Dissertation - HQ

84 Spatial distribution of larvae
Figure 4.9 Principal Component Analysis of the concentrations of the ten most
abundant reef fish families and various physical factors: depth of the chlorophyll
maximum and of the halo-, thermo- and pycnoclines, mean salinity, temperature,
density and fluorometry above the clines, current and wind speed, latitude and
longitude). Only factors with noteworthy projections are plotted.
captured would have been discarded). The regression tree only revealed
current speed as influential, with more larvae where current speed
was high. However, here also, the explanatory power was low (residual
error = 0.85 but residual cross-validated error = 1.35, meaning that, in
many permutations, the split occurred differently, thus is not robust).
In addition, time of day was close to being influential too. Larvae
were indeed captured in greater numbers at night than during the day
(Wilcoxon rank sum test, p = 0.0001). However this was probably simply
related to higher net avoidance during daytime.
Single factor regressions
The exploratory analysis revealed that, beyond intrinsic taxonomic and
ontogenetic variations, two factors seem to influence the density of the
larval reef fish community around Tetiaroa: longitude and current speed.
But both effects were weak and need more thorough investigation.
Current speed explains Current speed was highlighted by both exploratory methods. For
7.6% of variance each rotation considered independently, Figure 4.10 also suggests that
current is usually fast in regions of high concentration. However, larval
concentrations are very low in rotation 4 compared to the others, while
current is faster almost everywhere. Overall, a GLM regression of
standardised larval abundances against current speed shows that its
effect is significant (p = 0.00325), but only explains 7.6% or the variance.
More larvae to the
West of the atoll
The effect of longitude, highlighted in the PCA, is significant whether
it is tested as a comparison between abundances to the East and to the
West of the atoll (Wilcoxon rank sum test, W = 3190, p = 0.017) or as a