July 16, 2018 — Across Bristol Bay, scales from fish are being picked, licked, and stuck on cards to be sent to researchers. The reason? To figure out the ages of the salmon making their way up the rivers during the run. One researcher has spent almost 30 summers examining scales and figuring what fish are head where.
Cathy Tilly puts a thin sheet of plastic over a paper card with rows of fish scales on it and then places it into a hydraulic press.
She described the process, “then I can start pumping the pressure up and we go up to 25,000 psi and count to 15.”
It takes that much force to make imprints of the scales in the plastic.
She continued, “Okay and then we use a dump valve to lower the clayton. Pull these metal plates out. Peel it up and what you are left with is an impression of the fish scales.”
After pressing the scales, Tilly takes the small card with the impressions and examines it underneath a microfilm reader.
She said, “Most people describe them as looking like a thumbprint or as tree rings.”
Tilly is figuring out the age of a salmon. Like trees, salmon have rings on their scales that show how old they are. Tilly looks at these markings that indicate the fish’s growth to figure out how many years they spent in freshwater rivers where they were born and how many they spend in the ocean.
Tilly and one other person age all the scales collected from the Bristol Bay sockeye run. That means they each look at tens of thousands of scales in a summer.