Disease and Fish Health
Diseases of farmed fish are risk factors for both the environment and fish farmers. Improperly cleaned or maintained cages, stocking fish at an unsustainable density, leaving dead fish within the cage, poor water quality, and improper feeds and feeding protocols are some of the factors that can lead to fish diseases. Disease can be a major risk for wild fish species as open water where cages are located are a potential pathway to transfer disease from within the cage to the nearby marine environments. If diseases within farmed fish are not addressed, they can spread to wild fish.
Disease can greatly impact the growth and overall condition of the fish in the cage by reducing growth, increasing deformities, increasing mortalities, increasing harvest time, and reducing harvest biomass and profitability. It is more economically and environmentally sustainable to prevent diseases rather than mitigating the spread of diseases that have already occurred. ref
A critically important concept to be aware of in finfish aquaculture is stocking density, which is the total weight of fish in a certain volume of cage. The weight is usually given in kilograms and the cage volume in cubic meters. For example, a floating net pen with dimensions of 5 x 5 x 3 meters (L x W x H) would have a total volume of 75 cubic meters, and it could be stocked with 2,000 fish of 100 g each. If the total weight of fish, or biomass, is 200,000 g, or 200 kg, then the stocking density would be 2.6 kg of fish for every 1 cubic meter.
Each cage type, species, and environment will allow for different stocking densities, but a general rule is that there is an inverse relationship between stocking density and growth - the lower the stocking density, the quicker the fish will grow. Higher stocking densities (more fish present in the cage) will generally increase overall fish stress and potentially increase prevalence of diseases and parasites. It takes careful planning, observation, and record keeping to finetune the optimal stocking density in a specific cage. ref
Cage Cleaning and Maintenance
Cage and net cleaning should be undertaken regularly between harvests and when needed. Depending on the selected site, nets will often accumulate “biofouling” or natural attachments from the local environment by algae, sponges, or even corals. The accumulation of marine organisms on the nets can reduce seawater flow within the cage and reduce oxygen replenishment and the efficient removal of waste. Additionally, parasites that can grow and prey on the cultured species can attach to gear and nets. Management and operators need to regularly communicate about net maintenance and cleaning as lack of cleaning and maintenance can also lead to holes and breakage, which could cause fish to escape and reduced harvests. ref
Routine maintenance generally includes brushing nets underwater when nets are empty between stocking. Total and more in depth cleaning can include removing the nets from the cage and cleaning them on land by allowing them to dry under the sun for an extended period, spraying them with a high-pressure freshwater hose, or treating them with specific chemicals. It is important to kill and remove as much of the marine growth as possible and fully rinse off any chemicals before placing nets and cages back into the water. In addition to net cleaning and depending on the cage type, the floating systems and walking platforms must also be serviced and maintained.
Type of Feed
The use of whole fish, fish trimmings, or other animal parts to feed the fish in cages is highly discouraged. This manner of feeding is unsustainable, uneconomical, and can have lasting and damaging effects on the environment. Instead, feeding fish with commercially produced feed pellets is recommended. The pellets have the required nutritional components to promote growth, survival and overall condition of the farmed fish with a balance of protein, lipids, energy, minerals, and vitamins. Depending on the species that is being farmed there may exist a feed designed and tested for that specific finfish. It is important to observe the feeding behavior of the fish to avoid overfeeding. If overfeeding occurs, uneaten pellets sink to the seafloor potentially damaging benthic habitat. Additionally, any feed that does not get consumed by the farmed fish is wasted money - determining the right feed efficiency is a win-win for the farmer and the environment. ref
Fish health can be affected by many different environmental, nutritional, and infectious factors. It is the responsibility of the farm operator and manager to oversee the health of the cultured animals from the fry that are purchased to the fish that is grown and harvested. As soon as any adverse behaviors or physical traits are noticed, immediate actions must be taken to determine and solve the underlying issue. If the operator or manager does not have the necessary technical training to accurately assess and cure possible ailments, it is the responsibility of the operator and manager to seek assistance. Aquatic veterinarians can be presented with visual evidence (photos or videos) so they can provide recommendations.
Distance Between Farming Operations
Direction and speed of currents will determine which way and how fast waste is removed from a cage and potentially carried into an adjacent cage. Direction of the current can also mean that certain cages will be exposed to water with higher oxygen content, with downstream cages potentially receiving reduced oxygen. If cages are too close to each other, then there is a higher risk that a disease or parasite outbreak can quickly spread and affect neighboring cages and farming operations.
It is the responsibility of the farm operator and manager to oversee fish health and evaluate whether the fish have any diseases or parasites. In terms of fish behavior, the following traits can indicate disease or parasites: failure to feed properly, flashing (turning on their sides), rubbing on the bottom, and/or reduced vitality or gasping at the surface. In terms of physical signs, the following traits can indicate disease or parasites: blistered areas, swollen bellies, popped-out eyes, bloody areas on fins, discoloration or erosion of body parts, and/or excessive mucus or growths on the body. ref