A wise man once said “you can judge a man by the company he keeps” and it seems that the same can now be said of fish. Just like humans, many fish like to surround themselves with active companions, even though a posse of active friends also makes for stiffer competition when looking for food or mating partners.

 A recent scientific study measured the metabolic rates of Eurasian minnows (Phoxinus phoxinus) and discovered that those with higher metabolisms (and therefore more energy) were preferred as shoaling partners by other minnows. Metabolic rates have a large influence on fish behaviour and higher metabolisms usually mean bolder behaviour such as exploration and greater success at finding food, but also a greater risk of ending up as food.

 Scientists aren’t yet completely sure of the reasons behind the preference, but they currently believe that the less active minnows enjoy the benefit of swimming with more energised swimmers by swimming in their wake and thus saving energy. The fish might also be simply attracted to more active shoalmates because they look more visually stimulating, which could be increasing their attractiveness to other fish. 

Regardless of the real reasons, the research shows that while fish can choose their swimming buddies based on visual factors such as body size or colour, non-visual factors can play an important part too.