Viral Pathogens in Free-Living Salmonids: Aquaculture and Ecosystem Implications

Abstract

Salmonids play vital ecological and economic roles and have been introduced to many regions worldwide. When not held captive in farms, they can be classified as free-living salmonids, a group that includes native, feral, and naturalized populations, as well as fish that have escaped from aquaculture facilities. Compared to farmed salmonids, knowledge of viral infections affecting free-living populations remains limited, creating a significant gap in understanding the ecological impacts of interactions between aquaculture and natural ecosystems. To address this knowledge gap, we reviewed global reports and summarized the information on viruses infecting free-living salmonids. Most viral detections reported in the reviewed studies were found in native wild salmonids, with escaped salmon ranking second in detection frequency. Atlantic salmon (Salmo salar), sockeye salmon (Oncorhynchus nerka), and brown trout (Salmo trutta) were the most commonly reported host species. The most frequently reported viruses included infectious hematopoietic necrosis virus (IHNV), piscine orthoreovirus (PRV), infectious pancreatic necrosis virus (IPNV), and infectious salmon anemia virus (ISAV). Evidence from some studies suggests a bidirectional transmission of viruses between farmed and free-living salmonids, highlighting the complex interplay between these groups. Beyond their potential role as reservoirs for aquaculture pathogens, free-living salmonids may also be negatively impacted by viruses from farmed fish, contributing to the decline of native populations. By enhancing our understanding of the prevalence and impacts of viral infections in free-living salmonids, we can inform future research and management strategies to protect the health of both farmed and wild fish populations.