Sounds pleasant, doesn’t it? A fishy old notebook? But this is actually a pretty cool story!

One day in June 1919, workers in a busy Canadian cannery in Port Essington rushed to clean, cook, and can the bright red flesh of a huge number of sockeye salmon hauled from the nearby Skeena River. Watching the frenzy was a government “fisheries overseer” named Robert Gibson. Periodically, Gibson selected a fish, scraped off a few scales, and affixed them to the pages of a small notebook using the salmon’s own slime. Next to each sample—he collected a total of 125 on this day—Gibson wrote the weight, length, sex, and catch date. A U.S. fish biologist hired by British Columbia would follow up by calculating each fish’s age with the then-new technique of using a microscope to count the growth rings visible on the scales, much as botanists age a tree.

The dozens of notebooks Gibson filled with scaly fish slime were thrown in a box and forgotten until they were rediscovered 23 years ago. Now scientists are analyzing the salmon DNA in the notebooks in order to study and compare to today’s salmon population.

Read more: Century-old salmon-smeared notebooks reveal past bounty of fisheries