Dynamic 3-dimensional structure of thin zooplankton layers is impacted by foraging fish

The potential attraction of fish to thin layers of zooplankton and the role of predation by fish in the formation and persistence of these layers were assessed using 3-dimensional sonar observations. Zooplankton were found in intense layers with vertical scales of 0.2 to 4.6 m with a mode of 2.2 m. These thin zooplankton layers had complex 3-dimensional structure with significant, though gradual, undulations in their depth, thickness, and intensity. Fish spent significantly more time within zooplankton layers than expected, modifying their usual surface-coupled behavior when layers were present. Sonar tracks of individual fish showed them diving down through a zooplankton layer before spiraling slowly upwards through the layer. The upward portion of this behavior was correlated with a dramatic decrease in the intensity of zooplankton scattering at the scale of 1 m2, resulting in the appearance of holes in the layer. Continued observation of layers revealed that these holes slowly filled in with zooplankton an average of 4.3 min after the fish’s departure. Survey results show that when more fish were observed, more holes were observed, and when larger fish were observed, larger holes were observed, so that a total of up to 5% of a layer’s area could be comprised of holes. The thickness of layers was not affected by fish presence. Fish were attracted to zooplankton thin layers, showing that thin layers in natural systems can have significant ecological effects; however, despite fish-associated changes to the structure of layers, the layers were resilient over time to the apparent foraging fish.