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Lunar Effects on Salmon

According to folklore and almanac entries, there are certain activities best undertaken during particular phases of the moon. Even if one does not subscribe to such advice, there is no denying that some animal behavior is related to the 27-1/3 day rotation of the moon about the earth.

Evidence is mounting that salmon and related species of trout born in fresh water migrate back to the sea at times influenced by the phase of the moon. A possible explanation of this phenomenon is given in an article in the February 6, 1981 issue of Science, written by E. Gorden Grau of the University of California and several coauthors.

By measuring the amounts of thyroxine in young salmon, these scientists have found that the level of the thyroxine dramatically increases near the time of new moon. Thyroxine is the hormone produced by the thyroid gland to control growth. The surge of thyroxine seems to cause salmon to undergo smoltification, a process whereby the fish ready their bodies to enter salt water. Fish released from hatcheries too soon or too late after smoltification occurs have high mortality or fail to grow properly. Hence, there is definite practical value to knowing what triggers the surge of thyroxine in the young fish.

The peaks of the thyroxine hormone appear to occur within a day or two of the time of new moon; the time when the moon is fully dark and is located between the earth and the sun. Tides are greatest at new moon and also at full moon, so if the production of thyroxine in salmon is related to the gravitational forces causing tides, one might expect the thyroxine surges to occur both at new and full moon.

Instead of such gravitational charges triggering the thyroxine surge, the investigators suspect that the trigger has more to do with the amount of moonlight falling on the fishes' habitat. It is known that seaward migration of salmon is restricted to the nighttime hours, and the migration appears to occur mostly during the two weeks of the lunar month starting at new moon.