of a
two orders of magnitude difference in optimal self-recruitment rate
between Acanthuridae and Pomacentridae, and explains the decrease
in self-recruitment when the island-mass factor increases. Early feeding
was initially though to be key in temperate systems. It led to the
definition of the critical period by Hjort 1 , of the match-mismatch by
Cushing 12 , and to many developments of those hypotheses since. In
tropical systems, where feeding requirements are high (larvae have to
feed daily in the model, even at the end of the larval stage when they
are observed to sustain much longer periods of food deprivation in
laboratory experiments 236 ) and with a conservative estimate of predation
rate (the estimate used here is a mean for perciform fishes, mostly
temperate ones 232 , while mortality is likely to be higher in warmer
waters 72 ) it seems that early predation is more important. In fact, a
similar balance between death by starvation or predation was also
considered to explain the variations in the abundance of Cod on Georges
Bank 260 . In addition, the relative influence of early stage predation vs.
late stage predation may be even greater because predation is size
dependent and young, small individuals are more vulnerable 237,238 .
The primary effect of feeding is the oscillation between surface and
bottom, in the quest for high plankton densities. However, most fish
larvae are probably visual feeders and require light to feed 261 and no
restriction on feeding based on the time of day was introduced in the
model. Therefore such oscillations may be restricted to dawn and dusk
periods, when light is still present and plankton already migrates. To
resolve such movements however, the time step of the model should be
decreased enough to capture up and down movements during those
two periods (probably down to an hour). On the other hand, predation
may also participate in the vertical movement of fish larvae, because it