Salmon said: The way home, no matter how difficult it is, is always "fragrant"!

Author: Black Cat Nino

The article comes from the Science Academy official account (ID: kexuedayuan)

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Every year before the Spring Festival, the world's largest human migration takes place - the Spring Festival travel rush. No matter how far away from home, people will try their best to buy tickets to go home, just to go back and have a reunion dinner. In the aquatic world, salmon (including Pacific salmon and Atlantic salmon, also known as salmon) are similar to the Chinese, with a strong sense of returning home.

Salmon are born in freshwater, and when they grow to a certain size, they migrate to the ocean to live. When they mature, they will carry out reproductive migration. They travel thousands of miles upstream in groups, often having to overcome the potential energy of gravity and jump over waterfalls, returning from the ocean to freshwater to reproduce. What is amazing is that they have a precise navigation system, and most of them can return to the river where they were born and raised. How do they do this?

In fact, they "smell" their way home.

Salmon swimming upstream (Photo credit: veer photo gallery)

Do fish navigate by smell?

In the 1950s, scientists Hasler and Wisby discovered that after training, salmon could distinguish whether researchers placed food or electric shocks in different rivers. Once their olfactory organs were damaged, they lost the ability to distinguish. They then proposed the hypothesis that salmon migration is based on smell: young salmon living in freshwater have a deep olfactory memory of some chemicals in the environment (including the water environment in which they grow and the environment in which they migrate to downstream rivers). When they grow up, they will use these memories to migrate and find their former homes.

This is a wild idea, how can we prove it?

Scientists used artificially bred salmon of the same age as experimental subjects and conducted a rather interesting experiment. They divided the salmon into groups, raised them in pools containing different compounds and marked them. The first group of pools contained morpholine at a concentration of 5 x 10-5mg/L, the second group of pools contained phenylethanol at a concentration of 1x 10-3mg/L, and the third group of pools served as a control without adding any compounds. To avoid water quality effects, the three pools used the same spring water. (Note: Studies have found that salmon can detect low concentrations of morpholine and phenylethanol, and these two compounds do not appear in natural waters.)

When the tagged salmon grew up, scientists released them back into Lake Michigan.

(Image source: Reference 1)

A year and a half later, the experimenters released morpholine and phenylethanol in more than a dozen rivers flowing into Lake Michigan, set up observation points, and caught salmon in the rivers to see if the migrating adult salmon would return to the rivers containing specific juvenile compounds. The experimental results showed that the vast majority of salmon in the first and second groups (>90%) migrated to the rivers containing the corresponding compounds (morpholine was released in 10#, phenylethanol was released in 7 and 8#, see figure). The salmon in the control group that year migrated to multiple observation rivers and did not specifically return to the river where the compound was released. This experiment shows that salmon can use their sense of smell to find their way home.

Further research found that if the noses of adult salmon were artificially blocked with vaseline, cotton, etc., the proportion of them returning to their original birthplace was significantly lower than that of the control group, that is, it was difficult for them to find their original birthplace. This further illustrates the importance of smell for salmon reproduction and migration navigation.

(Photo source: veer photo gallery)

The memory of salmon: the special taste of the place where it was born

In the natural environment, there is a very complex mixture of chemicals. What smell guides the salmon back home?

Scientists have been looking for special substances that play an important role in salmon migration. The substances they have studied include bile acids, pheromones, microorganisms, amino acids, etc.

Through electrophysiological recording and other means, people have found that the olfactory system of salmon is quite sensitive to amino acids. Related animal behavioral studies have also found that young salmon have a clear preference and tendency for water environments containing extremely low concentrations of water-soluble amino acids (10-6 mol/L). If young salmon are exposed to certain specific amino acids, they will prefer environments containing these amino acids when they grow up. Amino acids, as the basic components of protein structure, are widely distributed in the natural environment, so they are also widely considered to be the preferred memory imprint molecules for salmon. In addition to the smell in the growth environment, some scientists believe that some smells released by their own kind may also guide the migration of salmon.

So, when exactly do salmon remember these flavors?

As we mentioned earlier, salmon generally enter the ocean after they become adults. So the time window for the formation of memory imprints is most likely before they migrate into the sea. People speculate that the juvenile development period of salmon is the critical period for them to form olfactory memory imprints. During this period, researchers found that the level of thyroid hormone (Thyroid hormone) has increased significantly. The increase in thyroid hormone levels has been found to be closely related to the growth and development of olfactory-related cells in young salmon and the sensitivity of detecting tastes. Therefore, researchers speculate that the increase in thyroid hormone levels may promote olfactory learning and the formation of memory imprints. Interestingly, related functional studies provide strong support for this speculation. Studies have found that while giving young salmon a taste, artificially increasing their thyroxine levels helps them maintain long-term memory of the taste.

In short, perhaps the smells of the flora and fauna in the river and the ecological environment on both sides of the river are intertwined to form the special smell of the salmon's birthplace. At the same time, the physiological process of the precise regulation of hormone levels in the salmon's body is likely to promote the olfactory imprint of the smell of the birthplace in young salmon.

Small salmon (Photo source: veer photo gallery)

Conclusion

It is worth noting that scientists generally agree that salmon use their sense of smell to navigate their migration after they reach the coastline and enter fresh water. However, the scientific community is still unclear about how salmon navigate in the vast ocean and migrate to fresh water. It is currently believed that salmon in the ocean may use multiple sensory organs including vision and smell to make judgments. Information such as the sun, magnetic field, temperature, ocean currents, food, and smell in the water may all be used by salmon to find their way home.

Over the past half century, human beings have been increasingly damaging the natural environment. The construction of dams and polluted rivers have seriously affected the migration and reproduction of salmon. In 2018, a study by the University of Washington and the NOAA Fisheries Northwest Fisheries Science Center showed that ocean acidification caused by increasing carbon emissions is likely to damage the sense of smell of salmon, thereby affecting key behaviors that rely on smell, such as avoiding predators, navigation, and reproduction. Here, the author calls on everyone to protect the earth's environment and give animals a living space.

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