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Unlike mammals, the red blood cells of fish have DNA, and can become infected with viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the disease can also develop without the fish showing any external signs of illness, the fish maintain a normal appetite, and then they suddenly die.
The transfer of parasites from open-net cage salmon farming, especially sea lice, has reduced numbers of wild salmon. The European Commission (2002) concluded, "The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same ...
Salmonella species can be found in the digestive tracts of humans and animals, especially reptiles. Salmonella on the skin of reptiles or amphibians can be passed to people who handle the animals. [39] Food and water can also be contaminated with the bacteria if they come in contact with the feces of infected people or animals. [40]
The adult salmon then return primarily to their natal streams to spawn. Atlantic salmon spend between one and four years at sea. When a fish returns after just one year's sea feeding, it is called a grilse in Canada, Britain, and Ireland. Grilse may be present at spawning, and go unnoticed by large males, releasing their own sperm on the eggs.
Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth.
The dominant hosts for viruses in the ocean are marine microorganisms, such as bacteria. [14] Bacteriophages are harmless to plants and animals, and are essential to the regulation of marine and freshwater ecosystems [91] are important mortality agents of phytoplankton, the base of the foodchain in aquatic environments. [92]
Sea lice possibly survive on fish that remain in the estuaries or they transfer to an as yet unknown alternate host to spend the winter. Smolt get infected with sea lice larvae, or even possibly adults, when they enter the estuaries in the spring. How sea lice distribute between fish in the wild also is not known.
Salmonellosis is a symptomatic infection caused by bacteria of the Salmonella type. [1] It is the most common disease to be known as food poisoning (though the name refers to food-borne illness in general), these are defined as diseases, usually either infectious or toxic in nature, caused by agents that enter the body through the ingestion of food.