Ashley Ward from the University of Sydney in Australia and Suzanne Currie from Mount Allison University in Canada report chemical cues help fish find others of the same size to form a group with strength in numbers.
Grouping together reduces individual risk from predators, they said, because when predators are confronted by a number of almost identical-size prey animals they find it more difficult to select a target.
"Fish typically form shoals with fish of the same size," Ward said. "The key question that motivated our study is this: How on Earth does a fish know how big it is?
"For humans this is trivial -- we can stand on a flat surface and see whether we're taller or shorter than someone, or we can look in a mirror. These options don't exist for fish, so how do they choose to associate with fish of the same size?"
Ward and Currie studied freshwater fish and the chemical cues that are created as they constantly emit molecules into their surroundings and found shoaling fish preferred the chemical cues of same-sized fish to those of larger or smaller fish from their own species.
"We know the sense of smell is well developed in fish and that they are sensitive to tiny differences in the chemical signature given off by others," Ward said. "So could they smell how big they are themselves and use this as a template to assess the size of others? It seems they can."
The study has been published in the journal Behavioral Ecology and Sociobiology.