The effects of thin layers on the vertical distribution of larval Pacific herring, Clupea pallasi

Temporal and spatial heterogeneity of resources is likely to have dramatic effects on the behaviors of zooplankton and ichthyoplankton, which in turn are likely to have strong effects on ecological dynamics such as predation, growth, and mating. The objective of this study was to determine whether vertically thin layers of extreme prey concentration affect the vertical distribution of larval Pacific herring (Clupea pallasi). We employed 2-m tall experimental tanks equipped with video cameras that scanned the vertical extent of the tanks to investigate the effects of thin layers on the vertical distribution of 5- and 10-day-old herring larvae. Three treatments were established: (1) a thin layer of prey (rotifers, Brachionus plicatilis) through density (salinity) stratification, (2) homogeneous vertical distribution of both prey and density, and (3) density (salinity) stratification, but with a homogeneous distribution of prey. We found that in all treatments the majority of larval herring were at the surface, near the light, despite the absence of a peak in rotifer abundance at this depth in some instances. However, there were also clear effects of the thin layers—secondary subsurface peaks in herring abundance occurred at the mid-depths in the stratified tanks, in and around the thin layers. In addition, our results provide some evidence that thin layers specifically, rather than prey patches generally, influence the vertical distribution of larval herring, i.e., larvae may use the physical properties of thin layers to locate and distribute themselves, instead of reacting solely to the prey patches. Thus thin layers can affect directly the vertical distribution of larval herring, and perhaps indirectly their horizontal distribution, as herring larvae live in environments (e.g., estuaries) where advective transport is also often vertically heterogeneous.