A hierarchical analysis of the spatial distribution of larval fish prey

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We quantified the small-scale distribution of larval fish prey (copepod nauplii and copepodites) to (i) investigate the variability in prey abundance across a range of spatial scales and (ii) determine the statistical distribution (normal, negative binomial or Poisson) that best describes the distribution of prey at scales relevant to larval fish foraging. We used a hierarchal sampling program to collect zooplankton at scales from metres to kilometres using a rosette sampler that collected replicate 2.5 L samples (∼ volume searched by larval fish) at 10 m. A generalized linear model framework was used to investigate the underlying distribution of the zooplankton at different spatial scales. The majority of variance (51%) in zooplankton abundance was found at the metre-scale indicating a high degree of small-scale patchiness was present. The distribution of zooplankton was significantly different from a Poisson distribution but not a normal or negative binomial distribution. Thus, larval fish prey were not randomly distributed within the upper mixed layer, but were more aggregated than predicted by the standard Poisson distribution used in most foraging models. Our results suggest that efforts should be made to estimate the variability of prey abundance at the scale of larval fish foraging rather than using large-scale average abundance estimates, as small-scale prey patchiness likely plays a role in larval fish feeding dynamics.

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