Linking variation in planktonic primary production to coral reef fish growth and condition

Within low nutrient tropical oceans, islands and atolls with higher
primary production support higher reef fish biomass and reef organism
abundance. External energy subsidies can be delivered onto reefs via a
range of physical mechanisms. However, the influence of spatial variation
in primary production on reef fish growth and condition is largely
unknown. It is not yet clear how variability in food delivery onto a reef
interacts with reef depth and slope, and affects reef fish
productivity. Here we test the hypothesis that with increased
proximity to deep-water oceanic allochthonous nutrient sources, or at
sites where transportation of these water bodies onto reefs is facilitated
by shallower reef slopes, parameters of fish growth and condition will be
higher, and this pattern will be further emphasised in areas naturally
higher in primary production. Contrary to expectations, we found no
association between fish growth rate and sites with higher mean
chlorophyll values. There were no differences in δ15N
or δ13C values in fish collected at greater depths across reefs,
suggesting a homogeneous primary production resource. However, the
relationship between fish condition and primary production was influenced
by depth of collection, driven by higher fish condition at shallow depths
within a study site which is a ‘hotspot’ of primary production.
Carbon δ13C values were depleted at sites with increasing primary
production, and this trend was reversed by an interactive effect with
shallower reef slopes. Our results indicate that deep-water ocean
nutrient influences did not translate into observable increases
in overall population growth in planktivorous Chromis
fieldi within the 10–17.5 m depth range, but
show the importance of site specific variation in hydrodynamics and reef
physical characteristics influencing fish carbon isotopic composition and
condition.