Carp Pathogens of Recreational and Ornamental Aquaculture: What Lies Beneath the Surface?

The farming and rearing of freshwater fishes is increasing in scale in response to growing global food demands, but notably also through the increasing popularity of sport angling and expansion of ornamental hobbyist practices globally. Alongside rises in numbers of fish and the intensity of their rearing, increases in aquatic and ecosystem stressors have expedited the emergence and motility of many pathogenic diseases. Cyprinids are the most popular species for culturing, and although largely considered hardy, are susceptible to a diverse array of pathogens. Common carp (Cyprinus carpio) and ornamental koi carp (Cyprinus rubrofuscus) are valued for their ease of cultivation, fast growth rates, and variety of genetic appearances. Vaccination programmes have become a mainstay of industrial aquaculture where financially viable, but their use in fish reared as companion animals or for sport angling is not cost-effective and despite the magnitude and growth of these markets, much less is known about current threats and trends in this area of aquaculture. Here, we review the bacterial, viral, and fungal infections commonly found within carp bred for angling and ornamental aquaculture in the United Kingdom and Europe, alongside current threats such as antimicrobial resistance and the potential impact of climate change on temperature-dependent infections. We highlight the evolution of diagnostic techniques, alternative therapies to reduce antibiotic dependence, and the environmental impact of available treatments as priority areas for cyprinid research. Additionally, there is an urgent need for novel therapeutics in this area, as innovation is currently lacking.