Dissertation - HQ

General discussion 163
6.5.2 Why self-recruitment?
Relevance of self-recruitment in models
In these models, we only focus on recruitment back to the natal region.
In the island configuration of the last model for example, if a second
island was introduced downstream and that final gain was set to one at
both locations, then it is easy to guess that all optimal trajectories would
go from the upstream to the downstream island, because it would be the
energetically cheapest route to recruitment 69 . Yet, we would learn very
little from the behavioural mechanisms involved in such trajectories:
they would be different if the island was downstream and to the left for
example, or downstream and to the right. The trajectories and underlying
strategies would be highly specific to the spatial configuration of the
system and, as such, would be difficult to justify from an evolutionary
point of view. By contrast, downward vertical migration for example,
will almost always enhance retention, whether around an island or
along a coast 71,202 . The behaviours involved in retention, and ultimately
self-recruitment, appear consistent between organisms and locations 24 .
Because of that, they are subjected to long term evolution and can be
explained in terms of fitness and phylogeny, along Tinbergen’s view 51 ,
a requirement for the theory of optimal behaviour. The opposite of
retentive mechanisms would be behaviours enhancing advection, albeit
in no particular direction. Indeed, it is difficult to think of mechanisms that
would explain the long term evolution of behaviours such as “enhance
advection to the west only” (because it just happens that there is a
recruitment opportunity there). So this modelling framework does not
apply without modification to situations with many recruitment targets.
It would require to specify additional hypotheses regarding how much
intrinsic value self-recruitment has compared to allo-recruitment, or to
modify the optimisation method to include only partial information
about the environment, hence changing the focus from distal causes of
behaviour to proximal ones. This was not the purpose of this work.
Optimal allo-recruiting
strategies have no
evolutionary justification
Allo-recruitment induces
hypotheses outside our
scope
Sponaugle et al. 24 list many processes potentially affecting self- Modelling allows
recruitment, from adult spawning behaviour, to larval swimming and
orientation, to coastal complexity and flow characteristics. Most of the
paper consists in the discussion of isolated examples illustrating each
to integrate
self-recruitment
causes together
potential effect. By interconnecting larval behaviour and environment
description closely, the modelling framework presented here allows to
integrate those effects together, and we discuss the relative influence
of some mechanisms here. Currently, only a mechanistic modelling
approach allows such quantitative comparisons.
Relevance of self-recruitment strategies for connectivity
It is increasingly obvious that self-recruitment, or at least limited dispersal,
is more common than it was initially thought to be in marine
populations 41,43,44,240–242,281,282 . The very few direct field estimates of
Self-recruitment is
frequent even in a
connectivity context