Overfishing is a serious global issue, and many people have turned to farmed fish, or ‘aquaculture’ as a solution to dwindling wild populations. But intensive farming of any kind often comes with problems, and a new study shows that these fish farms are the perfect breeding ground for virulent diseases.

The researchers collected strains of Flavobacterium columnare from fish farms in Finland between 2003 and 2010, and from upstream and downstream of one fish farm during an outbreak in 2010. The team tested each strain’s virulence in captive zebra fish (Danio reri), and found that strains collected in more recent years were more virulent than older strains. This pattern was detectable even at short timescales – virulence increased significantly over just a few months during an outbreak, with downstream bacteria showing greater virulence than upstream strains. The high densities of fish used in aquaculture provide plenty of hosts for bacterial infections and extremely easy transmission, two conditions that tend to favour the evolution of heightened virulence in diseases, the authors say. This finding could have major implications for how we design fish farms to avoid disastrous disease outbreaks.

Want to Know More?

• Sundberg et al (2016) Intensive aquaculture selects for increased virulence and interference competition in bacteria Proceedings of the Royal Society B.