The largest shoals were almost perfect in identifying real predator attacks. However, their false alarms towards flybys did not increase. This means their decisions really became more accurate! And suprisingly they also became faster!

But diving down into the hypoxic water is costly, so they need to decide if a situation is worth it. We looked at shoals of different sizes and if they are able to discriminate between harmless flybys by any bird and actual predator attacks. Turns out as shoals get larger they can!

Or they join a group! Shoals of sulphur mollies are bound to the surface due to their hypoxic environment. This attracts various predatory birds. But fish shoals are not defenceless: When attacked they dive collectively and repeatedly producing surface waves, that can scare away their predators.

Animals are constantly forced to make decisions. Most crucially if they are in danger of predation or not. Solitary animals can become more sensitive and react to almost everything, which does not increase decision accurcy. Or they take their time, make better decisions but at the cost of speed.