FACT: Salmon are very sensitive to pollution - they require very cool, clean water. If you have salmon in your river, you probably have a clean river!Because salmon have to live in both freshwaters and the sea to complete their lifecycle, they have to cope with threats in both environments in order to survive.
Some of the problems that salmon face are explained below:
Pollution
To be healthy, Atlantic salmon need cool, clean water which contains a lot of oxygen. If the environment that salmon live in becomes changed in a way that is harmful to them, the environment can be said to be polluted. An obvious type of pollution could happen if chemicals enter a river or if hot water is released into a stream. In some places acid rain can lead to acid pollution also known as acidification.
Pollution that can be predicted can usually be controlled, but it is often when pollution happens unexpectedly that the greatest risk occurs. If you see pollution on a river in Scotland, you should contact SEPA.
Habitat problems
Salmon need good quality habitats to live in (see habitats). But sometimes habitats are affected by problems and become more difficult for salmon to continue their lifecycle in. Problems with spawning can be caused by cattle walking in the river and stirring up mud, which can stick spawning gravels together and make it difficult for salmon to make redds. There can also be problems when rivers flood areas and wash away the banks of the river. In this case, the river might become too shallow for salmon to live in.
QUESTION: Can you think of any waterfalls or fish ladders in your area?Access problems
Sometimes salmon cannot swim to some parts of the river because of waterfalls or dams. This can mean that parts of the river are not available for salmon to spawn in. The loss of the spawning area above the waterfall or dam could mean that less young salmon are produced. Fish ladders (special water filled staircases that salmon swim up) can be made next to dams to allow fish to move over obstacles.
Alien species
Not all aliens come from outer space! Sometimes, biologists call plants and animals aliens when they are found living somewhere where they wouldn't occur naturally. An alien species that causes a problem for salmon is the American Signal crayfish. This creature has been introduced to some rivers in Scotland although it normally lives in North America. The crayfish has sharp claws (which can give you a nasty pinch!) and is a predator eating insects, fish eggs, fry and larger fish. The crayfish is not a normal part of the foodchain in Scottish rivers and by eating these foods it changes the way that energy moves through rivers. It also creates burrows in riverbanks which make the banks weak and more likely to collapse.
By-catch
In the sea, there are fisheries for lots of different kinds of fish. Sometimes, when a fishing boat is trying to catch one kind of fish, it will catch other kinds of fish by mistake. The fish caught by mistake are called by-catch. Salmon smolts can sometimes be caught by accident by fishing boats looking for other types of fish, such as mackerel. Often by the time a fishing boat realises it has caught the wrong type of fish, the fish is already dead. Because salmon smolts move as a group in the sea, a fishing boat can sometimes catch (and kill) a lot of smolts all at once.
Fisheries for other fish
As mentioned above, fisheries for other types of fish can affect salmon. Salmon are predators. Fisheries for the fish that salmon eat can mean that they go hungry if the fishery takes too much! See foodchains for more information on what salmon eat.
High seas fisheries/ Coastal nets
There are specific fisheries in the sea that target adult salmon returning from their feeding areas. These fisheries take place in parts of the sea that do not belong to any one country and are called high seas fisheries. Adult salmon coming back to Scotland will tend to use the same general migration route across the sea before choosing to go down either the west or east coast to return to their home river. It is when they are crossing the sea in a big group that they are vulnerable to high seas fisheries. Once they have chosen to follow the coast back to their home river, they could be caught in coastal nets.
Disease/parasites
Fish can catch diseases from each other in the same way that humans catch
diseases from other humans. Some diseases have had a terrible effect on salmon.
One disease that affected salmon skin was caught by almost all of the salmon
in Scotland in the late 1970's. The effects of this disease on salmon numbers
can be seen in the catch
statistics for this period, where less fish were caught because so many
fish died. This shows what a threat diseases are to salmon. Parasites can
also affect salmon. The Scottish Executive is trying to guard Scottish salmon
from a parasite which has resulted in wiping salmon in over 20 Norwegian rivers.
The parasite, called Gyrodactylus
salaris, infects the gills, fins and skin of a variety of freshwater fish
and holds on to the salmon with tiny hooks. It can be brought into Scotland
by mistake if fishermen do not disinfect their fishing equipment properly.
FACT: Since the 1970's, it is estimated that the number of salmon returning to spawn has decreased by 50%.Climate change
Climate change is thought to have already had some effects upon Atlantic salmon and may be partly to blame for their decreasing numbers. There is evidence that the temperature of the sea's surface may affect smolt survival. It is also thought that climate change may affect salmon growth rates or make their food less available. Predator numbers may also be affected by temperature. Scientists do not know exactly what might happen if climate change continues and they are undertaking research to try and predict what might happen to Atlantic salmon under a variety of different climate conditions.