A shoal of fish stays together mostly by sight. Each fish watches its neighbors and adjusts, so when silt or algae cloud the water, a shoal ought to loosen and drift apart.

Small groups of sticklebacks did the opposite in a new experiment. In murky water they swam closer together, lined up more tightly, and reacted to each other faster. That closeness helped them reach food sooner.

Christos Ioannou, a professor of behavioral ecology at the University of Bristol, led the experiment. His team worked in a shallow tank with 72 fish, at a level of cloudiness far milder than the kind that turns a river brown after a storm. “When we put an unexpected piece of food in the water, it took the fish longer to find it when the water was cloudier. This makes sense and has been shown before in different water-living species,” Ioannou said. “However, we were excited to see that the sticklebacks were able to compensate for this challenge by changing their shoaling behavior.”

Water turns cloudy when fine particles hang in it, from clay and silt to floating algae. Scientists call that cloudiness turbidity. It’s rising in freshwater and coastal habitats around the world as farming, quarrying and construction wash more sediment into rivers. For a fish that hunts by eye, cloudy water shrinks the distance at which it can spot a meal, a friend or a predator. Earlier studies found that heavily clouded water breaks up fish shoals. At milder levels, though, shoals often held together, and in some cases they were tighter than in clear water.

That mild range is where sticklebacks spend much of their lives. Three-spined sticklebacks are small, spiny fish found in ponds and streams across the Northern Hemisphere. Their home waters swing from clear to periodically muddy to permanently murky. Ioannou’s team, based in Bristol’s School of Biological Sciences, tested shoals of two, six and ten fish. The arena was a white plastic tank about 4.5 feet (136 centimeters) long, filled with just 3 inches (8 centimeters) of water. Each fish was around 1.5 inches (3.9 centimeters) long. For the cloudy trials, the researchers stirred in 1.2 grams of kaolin, a white clay. That left the water moderately cloudy, at about a fifth of the level earlier studies found breaks shoals apart.

Food arrived as a surprise. A pipette wrapped in red tape appeared through a small hole in the corner farthest from the fish. Only once a fish came within two body lengths did the researchers squirt out a single bloodworm.

So the fish had to see the red tip first, with no smell or ripple to signal it.

Every shoal got six of these presentations, and a camera above the tank recorded each one at 25 frames a second. In total the team recorded 699 usable presentations from 120 trials. Bigger shoals attacked the food faster in both kinds of water. That fits a long line of work showing bigger groups are better at detecting things.

Cloudy water slowed shoals of every size, and the bigger groups’ edge neither shrank nor grew, so extra eyes didn’t cancel out the haze.