Which of the following is a common disease in aquaculture that affects fish populations?

Ichthyophthirius multifiliis, commonly referred to as "ich," is a protozoan parasite that is well-known in the aquaculture industry for causing significant diseases in fish populations. It infects freshwater and brackish water species, leading to symptoms such as white cysts on the skin and gills of the fish, which ultimately can result in respiratory distress, stress, and even mortality if not treated promptly and effectively.

The prevalence of ich in aquaculture can be attributed to several factors, such as crowded conditions in fish farms, poor water quality, and the stress associated with handling and transport of fish. These conditions make fish more susceptible to infection and hinder their immune responses, facilitating the spread of the parasite.

In contrast, the other options mentioned predominantly pertain to pathogens that are more commonly associated with terrestrial animals or humans rather than aquatic organisms. While they can cause serious health issues in various contexts, they are not typical diseases encountered in aquaculture settings.