Rainbow trout with glowing fungal infection

Unseen Threat: How Fungal Infections are Decimating Trout Populations

Samir D’Costa in Science & Nature August 2025 • 4 min read.

"Dive into the alarming world of aquatic mycosis and discover the devastating impact of fungal outbreaks on rainbow trout—and what it means for our ecosystems."

Rainbow trout, prized for both sport and consumption, face an increasing threat: fungal infections. These infections, caused by various aquatic fungi, are not just a problem for fish farms. They also pose a significant risk to wild populations, impacting biodiversity and ecosystem health. Understanding the dynamics of these infections is vital for conservation efforts and sustainable aquaculture practices.

In recent years, the rise of freshwater fish farms has increased our awareness of mycotal pathogens. These pathogens can severely limit fish populations, sometimes decimating up to 50% of the breeding stock. While the impact on aquaculture is concerning, the broader ecological consequences of these infections in natural environments are equally alarming.

A recent study investigated the growth of hydromycobiota—aquatic fungi and related organisms—on rainbow trout eggs in waters of varying quality. The findings reveal critical insights into how water conditions influence fungal infections and highlight the species most commonly involved. This article breaks down the science, offering a clear picture of what's happening and why it matters.

The Troubling Spread of Aquatic Fungi

Rainbow trout with glowing fungal infection

The study found that 16.6% of rainbow trout eggs were infected by mycotal species. This figure underscores the prevalence of fungal infections in trout populations. Researchers identified thirty different mycotal species on eggs collected from adult female trout. Some species, like Achlya polyandra, A. radiosa, Aphanomyces laevis, Leptomitus lacteus, Saprolegnia ferax, and Saprolegnia parasitica, were more frequently encountered, suggesting they are particularly adept at infecting trout eggs.

However, other species like Aphanomyces frigidophilus, Candida albicans, and Zoopage phanera were rarely found. This suggests that while some fungi are common culprits, others are less likely to infect salmonid fishes. This variation in species prevalence is crucial for targeted management and prevention strategies.

Key species identified in the study include:

Saprolegnia parasitica: A known pathogen in various fish species.
Saprolegnia ferax: Capable of killing eggs of sterlet and cyprinids.
Achlya polyandra: Frequently found on lampreys and Atlantic salmon.
Leptomitus lacteus: Infects numerous fish species across different water bodies.

The study also examined how these fungi utilize different nutrients. All tested species from the Achlya, Aphanomyces, Leptolegnia, Pythium, and Saprolegnia genera could assimilate glucose and starch. However, they could not assimilate glycine, leucine, lysine, ornithine, or arabinose. Interestingly, urease was only assimilated by species from Leptolegnia, Pythium, and Saprolegnia genera. These findings provide insights into the metabolic capabilities of these fungi, which could inform the development of targeted treatments.

Protecting Our Trout: A Call to Action

The research underscores the importance of maintaining water quality to protect rainbow trout from fungal infections. Eutrophic waters, like those in Pond Fosa, tend to support a higher diversity of mycotal species, increasing the risk of infection. By understanding the specific fungi involved and their nutritional requirements, we can develop targeted strategies to mitigate these threats and ensure the health and sustainability of our trout populations.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.5897/ajmr2014.7260, Alternate LINK

Title: Observations Of Mycotal Species Growth On The Rainbow Trout Eggs (Oncorhynchus Mykiss)

Subject: Infectious Diseases

Journal: African Journal of Microbiology Research

Publisher: Academic Journals

Authors: Czeczuga Bazyli, Czeczuga Semeniuk Ewa, Semeniuk Grell Adrianna

Published: 2015-02-20

Everything You Need To Know

What is the main threat affecting rainbow trout populations mentioned?

Aquatic fungi are impacting rainbow trout populations in both fish farms and wild environments. Some species like *Saprolegnia parasitica*, *Saprolegnia ferax*, *Achlya polyandra*, and *Leptomitus lacteus* are frequently found to infect rainbow trout eggs. The study highlights that controlling these infections is essential for maintaining biodiversity and supporting sustainable aquaculture.

Which fungal species were identified as the most common threats to rainbow trout eggs in the study?

The study specifically identified several fungal species as common threats to rainbow trout eggs. These include *Achlya polyandra*, frequently found on lampreys and Atlantic salmon, *A. radiosa*, *Aphanomyces laevis*, *Leptomitus lacteus*, which infects many fish species, *Saprolegnia ferax*, known for killing eggs of sterlet and cyprinids, and *Saprolegnia parasitica*, a widespread fish pathogen. Understanding which species are most prevalent is crucial for developing targeted prevention strategies.

What nutritional capabilities of the Achlya, Aphanomyces, Leptolegnia, Pythium, and Saprolegnia genera were uncovered that could inform treatment development?

The study reveals that the *Achlya*, *Aphanomyces*, *Leptolegnia*, *Pythium*, and *Saprolegnia* genera can utilize glucose and starch, but they cannot assimilate glycine, leucine, lysine, ornithine, or arabinose. Notably, only species from the *Leptolegnia*, *Pythium*, and *Saprolegnia* genera can assimilate urease. This information is vital because it provides insights into the metabolic capabilities of these fungi, potentially guiding the development of treatments that target their nutritional requirements.

How does water quality influence the risk of fungal infections in rainbow trout, according to the study?

The presence of eutrophic waters, such as those found in Pond Fosa, increases the risk of fungal infections in rainbow trout due to the higher diversity of mycotal species they support. This indicates that maintaining good water quality is crucial for protecting trout populations. Further research could explore specific water quality parameters that exacerbate fungal growth, leading to more refined management practices.

Although the study found that 16.6% of rainbow trout eggs were infected by mycotal species, what are the larger implications of this relatively small percentage?

The study revealed that only 16.6% of rainbow trout eggs were infected by mycotal species, but this relatively small percentage still represents a significant threat. The consequences include reduced breeding success, declines in fish populations, and potential imbalances in the ecosystem. Further impacts may involve economic losses in aquaculture and decreased recreational fishing opportunities. Understanding and mitigating these infections is vital for ensuring the long-term health and sustainability of both aquaculture and wild trout populations.