Fish health and welfare are at the heart of successful Scottish salmon farming. It is in everyone’s interests to operate to the highest of welfare standards.

The reproductive strategy of fish is to produce a large number of offspring so that a proportion will survive to adulthood. 

Fish, including farmed fish, are subject to a number of pressures in the environment, particularly naturally present pathogens. It can, therefore, be expected that some fish will experience challenges to their health. 

Farming fish in a controlled environment removes some of the environmental pressures and improves their survival rate.

In all animal production some losses, though regrettable, do occur.

Causes of mortality

Causes of salmon mortality are varied. Our dataset identifies reasons for mortality as follows:

  • Treatment losses: as in all animal production, it is sometimes necessary to use veterinary medicines or other interventions to protect animals. For salmon, these are mainly used to manage naturally occurring sea lice. In some circumstances, for example where fish might be experiencing other health challenges, treating fish can sometimes result in loss. Often, this is related to handling and moving the fish, rather than the treatment itself. Fish veterinarians and health managers continually monitor fish during any treatments. They also learn from experience and seek to improve best practice and avoid losses due to treatments in the future. 
  • Gill health: the gills of fish are sensitive organs that are in continual contact with the water. This means that gills are susceptible to anything in the water that might cause them harm, for example spiny micro-organisms, bacteria or algal blooms. As gills are used for respiration (or “breathing”), farmers minimise anything that causes the fish to use more energy, such as avoiding handling. They may also support the fish by providing additional oxygen in the water, use a medicine, or bathe the fish in freshwater to help treat against a specific problem. Farmers are also able to use specially formulated “functional feeds”, designed to support the fish in responding and recovering from a gill health challenge.

  • Viral challenges: as with any animal, there are several viral diseases that can affect fish - Pancreas Disease, Cardiomyopathy Syndrome, and Heart and Skeletal Muscle Inflammation, which typically affect the heart and other muscle tissue. For all three conditions, farmers may use “functional feeds” that are tailored to provide support against specific health challenges. Infectious Pancreatic Necrosis is a viral disease which can affect fish during the early stages of seawater production. There is a vaccine available that can help limit its impact.

  • Bacterial challenges: like other livestock, salmon can experience bacterial infections. Vaccines have been developed to protect fish against the most significant bacteria that affect salmon. These vaccines are further supported by farmers adopting high levels of biosecurity and good farming practice across the sector. As a result, the use of medicines to treat bacterial infections is very low, especially when compared to other livestock sectors.

  • Transfer losses: as in the wild, farmed salmon start life in freshwater and then transfer to seawater. Transferring between fresh and seawater can be a stressful time for salmon as their bodies go through huge osmo-regulatory and morphological changes . Farmers use technology to assess when their populations of fish have made the biological transformations necessary to allow them to live in the sea. They also use specialist lorries, boats and, in some cases, helicopters to transfer fish as swiftly and safely as possible, checking the fish and water throughout using cameras and specialist monitoring equipment. However, some fish in the population may not be quite ready for the transfer to seawater, resulting in some losses.

  • Predators: salmon farmers have a responsibility to protect their fish against predators and use a range of technologies and management measures to deter them. For example, nets over the top of the pens protect against birds and acoustic technology produces noise to deter seals. However, protecting against predators in difficult and, despite the management measures available, some predator attacks can and do happen. Predators of salmon include seals and various species of bird and salmon farms are attractive places for them to attack. 
  • Storms: Farmers monitor weather forecasts daily to stay up to date with changing conditions and prepare their farms for impending storms. Salmon farms are designed to ensure they are robust and protect the fish from the elements. However, farmers cannot control the weather and severe storms, in particular, can cause losses. They can also change farming practices to protect their fish. For example, they may temporarily stop feeding the fish, encouraging them to swim in deeper, more protected water.

  • Jellyfish and algal bloom: salmon can come into contact with jellyfish or potentially harmful algal blooms that occur naturally in the water. Such blooms can spread over large areas, driven by water currents and tides. Salmon farmers monitor harmful algal blooms using satellite images. This provides early warning of blooms that might impact their farm. Similar technology is not yet available to monitor jellyfish blooms. If farms are affected by jellyfish or algal blooms farmers can use “bubble curtains” and oxygenate the water to help disperse the blooms and ensure their fish have the oxygen they need. However, the success of these methods depends on the severity of the incident.

  • Environmental: salmon require high quality, oxygen-rich, cool water in which to thrive. However, even in the best possible locations, there can be circumstances where environmental factors and water quality change. For example, heavy rainfall can cause significant “run-off” from surrounding land, bringing dissolved nutrients or suspended particles. Similarly, algal blooms can impact oxygen levels in the water.

  • Handling: salmon farmers know that their salmon grow and develop well if they are not disturbed. However, from time to time it is necessary for farmers to handle their fish. In a salmon farming context, handling refers to farming practices that involve the fish being disturbed in some way (e.g. being removed and transferred to another farm or pen, being corralled within their pen to allow fish to be harvested, etc). Salmon farmers, veterinarians and fish health teams have considerable experience in monitoring their fish during handling events and always minimise stress as much as possible. Fish will also be monitored more closely for a period of time after any handling event.

The Scottish salmon industry works to ensure the highest possible survival rate for their salmon. Salmon farmers and fish health professionals ensure they adopt the most stringent standards of biosecurity and best practice where fish health and welfare are concerned.

Dedicated teams of fish health professionals and specialist veterinarians continually monitor the fish and develop management strategies to protect against health challenges and to respond rapidly, if needed. Ensuring that Scottish farmed salmon are healthy and have the highest standards of welfare is critical to ensuring we have the best product for consumers.