Dissertation - HQ

80 Spatial distribution of larvae
Very patchy distribution
Family and ontogenetic
stage drive overall
concentrations
3,624 were measured and the median body length was 3.5 mm; 10% and
90% quantiles were 2.47 mm and 6.3 mm. The small size of captured
larvae and the fact that pre-flexion stages largely dominated the samples
in most families (see Table 4.1) probably indicate avoidance of the net
by larger, older, hence more behaviourally capable larvae. In addition
the early ontogenetic stage limited most identifications to family level.
Indeed, given the important biodiversity in the region, fin rays and
spines counts are often required to identify genera and they are not
fully developed in pre-flexion and flexion stage larvae. Therefore, all
following analyses were performed at family level.
The spatial distribution of coral reef fish larvae was very uneven (Figure
4.6). Two regions of high abundance were present: in the Northwest
during rotation 1 and in the Southwest during rotation 2. Rich stations
were in general associated with extraordinary abundance of pre-flexion
Acanthuridae. Those few stations explain the overall dominance of this
family. When discarded, the first 5 families had similar total abundances
(650-800). Rotations 3 and 4 were overall less structured and samples
were thinner (the difference in concentrations between rotations is highly
significant – Kruskal-Wallis, χ 2 = 44.05, df = 3, p < 10 -8 )
A global regression tree highlights only two factors for the explanation
of the overall abundance of reef fishes: family and ontogenetic
stage (Figure 4.7). In other words, the role of other factors (geographic,
hydrographic, etc.) was negligible compared to the combined influence
of taxonomy and ontogeny. The primary driving factor was family, acknowledging
the fact that Acanthuridae were most abundant. Then, in
the only other significant split, pre-flexion Acanthuridae were separated
from flexion and post-flexion ones which were less abundant (Figure 4.7;
this split also occurred in most other families when Acanthuridae were
excluded from the analysis). These two simple splits explain 13% of
the variability in abundance (residual error cross-validated by permutations
= 0.869). Therefore, it is necessary to assess the influence of those
two biological factors before getting to biophysical correlations.
4.3.3 Intrinsic biological variability
Families are
distributed differently
As taxonomy is such a large determinant of abundance, and the ecology
of different species of fish has been shown to determine aspects of
the distribution of their larvae 162,168 , it is natural to first investigate the
spatial distribution of the different families. The distribution of the
concentrations of the five most abundant reef fish families (Figure 4.8)
reveals that Acanthuridae, Holocentridae, and Lutjanidae were concentrated
in the areas of high overall abundance (Northwest quadrant in
rotation 1 and Southwest quadrant in rotation 2). Labridae and Scaridae
seem more evenly distributed even if they were also abundant in
these regions. Syrjala’s tests and SADIE analysis failed to reveal any
significant dissociation between those families. However, the SADIE
association indexes show higher association within the Acanthuridae-