News

When predators strike, fish need to decide fast: dive or stay? It’s a life-or-death choice, and getting it wrong, whether by overreacting to harmless disturbances or missing a real threat, can be costly. Now, researchers from the Cluster of Excellence “Science of Intelligence” (SCIoI) with Humboldt-Universität zu Berlin and the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), have shown that wild fish shoals become not just faster, but better at making these decisions as they grow in size.

To survive as a species, aquatic organisms have developed a wide variety of strategies. Millions of years ago, sturgeons used their bony shells to fend off predators and relocate their habitats, sulphur mollies survive in toxic, almost oxygen-free water, and fish stocks in the River Oder are gradually recovering from the 2022 disaster. However, there are limits to the resilience of organisms, and these can be seen even in the tiniest single-celled creatures.

In the vast, open ocean with no place to hide, sardines group together for protection. When they cannot rely on speed to escape, the sardines’ best chance is to outmaneuver the predator altogether. However, predators also seek ways to find weak spots in the group’s defence.

Why do animals actually hunt in groups, when they have to share the prey afterwards? Researchers have shown in a field study in the ocean off Mexico: the faster the prey school moves, the higher the capture rate of the striped marlin. This is because if the prey school is moving fast, individual prey fish are more likely to become isolated from the swarm. These isolated fish are then easily caught by the non-attacking marlins. This shows an advantage of group hunting for the predatory fish.

The striped marlin is one of the largest and fastest predatory fish in the ocean. This  species forms groups when hunting schools of small fish, aided by its long, spear-like rostrum. A study by the Cluster of Excellence “Science of Intelligence“ (SCIoI) with Humboldt-Universität zu Berlin and IGB now explains how the fish might coordinate the rapid sequence of their attacks without injuring each other. The key to this, they suggest, is rapid colour changes - like a traffic light. Bright body stripes signalise: "Now it's my turn to hunt".

Clonal fish that are reared under nearly identical, highly standardised environments systematically differ in the number and size of offspring per reproduction cycle, two crucial indicators of biological fitness.

Animal behavioural research is fascinating, not least because it reveals information about how we humans interact with each other as social beings. Behavioural biologists at IGB are deeply involved in exploring the ways in which communities, such as schools of fish, make decisions and the factors that influence individuality. 

A team of behavioural ecologists from IGB and the Humboldt-Universität zu Berlin (HU Berlin) is investigating how animals hunt in groups. Animals in water hunt differently than animals on land, which is partly due to the different size ratio between predator and prey. One example is striped marlin, which belong to the billfish family. Motivation may play a greater role than dominance hierarchy during resource division, but the large fish get out-competed by sea lions when the two predators attack the same prey.