Quantifying the structure and dynamics of fish shoals under predation threat in three-dimensions

Detailed quantifications of how predators and their grouping prey interact
in three dimensions (3D) remain rare. Here we record the structure and
dynamics of fish shoals (Pseudomugil signifer) in 3D both with and without
live predators (Philypnodon grandiceps) under controlled laboratory
conditions. Shoals adopted two distinct types of shoal structure;
‘sphere-like’ geometries at depth, and flat ‘carpet-like’ structures at
the water’s surface, with shoals becoming more compact in both horizontal
and vertical planes in the presence of a predator. The predators actively
stalked and at- tacked the prey, with attacks being initiated when the
shoals were not in their usual configurations. These attacks caused the
shoals to break apart, but shoal reformation was rapid, and involved
individuals adjusting their positions in both horizontal and vertical
dimensions. Our analyses revealed that targeted prey were more isolated
from other conspecifics, and were closer in terms of distance and
direction to the predator compared to non-targeted prey. Moreover, which
prey were targeted could largely be identified based on individuals’
positions from a single plane. This highlights that previously proposed 2D
theoretical models and their assumptions appear valid when considering how
predators target groups in 3D. Our work provides experimental, and not
just anecdotal, sup- port for classic theoretical predictions, and also
lends new insights into predatory-prey interactions in three-dimensional
environments.