What causes fish parasites?

Answered by Randy McIntyre

Fish parasites are caused by a complex life cycle involving multiple hosts. The parasites, specifically roundworms, infect fish when they feed on intermediate hosts. These intermediate hosts can be various marine invertebrates such as crustaceans, mollusks, or even smaller fish. When the fish ingest these intermediate hosts, they become infected with the larvae of the parasites.

The definitive hosts for fish roundworms are typically marine mammals like seals and dolphins, as well as birds that feed on fish such as cormorants and seagulls. These hosts play a crucial role in the life cycle of the parasites. When the marine mammals or birds consume infected fish, they become the final hosts for the roundworms.

Once inside the definitive host, the roundworms mature and reproduce. The adult parasites release eggs into the host’s digestive system, which are then excreted in the faeces. These eggs are resistant to environmental conditions and can survive in water for extended periods.

When the infected marine mammals or birds defecate in the water, the eggs are released into the aquatic environment. The eggs hatch into larvae in the water, forming the infectious stage of the parasite’s life cycle. These larvae need to find an intermediate host, typically small invertebrates, in order to continue their development.

The intermediate hosts ingest the larvae while feeding or filter-feeding in the water. Inside the intermediate host, the larvae develop further, often infecting various tissues or organs. This stage of the life cycle is crucial for the parasite’s survival and allows it to reach its next stage of development.

Once the intermediate host is consumed by a fish, the larvae are released in the fish’s digestive system, where they can penetrate the intestinal wall and migrate to various organs or tissues. The larvae then develop into adult parasites, completing the life cycle.

It is important to note that the presence of parasites in fish is a natural occurrence and not necessarily an indicator of poor fish quality. However, heavy infestations can negatively impact fish health and potentially affect the commercial value of the fish.

Fish parasites are caused by a complex life cycle that involves multiple hosts. The definitive hosts, such as marine mammals and birds, shed eggs in their faeces, which hatch into larvae in the water. These larvae infect fish when they feed on intermediate hosts, completing the life cycle of the parasites. Understanding the life cycle of fish parasites is crucial for managing their impact on fish populations and ensuring the sustainability of aquatic ecosystems.