Dissertation - HQ

166 Oceanography vs. behaviour
the ocean at random) and such behaviour would be advantageous
selectively. As a consequence self-recruitment and reduced dispersal
should be high; and they are. Thus, long distance dispersal may only
be regarded as a side effect of having a larval phase, rather than as a
cause for it. Regarding our models, this means that swimming decisions
maximising self-recruitment also make sense in a connectivity context.
6.5.3 Model validation
Distribution patterns
agree with observations
Vertical swimming
found in real-world data
Model predictions
After justifying the hypotheses of optimal behaviour and focus on selfrecruitment
a priori, we evaluate their performance when implemented
in a model. As already noted, in the model, Pomacentridae are distributed
closer to shore than the other species, which have pelagic eggs.
This qualitatively agrees with observations near coral-reefs 168,259,293 . Similarly,
fish larvae are observed to accumulate in the lee of emerged
land 74,130,215,259 or at the edge of eddies 164 and these are also features of
the model.
Besides larval trajectories, that are the focus of all early life history
models, our model also predicts behavioural strategies. One feature
highlighted by the second model is that swimming downward, particularly
in areas or intense surface flow, is an effective means of retention, or
at least places larvae in environments where other behaviours make retention
possible. This also agrees with the observations that ontogenetic
downward migration increase retention 71,84 (see also chapter 5).
Nevertheless, data allowing validation of such models is still very
scarce and we have to resort to predictions about what should be
observed. Because not all larvae behave optimally, young larvae are
expected to be distributed differently from what the model predicts (all
strategies would still be present at this stage, including non-optimal
ones). The agreement between model and observations should progressively
increase for older larvae, as mortality filters out the bad strategies.
Yet, these models are more about processes than about the resulting
patterns, so it would be more interesting to focus on decisions than on
trajectories. Currently information regarding swimming decisions is virtually
absent, except for the latest larval stages 25 . Hence the occurrence
of early swimming, or its shoreward orientation in the island case for
example (Figure 6.17), cannot be checked. New observational devices
should prove useful in that respect (chapter 2).
6.5.4 Consequences of larval behaviour on connectivity
Events early in larval life
determine retention
Models help to get a mechanical understanding of processes. Here, the
processes that increase self-recruitment can be identified and their consequences
in terms of population connectivity, inferred. First, mortality
by predation early in larval life seems key in determining the magnitude
of self-recruitment, in particular in environments where resources are
very concentrated near shore. In addition, early swimming is the most