Using an ultrasound, a tool that can capture pictures of an animal’s insides, biologists can look at the fish’s anatomy. They can even tell if a fish is ready to spawn, or deposit, eggs in the water.

There are also some outward characteristics that help us tell the gender of a fish. Wheeler is very curious about salmon. He said male salmon get darker, and their colors are more intense than females. The males also have a big hook on their jaw and sharp teeth.

Researchers can also look at DNA, or genetic information, to learn different things about fish.

“It requires taking a little piece of their fin, and the fins grow back. It’s like taking a little piece of fingernail,” Wheeler said.

It takes time and scientific equipment to analyze the DNA from the fin, but it can help us learn if a fish is male or female.

Wheeler adds that even fins can sometimes signal if a fish is male or female. Male sharks have a pair of fins called “claspers.” Male mollies also have a special set of pointed fins near their rear ends.