Disease
All fish carry pathogens and parasites. Usually this is at some cost to the fish. If the cost is sufficiently high, then the impacts can be characterised as a disease. However disease in fish is not understood well. What is known about fish disease often relates to aquaria fish, and more recently, to farmed fish.
Disease is a prime agent affecting fish mortality, especially when fish are young. Fish can limit the impacts of pathogens and parasites with behavioural or biochemical means, and such fish have reproductive advantages. Interacting factors result in low grade infection becoming fatal diseases. In particular, things that causes stress, such as natural droughts or pollution or predators, can precipitate outbreak of disease.
Disease can also be particularly problematic when pathogens and parasites carried by introduced species affect native species. An introduced species may find invading easier if potential predators and competitors have been decimated by disease.
Pathogens which can cause fish diseases comprise:
- viral infections
- bacterial infections, such as Pseudomonas fluorescens leading to fin rot and fish dropsy
- fungal infections
- water mould infections, such as Saprolegnia sp.
- metazoan parasites, such as copepods
- unicellular parasites, such as Ichthyophthirius multifiliis
Read more about this topic: Fish Diseases
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—Gilbert Keith Chesterton (1874–1936)
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—Mary McCarthy (1912–1989)