Fish 'Smell' How Big They Are; Schools Swim with Same-Sized Mates

You've probably seen it before: schools of fish that act as one being, turning and twisting away from predators or merely floating in your local aquarium. Yet what is so striking about these fish aren't necessarily their actions, but rather how they manage to group together with individuals that happen to be around their same size.

New research by Ashley Ward from the University of Sydney and Suzanne Currie from Mount Allison University may just answer how they do it. Fish use a variety of different sensory cues to locate shoal-mates, but scientists weren't sure exactly how fish chose others of the same size. It's generally beneficial for fish to form groups, since it reduces individual risk from predators. The schooling method that fish use, in fact, can make it far more challenging for hunters to target a single fish, which allows the school to be more likely to escape.

In order to find out exactly why fish choose to school with same-sized groups, the scientists examined which sense fish use to both assess the size of other individuals and determine how big they are themselves. They studied two freshwater shoaling fish species, including three-spined stickleback and banded killfish. During a series of experiments, the researches exposed these fish to a variety of chemical cues from fish that ranged from the same size to other sizes.

Since fish constantly emit chemical cues into their surroundings, their sense of smell is highly developed. They're sensitive to small differences in the chemical signatures given off by others, which make it potentially possible for fish to "smell" how big they are themselves.

The researchers found that both species of the fish preferred the chemical signatures of fish that were the same size rather than of fish that were either smaller or larger. In other words, the fish were indeed able to smell how large another fish was.

This research could help scientists better understand fish behavior in the wild. As fisheries collapse and as various species become scarcer, learning these animals' habits and abilities is crucial.

The study was published online in Springer's journal, Behavioral Ecology and Sociobiology.