Japanese researchers Yusuke Miyazaki and Atsunobu Murase studied 261 pieces of gyotaku from tackle-and-boat stores in areas with threatened fish species. They said they learned a lot about the fish in those regions.

The gyotaku pointed to a new way to estimate how large populations of now threatened or extinct fish once were. The researchers recently published their results in the open-access journal ZooKeys.

Gyotaku are already used to teach kids about fish anatomy and as inspiration for modern artists. But their use as a data source could help preserve the kinds of fish they so beautifully document. It’s hard to find good sources for historical information about bygone animal populations, and many documents are lost, destroyed or threatened.

The fish rubbings, on the other hand, contain a surprising wealth of information. The fishermen had often included dates, names, the type of tackle they used, and the name of the species along with the image — a kind of citizen science that’s baked in to claiming bragging rights or drawing attention to a memorable catch.

The researchers said that other scientists could one day validate the finds using DNA that made its way onto the rubbings, or make larger data sets for other research.

Until then, Japan’s tackle shops and fishermen’s homes could hold a gold mine of historical information.