Dissertation - HQ

118 Oceanography vs. behaviour
Fitness during the
larval phase is
survival (and growth)
Self-recruitment is
essential to metapopulations
dynamics
For example the swimming endurance of some individuals may be
more than twice the mean of the population 236 . Finally, all sections
of chapter 1 highlighted how the different aspects of larval behaviour
could influence the outcome of the pelagic stage: oriented swimming
may enhance retention in food rich areas and facilitate recruitment
to suitable nursery zones, efficient foraging would increase growth
(because food is limiting), schooling would affect swimming, feeding,
avoidance of predators, and enhance recruitment. Surviving the larval
phase is a pre-requisite for future reproduction, and fitness can therefore
be summarised in term of survival, or recruitment success. Given
the very low recruitment rates observed in fishes (around 10 -5 45,61 ),
selective pressure is likely to be strong during the pelagic phase, so that
any variation in these behaviours which enhances survival would be
strongly favoured. In addition, faster growth increases the probability
to survive the larval phase 237,238 and to persist once installed 48,50,170 .
Therefore selective pressure on energy intake and energetic efficiency
is also probably intense. Overall, the behaviour of marine larvae, and
of larval fishes in particular, can be viewed, studied, and predicted
through the prism of the theory of optimal behaviour, thus providing a
means to avoid the “simplifying assumption” 22,25 of passive transport.
In marine metapopulations connected by larval dispersal, self-recruitment
was initially though to be uncommon and local populations were
expected to be replenished largely by larvae originating from elsewhere
22,239 . However, as detailed in section I.2.3 (page 9), self-recruitment
is in fact essential to metapopulations dynamics. First, one self-sustaining
population may be sufficient to maintain many other sink populations.
Second, the shortfalls in self-recruitment of all local populations multiply
in the persistence condition of the whole metapopulation 30 . And
indeed, self-recruitment was found to be higher than expected in marine
populations (between 20 and 60% 43,44,240 ). Larval behaviour was
often evoked to explain such high proportions of retention 24,57 . Nowadays,
the paradigm has shifted from early ideas of open populations
to current conceptions of restricted dispersal 241,242 . Therefore, it seems
important to examine the behavioural processes that favour or impede
self-recruitment.
This chapter presents a framework for the inclusion of behaviour in
models of the early life history of fish, and two applications. Rather than
trying to implement the very few known facts about larval swimming,
orientation, and feeding as behavioural rules in the model, the rules
emerge from the interactions with the environment through the application
of optimal behaviour theory with biologically sensible constraints.
Within the three categories of early life history models 222 , the purpose
of this modelling framework is clearly inferential, possibly hypothesis
generating, but not explanatory or descriptive. First the framework is
presented through a very simplified model, then two complete models
are constructed along its guidelines. The first is used to investigate the
trade-off between predation and feeding in two species with contrasting