What type of pathogen is responsible for causing opportunistic infections in freshwater fish?

Oomycetes are indeed a significant group of pathogens that can cause opportunistic infections in freshwater fish. These organisms, often referred to as "water molds," thrive in aquatic environments and can become pathogenic under certain stress conditions, such as poor water quality, overcrowding, or altered immune responses in fish.

Many oomycetes, such as those in the genus Saprolegnia, can infect freshwater fish when the host is weakened, often leading to skin lesions and other health issues. The presence of these pathogens is sometimes linked to environmental factors that compromise the health of fish, allowing oomycetes to exploit the vulnerability.

While bacteria, viruses, and protozoans can also cause diseases in fish, oomycetes are particularly noted for their role in opportunistic infections, especially in the context of freshwater species, making them a primary concern for aquaculture and fishery management in freshwater systems.