Scent Perception in Fish
The word migration generally reminds one of the way birds change location on a semi-annual basis. Yet there are other creatures that migrate on land and sea, as well as just in the air. Young salmon hatch from their eggs in rivers at the end of winter. Some species migrate to the open waters and the sea immediately after hatching, others do so after feeding for a few weeks, and still others after spending a few years in the river. Salmon that have spent a few years in the oceans and reached reproductive maturity now engage in a journey that astonishes human beings.
On this journey, the salmon's objective is to return to the stream where it hatched, and lay its own eggs there. This journey is very much harder than the first one, because the salmon must swim up against the powerful flowing current and leap up over cliffs and waterfalls. Every salmon travels hundreds or even thousands of miles to reach the river or branch where it hatched. Red salmon travel more than 1,609 kilometers (1,000 miles) in the sea and rivers. King salmon and dog salmon swim for more than 3,218 kilometers (2,000 miles) in the Yukon River. Atlantic salmon repeat this migration every year, while other species performing it only once.
In making this long and wearying journey, the salmon has no navigational aids such as map or compass to help it find its way. Even though it has no training, it has no difficulty in finding the mouth of the river it swam down in its youth, and from many river branches, unerringly selects the one that will lead it back to where it was born. The salmon achieves these seemingly impossible tasks because it possesses a perfect scent perception system that functions as a directional locater.
These abilities of salmon were first revealed by experiments carried out in the 1970s. Allan Scholz of Eastern Washington University exposed salmon to one of two odorant chemicals, then tagged the fish and released them. Two years later, when the time came for these salmon to spawn, he scented one of the nearby river branches with one of the scents and another branch with the other scent. The salmon were observed to return to whichever branch contained the scent they had been exposed to when young.
The salmon possesses a dual-nostril nose. Water enters through one nostril and exits through the other. These holes have been so designed as to open and close in synchronization with the fish's breathing. In this way, the salmon can immediately analyze scent molecules dissolved in the water. It perceives the characteristic scent of every individual tributary, which odor stems from plants, animals and soil, and completes its journey by comparing these with the scent memory it recorded on its journey to the sea when young. In short, the fish's sense of smell serves to guide it on its journey of thousands of kilometers (miles).
No doubt the sensitivity of the salmon's sense of smell is one of the countless proofs of the splendor in God's creation. In one verse, God reveals:
Among His Signs is the creation of the heavens and Earth and all the creatures He has spread about in them. And He has the power to gather them together whenever He wills. (Surat ash-Shura: 29)