Mechanism:

Mechanism looks at the mechanical relationships between the cause and effect of a behavior.

The mechanism for salmon navigation is BIPHASIC, meaning that it involves TWO independent sets of mechanisms.

1) The FIRST PHASE involves the magnetic field of the earth: This magnetic field is used by salmon as a compass to navigate from up to thousands of miles away to it's home river!

2) The SECOND PHASE mostly involves olfactory imprinting: Once the fish is close enough to it's home river...It picks up on scents that it learned years earlier before it went out to sea and uses these to return to the exact site it lived in during youth. This is the same learning observed by Konrad Lorenz in his work with Geese.

 

Phase 1 expanded...

What is magnetic field detection?

Magentoreception is the ability to perceive the Earth's magnetic field. This used to be dismissed as completely impossible, but now, we know that this ability is found in all all major groups of vertebrates: this is the subphylum Vertebrata that we are a part of. The list of animals with magnetoreception includes flies, chickens, newts, lobsters, the European robin etc.. Magnetoreception is a great example showing the relationship between the living (animals in this case) and non-living (the planet in this case.)[17]

Earth's magnetic field?

Our planet has a magnetic field, in the same way that a di-pole magnet does. There are magnetic field lines that leave the southern hemisphere and curve around the globe and reenter the planet in the northern hemisphere(Look to picture 1). At each location on the globe, these same magnetic field lines intersect the Earth's surface at a specific angle of inclination. These angles are being represented on picture 1 on the right side of the globe, by the arrows at different angles. Picture 2 shows a close up of what I have just described, fish could hypothetically pick up on this inclination angle as well as the field intensity which is strongest near the magnetic poles and is weakest near the equator. In order for this to occur, salmon would have to imprint the magnetic signature of their natal region before leaving to sea, this imprinting could possibly occur similarly to olfactory imprinting. This makes the compass usable for navigation.

 

Picture 1: The earth's magnetic field.

 


 

 

 

 

 

 

Picture 2: Representation of one magnetic field line intersecting the surface of the earth. This is a close up on one arrow of the right side of the globe (each of those represents a possible field line exiting or entering through the surface of the earth) on picture 2. These different angles of inclination function as "magnetic signatures" that are used to identify the fish's position in relationship to their natal region.

 

 

 

Magnetic field detection in Salmon:

There is now accumulating evidence that salmon also use magnetoreception. Iron compounds, also known as manetite, from the head of stockeye and chum salmon have been found which may make possible geomagnetic navigation of long-distance migrating salmon (Picture 3.). [1-2] However, these magnetite crystals in the head of the fish are only half the story because it is unclear how these metalic crystals are used. The simplest explanation of how these particles are used for magnetoreception is that their magnetic movements are sensed by secondary receptors like stretch receptors or mechanoreceptors.

 

 

 

 

 

 

 

 

Picture 3: Chain of iron particles from the head of sockeye salmon.

 

Future possibilities...

A notable inventor named Jacque Fresco has shown that magnetoreception might someday become more applicable to humans: "when our technologies are more closely aligned with natural law, airplanes might use geomagnetic fields for navigation..." [18- p.14] and with further developments in science, we might develop technologies that allow us to sense our own location on the earth using magnetoreception. In any case, as you read this, the earth's magnetic field of the earth is being perceived by many species of animals.

 

 

 

Phase 2 expanded...

Olfactory cues.

Once salmon reach their natal river, it is primarily olfactory cues that they use for navigation (picture 4). Prior to their seaward migration, juvenile salmon imprint on odors associated with their natal site and 18 months later, they will follow these same odors back to their natal site (3). Scholz et al. exposed 1 1/2 year old coho salmon to imprinting chemicals for a month before releasing them into the sea, 18 months later, during the spawning migration, when these fish would be returning from sea, they poured the imprinted chemicals into the river to simulate a home stream; most of the chemically trained fish returned to the chemically scented stream, supporting the odor hypothesis for salmon homing. Hasler et al. Later made the experiment more realistic by having marked salmon embryos develop normally in a pond, examinations on the year that the salmon were expected to return from sea showed that more marked salmon returned than would have occurred by chance; further supporting the hypothesis that the homing process is guided by odors resulting from the local geology and flora.

 

Picture 4: Sockeye salmon swimming up Alaskan stream.