The Atlantic salmon (Salmo salar) is an anadromous migratory fish found in the temperate and arctic regions of the Northern Hemisphere. The word ‘anadromous’ is a posh term for the fact it is the fish’s habit to migrate from the sea into freshwater to spawn. These fish make an astonishing journey during their lifetime, crossing the whole of the Atlantic Ocean and returning.
The life cycle of salmon is fairly complex. Initially, spawning takes place in streams. Eggs are buried in gravel, the eggs then hatch, and young salmon, known as alevins emerge from the egg. Alevins remain within the streambed until their yolk sac is exhausted. When development is complete, fry (baby fish) emerge from the gravel. The fry feed on the larva of insects. After one to three years of growth, juvenile salmon, called parr, migrate to estuaries, metamorphosing into smolts. Still with me? The transformation from parr to smolt is a complex change, in which the fish acquires the ability to survive in seawater. After this change salmon are then ready to move to the sea to take advantage of the rich feeding grounds.
After one or more years at sea, salmon mature and make their return migration to their hatching stream. Atlantic salmon may complete several migrations and spawning, unlike all the Pacific salmon (Oncorhynchus) species which die after spawning. Their extremely accurate ability to return back to the same stream that they were hatched from has mystified science for many years.
It is thought that one of the most important navigational tools for Atlantic salmon is their sense of smell. During certain stages of development salmon are thought to imprint on smells, a hormone (Thyroxine) secreted by the thyroid gland. This hormone ensures the fish only imprints on odorants that will help it to migrate. Letting it only remember the green green waters of home!
The Thyroxine hormone is also thought to increase the chances of migration. It does this by increasing or decreasing a salmon’s water salinity preference, making them move downstream towards the sea or freshwater. The thyroid gland is stimulated by the nervous system, producing hormones in response to what’s going on around it. These are thought to include water flow rate (influenced by rain) and the temperature of the water. This makes the migration seasonal in nature, by migrating only at certain times of the year, the salmon increase the survival of their young.
During migration it is thought that salmon can detect smells from their river on the currents in the ocean, using their odour memory to guide their return journey. It is also theoght that salmon can detect smells from other fish, detecting whether they are related. It is thought the shoals that form in the sea may become family specific due to this, accentually meaning that salmon follow related fish as they are travelling to the same location as them.
Although smelly cues must help salmon locate their river when in fairly close proximity, salmon must have other navigational tools when in the open sea where any odour would be diluted, becoming undetectable. Research has shown that some fish species can be sensitive to the sun’s position. It is highly probable that salmon use the sun’s position as a way of determining direction, using it as a navigational tool.
It is suspected that salmon also have the ability to sense magnetic fields, using this to navigate. Magnetic minerals in the creature’s brain may function as a biological compass. Salmon fry will change their orientation when subjected to an artificial magnetic field, illustrating their ability to sense these fields. It is thought that the fish imprints a memory of its magnetic latitude and longitude at the time it enters the ocean, controlled through the use of hormones in the same way as odour memories.
There is also thought to be a strong genetic link to the migratory habits of salmon. This factor has been highlighted by the wild population breeding with escapees from fish farms. These captive bred fish seems to weaken the migratory instincts of the wild population.
Atlantic salmon can make several migrations and spawning in its lifetime, this could mean that a certain amount of the journey could be learned or memorised, but recent evidence seems to have discounted this idea. Evidence of the strength of the genetic instinct to migrate in these animals was recently documented in Sebago salmon (Salmo salar sebago) found in land locked Lake Sebago, USA. The population immediately began migrating again when a dam was removed, giving the population access to the sea again. The instinct to migrate survived for 150 years in the landlocked salmon. This evidence suggests to me that learning from other fish and remembering routes is not the source of the migratory behaviour exhibited.
The exact mechanism as to how the fish find their way home may never be fully understood by science, but the accuracy and precision of the species ocean navigation is a startling reminder of the complexity of the natural world around us!