Idaho Fish and Game runs 15 screw traps throughout the state to study out-migration of juvenile salmonids (read about the journey of one particular tagged Chinook salmon here). To understand fish migration patterns, we must be cognizant of several things: the season, water fluctuations, weather, and even moon cycles.
During the fall, we see large out-migrations of juvenile Chinook salmon and steelhead. During this time, we capture hundreds to thousands of fish in a single day. We’ve found that on our trap on the South Fork Salmon River, this push of fish often comes near the new moon in October. Likewise, we find that during the full moon phases, we catch fewer fish. This knowledge has produced a spark of interest in attempting to define the relationship between fish migration and the moon’s phases.
To examine this relationship, we took data we collected from the prior two years at our screw trap. We separated the number of juvenile fish we captured into days that were within three days of a full moon and days that were within three days of a new moon. Then, we compared the average number of Chinook salmon and steelhead captured in our trap (see figure below). We found that we captured almost twice the amount of fish during new moon phases as compared to full moon phases. The next question we want to look into is why?
This topic has been widely researched in the natural resource world. From the tiniest specimens of zooplankton located within the water column, to the pollination of plants like joint-pine, to flying creatures like bats, and predacious large cats, the impact of the moon’s phase is ecologically important. Generally, these behaviors are attributed to safety measures for themselves or using the light or darkness to their advantage for feeding. The Chinook salmon and steelhead we catch at our screw traps are no different. The cover of darkness provided by new moons likely provides them with some safety as they start their long migration to the ocean.