If asked to think of the ecological role fish play in rivers and streams, there’s a good chance you will immediately think of the fish as a predator or possibly prey. But fish can play another crucial role in the ecosystem—they are mini fertilizing machines, delivering little packages of nutrients whenever they poop. This parcel of nutrients can stimulate plant growth—including phytoplankton, which forms the base of the food web.

Just how much nutrients one fish poops depends on many factors, including size. Per gram, small fish use fewer nutrients to grow compared to larger fish. The result? Gram for gram, small fish excrete more nutrients. But since small fish are, well, small, the actual amount excreted (the absolute excretion rate) is lower than in bigger fish.

Now, a new collaborative study from researchers Stéphanie Guernon, Matthew Yates, Dylan Fraser, and Alison Derry published in Canadian Journal of Fisheries and Aquatic Sciences shows that the genetic differences between populations of the same species that influence traits like body size are also important in determining the nutrient level in fecal matter.

This story was written for (and can be read in full at) Canadian Science Publishing.