In Chilean salmonid farmers’ worst nightmares, Piscirickettsia salmonis must be a regular feature. This bacterium causes Salmon Rickettsial Septicaemia (SRS), a highly infectious disease whose symptoms include haemorrhaging, lesions, ulcers, anorexia, and many cases death.
SRS is an epidemic in Chile, costing the salmon aquaculture industry over US $300 million each year. Vaccines have been largely ineffective and antibiotics have given mixed results. Mandatory three-month fallowing, on the other hand, seems to routinely reduce the chance of reinfection.
The underlying principle behind fallowing is fairly straightforward. Net-pen systems commonly used by finfish farmers allow pathogens to spread into the wider environment. Some are carried away by currents, some remain suspended in the water column or on the seafloor under and near the farm. In all cases, they wait for a new host to come close enough for reinfection. Fortunately for the fish, and the farmer, pathogens can’t survive forever without a host – which is where fallowing comes in. Take the hosts away and all the equipment out of the water for disinfection, and the odds of contagion rapidly drop.
Chilean management organises salmon and trout farmers into ‘neighbourhoods’, each with a fixed-date two-year production period. At the end of this time all farms within the neighbourhood must fallow for three months. “It is very costly for the industry to stop producing for three months every two years, explains Derek Price, a PhD candidate at the University of Prince Edward Island and lead author of a recently published study which demonstrates fallowing’s effectiveness.
For large companies with many sites spread among different neighbourhoods with different fallowing times, some of the economic impacts of this three-month downtime can be dampened, but for smaller companies with only a few sites, the economic implications are not inconsequential – particularly if all those sites are located within a single neighbourhood. Since industry is impacted by these regulations, “we wanted to make sure that the fallowing was effective,” Price reflects.
The study, which used data provided by the aquaculture industry to draw its conclusions, found that all things being equal, three-months’ fallowing was sufficient to significantly reduce the risk of a farm being reinfected by SRS from bacteria that had multiplied at the farm site during the previous production cycle. Fallowing for a longer period doesn’t reduce risk further, whilst fallowing for a shorter period increases the risk substantially.
Whilst the study suggests Chilean salmon farmers are currently fallowing their sites for a sufficient period of time, the story is different for rainbow trout farmers. Salmon farmers typically produce only one round of fish in each two-year production cycle, but thanks to shorter growth periods, rainbow trout farmers potentially could produce several. Between each round of trout, farmers only have to fallow for one month. Price points out that, since some strains of Piscirickettsia can infect both salmon and trout, this is a problem: “One, you are resting for only one month [which the study indicates may be too short a time], and two, there are many farms around you who may have gotten infected.”
According to the study, a trout farm which has suffered from SRS has a higher risk of reinfection from bacteria originating from its own site if it only fallows for one month after infection, compared to those that fallow for three months – or which have been lucky enough not to suffer infection in the first place. It also highlighted the problem of infected neighbouring farms. The study estimates it takes three infected farms within 10km to significantly increase the chance of infection impacting another.
The findings are unlikely to be a complete surprise to trout farmers, many of whom already felt that the one-month fallow period was not enough. Some trout farmers have also argued that they should have their own, all-trout neighbourhood which can be managed on their own timescale rather than that of the salmon industry. For Price, this makes a lot of sense for disease control: “If they were allowed to have their own management areas, they would be able to fallow all at the same time, and restock at the same time.”
The investigation sits within a larger body of Piscirickettsia-management research by Price, who works closely with Chile’s salmon industry to come up with effective solutions to SRS. His work includes exploring why antibiotics fail to treat the disease.
“If you treat early you get better results,” he says, referring to his previously published work. “The earlier you intervene, the less likely an infection is to spread – both within the individual and the population.”
Price is also looking at why antibiotics sometimes fail to reach sufficient concentrations in the fish to fight disease, and at understanding the biocapacity of a body of water for sustainable aquaculture.
“There may be an optimal level of production at which we can control these diseases,” he notes.
