Gill problems account for a large proportion of production losses in the sea phase. Like other species, fish are dependent on having sufficient access to oxygen, and it is the gills that ensure that they can utilise the oxygen dissolved in the water they live in. Many infection agents and particles in the water can harm the gills and thereby be a threat to the welfare, health and survival of the fish.
Gill problems in the saltwater phase are often spoken about as a multi-factor disease, and are connected to a range of infectious and non-infectious factors. This means in cases with reduced gill health, you often find a variety of causes, but in other cases it may be that no cause will be found at all. This means that prevention of gill problems is the key with operational measures together with general strengthening of fish health and the immune system.
As gill problems often involve multiple factors, we often talk about gill problems using terms such as "autumn sickness", "chronic proliferative gill inflammation" and "epitheliocystis", which actually points to when symptoms occur, and how the types of symptoms have more than one cause. AGD – or amoebic gill disease - is however a disease that is primarily related to one specific agent – the amoeba Paramoeba Perurans, but this is also seldom found in isolation.
Heart health for farmed salmon has many challenges such as malformations and infections. These can weaken the heart's ability to compensate for the increased need for oxygen in demanding situations. If the fish is also suffering from gill disease, this may result in reduced oxygen intake, and further stress on the circulatory system. In situations where the fish is exposed to exertion and reduced access to oxygen, this can lead to circulatory failure and death from being handled.
To ensure that the fish are robust with good circulatory health, a good layer of mucous and a strong immune system can have a preventive effect against gill problems.