The complex lifecycle of salmon

While driving the Sterling Highway I noticed the giant full moon glowing in the early morning sky. The haze to the west behind Mount Iliamna’s volcano is glowing bright peach. I pull over and sit there gazing at the blazing sky with a whitish peach full moon hanging over and beside the mountain. It was a strangely peaceful and eerie setting.

People have been claiming the phase of the moon affects us all. They say it affects crime rates, dog attacks, fertility and even fishing. You’ve probably heard someone say, “all the crazies come out with a full moon.” I can see moon gravity affecting fishing because of the tidal effects but how could a moon phase or its visibility affect fishing?

I drop the car into drive and pull back onto the highway. It’s hard to stop thinking about something and drive instead. Lately Alaska’s had some really poor king salmon returns. I began wondering if a moon phase could affect salmon survival?

I read about a couple guys down at the University of California (Dr. Gordon Grau and Richard Nishioka). They found a real connection between moon phases and the amount of time a salmon smolt takes to “smolt out” from its freshwater home into a new saltwater home.

Apparently, the amount of “smolt-out” time determines survivability.

I saw that Sea Grant had funded a Professor Howard Bern’s research into peak thyroxin levels in salmon smolt, which happen during a full moon. Thyroxin is a thyroid hormone that compels a smolt to leave its old freshwater home and enter its new saltwater home (smolt-out).

I saw that Grau, Nishioka and Bern’s were all saying smolt thyroid hormone levels peak out during a full moon and that peaking causes them to “smolt out” into the saltwater. Thyroxin appears to kick a smolts metabolism into high gear and produce the energy necessary for a smolt to make the fresh to saltwater transition. I learned this thyroxin peak is also involved in the smolt imprinting process, which allows smolt to remember the smell of the water they were born in, so they can later navigate back home to spawn.

When salmon smolt decide to leave freshwater and head down stream to the saltwater they don’t just point their noses downstream and swim. They point their noses up stream and swim just slow enough to allow themselves to slip downstream. This slow backwards slipping to the saltwater makes them extremely vulnerable to birds and predatory rainbow trout. Predators have forced salmon to become inventive by “smolting-out” at night under the dim glow of a full moon where predators cannot see them. The moon and the darkness surrounding it helps accelerate salmon metabolism while assisting their navigation and predator avoidance.

The salmon life cycle is complex when operating correctly; it can become beyond complex when operating incorrectly. What happens if billions of thyroxin-deprived hatchery salmon smolt are dumped alongside millions of wild salmon smolt? Can a defective hatchery smolt-out contaminate a wild smolt-out and thereby doom them all? What happens to a smolt when the visibility of the moon changes with climate change? Does increased cloud coverage doom smolt who remain in the freshwater? How sensitive are wild smolt to the imperceptible defects of hatchery smolts? Can wild smolt die smolting-out with a defective hatchery leaders who is usually larger and more dominating?

Could thyroxin-defective hatchery smolt fail to properly imprint and decoy healthy wild smolt in the saltwater to anywhere but home to spawn?

The questions can go on and on, while getting extremely complex and taking decades to respond to. Meanwhile, we continue dumping billions of hatchery and wild salmon together while hoping nothing goes wrong.

— Don Johnson, Soldotna