Homeward Bound

By Bill Hauser

How do mature salmon find their way from mid ocean to their home river?

There is no easy answer but first, here is a quick overview of a generalized salmon life cycle beginning with the egg.

Eggs are deposited during summer and incubated until mid-winter when the fish hatch and in spring, become free swimming fry. After some development and growth, they become juveniles called parr or fry until they become smolts that migrate to the ocean. In the ocean, they grow and mature until they migrate back to freshwater to spawn. This sounds easy enough but the details are often more murky.

Okay. The salmon have survived hatching, rearing, the smolt transformation, one to several years at sea, and now they are ready to head for home. What is next? Anything missing? Think about this. These fish are roaming around the ocean. How do they know where they are? How do they know which direction to head to get to their home stream?

They have two challenges. Think about it. Put yourself in a sailboat in the middle of the North Pacific Ocean and you want to get back to the boat harbor in Homer. What information do you need? You need to know where you are and your starting location. And you need to know what direction to travel. How do you figure this out? Without your GPS! If you know your position on a navigational chart it is easy to use a compass to get to Homer.

Effectively, that is all there is to it. That is how the salmon get back to their home stream… know their starting point and compass angle. But this is where the details get a little fuzzy. Fishery scientists do not have the details locked down but some facts are known for sure. Salmon - and most fishes – are capable of using the sun angle as a form of compass for orientation. (This is demonstrated with a simple experiment. Put fish in a circular tank in the dark except with one light at some angle. Feed the fish from one location at some constant angle from the light. After the fish has been trained to find a reward of food based on the light (sun) angle, offset the angle of the light. The fish will search for food at the same offset angle.)

Salmon can also use the earth’s geomagnetic field to orient. (This can be shown with the same experiment by substituting a weak magnetic field for the simulated sun. also, it has been shown that birds and sea turtles orient using the geomagnetic field and other researchers have found magnetic particles in noses of rainbow trout that could be used to detect the geomagnetic field.)

Further, it is clear from tagging studies that salmon are not moving in a random searching pattern. Rather, they traveled steadily in a more or less straight line. Also, sockeye salmon are sprinkled all about the eastern North Pacific and Gulf of Alaska but 80 per cent are able to return to Bristol Bay within a two week period.

I must confess that I have been stalling here… I still cannot answer the first part of the equation. I have not found exactly how the fish know what compass angle they need to follow. I do not know how they know their starting location. I only know that they are good at it and have been good at it for eons.

A clue has appeared in a research report from February, 2013 that demonstrated that salmon are capable of memorizing or imprinting on the earth’s geomagnetic field. Another report that is just published in February 2014 shows that juvenile Chinook salmon actually inherit a map of magnetic fields and use a combination of magnetic intensity and inclination angle to determine their geographic location. Researchers have also shown that sea turtles use this same ability to navigate over long oceanic distances. Other animals, including whales, have been documented to use the geomagnetic field for orientation and navigation.

Finally, some scientists have discovered micro particles of magnetic material in the snout of rainbow trout. These particles align with the geomagnetic field at the cellular level.

So. The final analysis is still out but scientists are slowly working this out. The bottom line… we are not quite sure how, exactly, the salmon can navigate so successfully. But we do know that they can!

As the salmon approach the coast, other cues become useful for orientation and homing. Changes in salinity, currents, water temperature, and odor can be used.

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