Marine Science Chapters

5.1.2

Salmon Reproduction







Adult salmon return to their natal stream for reproduction. This migration occurs in a slightly different time frame for each species and for each stream. In general it occurs during summer or fall in the five common North Pacific salmon. It may be triggered by day length, water temperature or other environmental changes. Some salmon swim thousands of miles to get to the mouth of the stream where they were born.


Pink salmon going upstream
Pink salmon going upstream.








Salmon stop feeding as they enter fresh water. Their stomach is no longer needed and it begins to disintegrate internally - leaving more room for the developing eggs and sperm. They begin living off the stored fat in their tissues that has been accumulating during their life in the ocean. When this starts the flesh of the salmon begins to lack flavor, becomes pale in color and is mushy in texture. Most salmon used for food are caught in the ocean where they have the best flavor, color and texture before they start upstream.


Salmon right before entering their stream
Salmon right before entering their stream.

Salmon change their physiology before entering fresh water. In the ocean their bodies tend to dehydrate in the seawater. They constantly drink seawater, using a salt gland to extract the salt, so that they do not get dehydrated. The opposite is true in fresh water, their bodies tend to take on water and become bloated. So, in fresh water salmon stop drinking and begin to process large amounts of urine to rid their bodies of the extra water.


Male salmon with kype
Male salmon with kype.

Male salmon with kype




Male salmon usually develop hooked snouts called kype. Although more pronounced in some species than others, kype is used by males as a means to show their dominant characteristics. In some species males develop other body differences as well - in the pinks the males develop a pronounced hump on their back, in the sockeye the males also have a hump and become very red.
Male salmon with kype.










Adult salmon swim upstream to the same area where they hatched. This is thought to be accomplished by their outstanding sense of smell that 'imprinted' the smell of their natal stream on their brain at birth. This journey may be only a few hundred yards or over two thousand miles depending on the species and the stream. Fish that swim long distances are exhausted by the time they arrive but still have enough energy for reproduction. Some species must jump again and again to get up and over waterfalls in their stream. Only the fittest fish make it to the spawning grounds.
Salmon starting upstream
Salmon starting upstream.


Male sockeye salmon
Male sockeye salmon.

Male salmon arrive first and start to stake out territories. They can be seen biting each other and displaying their kype. The best areas are in gravel beds where the gravel is not too small nor too large and there is the right amount of water flow to oxygenate the eggs without washing them out of the gravel.


Sockeye salmon pair
Sockeye salmon pair.

When the female salmon arrive the spawning process begins. It is the female that initiates spawning by digging a shallow nest for her eggs in the gravel. She does this by turning slightly on one side and fanning her tail. This disturbs the gravel and much of it is pushed downstream by the current. The depression in the gravel is called a redd and will be the nest for the salmon eggs.


After the redd (nest) is ready the female signals that she is ready to lay her eggs by touching her anal fin to the gravel. There may be other cues as well because this is the time males appear and there is a final competition for dominance. The winning male joins the female salmon and side by side they release their gametes (eggs and sperm) above the redd.


Salmon reproductive opening
Salmon reproductive opening.

Salmon have external fertilization. They release their gametes (eggs and sperm) through an opening right in front of their anal fin.


Orange salmon eggs White salmon milt
Orange salmon eggs (left) and white salmon milt (right).

Female salmon release orange eggs. She may lay two to ten thousand eggs. Each of these is about the size of a small pea. Salmon eggs are rich in yolk - the material used as the food for the developing fish.


Male salmon release white milt. The milt contains millions of sperm.


Salmon gametes mixing
Salmon gametes mixing.

Fertilization occurs as the salmon eggs and milt mix. The fertilized egg then settles into the redd (just below grade so that the flow of the stream does not move the fertilized egg).


The pair moves upstream and repeats the process. This time the gravel that is dug for the second redd washes downstream and lands on the first redd thus covering it with just the right amount of gravel. What a neat trick these salmon have for protecting their nests. A pair will do this until they are out of gametes or out of energy. Males may spawn with several females if they are strong but females usually just spawn with one male. All of this may take only a day or two once the adults have reached the spawning grounds.


Salmon stream with dying salmon
Salmon stream with dying salmon.

Dead salmon Dead male salmon
Dead salmon lining the streamside (left). Dead male salmon with kype along streamside (right).

After spawning the salmon die. All North Pacific salmon die after spawning. The females may guard their nests for a week or two after spawning if they still are alive. They get weaker each day and eventually are washed downstream and end up on the river banks.


Grizzly bear eating salmon eggs
Grizzly bear eating salmon eggs.

Dead and dying salmon are food for predators such as bears and eagles. They are easy catch in their spawning grounds as the streams are shallow and the fish are listless. Bears may also develop a taste for salmon eggs and can be seen sitting in the spawning streams scooping up handfuls of the orange salmon eggs to eat.


The fertilized eggs hatch in three to four months. The young fish hatch in a form called an alevin where they still have a large amount of yolk attached near their throat. This yolk may last them for a month or more so that they are not immediately in need of food.


Baby salmon eat insects in their stream. They will eat anything they can find and when they are about one inch they are called fry. Each species spends a different amount of time in the stream as a baby. Also, each stream is a little different.


All baby salmon eventually migrate downstream to the ocean. Once in the ocean their real feeding begins. They feed on fish, shrimp, squid and anything they can catch. Most of the smaller salmon are also able to feed on plankton with their gill rakers. As they grow they add a lot of muscle and oil from the abundant food in the ocean.


Some salmon stay close to their stream and others migrate far out into the ocean. This depends a little on the species of salmon but more on the availability of food. Every species has its own approximate number of years it spends in the ocean. Then each surviving adult returns to its natal stream for reproduction and the process continues year after year.



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(Revised 21 June 2006)
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