Red salmon became the subject of the study after Finney realized the nutrients those fish gather in the ocean could be tracked back to freshwater by measuring a stable nitrogen isotope in lake sediments. Red salmon spawn only in systems connected to lakes. Some of the fish spawn in the lakes, but most pass through to spawn in tributaries. After fish die, the decomposing carcasses wash back into the lakes, becoming a huge source of nitrogen fertilizer. By analyzing core samples from lake beds, scientists were able to get a yearly look at the nitrogen deposition from the rotting carcasses.
More nitrogen means more rotting carcasses.
Other physical markers, such as ash deposited by volcanic eruptions, helped set a date for the mud layers. Over the past 20 years, nitrogen levels stabilized as state biologists tried to stabilize annual spawning returns. Before the 1970s, however, the runs yo-yoed through time. The researchers took the peaks and valleys of those runs and compared them with recorded climatological data. The correlation became clear.