STRESS

Like all other animals, fish are subject to stress. There are three ways to induce stress response in fish: chemical, physical, and perceived. These stressors trigger stress in fish, which lead to primary response, secondary response, and tertiary response. Primary response can lead to secondary response, which then can lead to tertiary response. Primary responses can include increases in corticosteroids and changes in neurotransmitter activity. Secondary responses can include changes in immune function, metabolism, and disturbances in osmoregulation. Tertiary responses include changes in behaviour and performance such as growth.

In fish, increased cortisol levels inhibit functions such as growth, reproduction, and a weakened immune system. Cortisol also controls hydromineral balance through the effects of catecholamines. A fish experiencing stress will have increased cortisol levels, which will put it at a physiological disadvantage and subsequently have a negative impact on the probability of survival and lifespan.

However, not all fish respond to stress in the same way. In fact, stress response and cortisol levels may vary between species of teleosts. Not only that, a fish may show different responses to stress depending on which developmental stage it is in.

DISEASE

Fish are susceptible to bacterial and viral diseases. Bacterial infections of fish are usually caused by either gram-positive or gram-negative bacteria. Fish that are stressed have a weakened immune system. The bacteria already existing in the fish’s environment will take advantage of the compromised immune system, resulting in disease outbreak. It has been determined that contamination of the aquarium environment is likely to be the cause of bacterial infection in aquarium fish. Viral diseases are more difficult to identify and treatment is impossible once the disease is in progress. Vaccines are few and expensive, and most viral diseases do not make such a significant economical impact that there is an urgent need for a vaccine.

Some species of fungi produce aflatoxins, which are mycotoxins that cause aflatoxicosis. Aflatoxins can occur in fish feed, and thus have an impact on farmed fish. In the US, the Food and Drug Administration has placed strict regulations on feed ingredients, but there are no regulations on levels of aflatoxins in fish feed.

Parasites such as nematodes and pentastomes are capable of infecting many species of fish. Fish can become infected with nematodes through infected live feed and fish-fish contact.

PAIN

Research shows that fish may be able to feel pain. Fish that were administered a noxious substance were affected behaviourally and physiologically. It has also been found that morphine has an analgesic effect on fish. Other responses include an increase in ventilation rate and decreased activity.

However, nociception and pain response may be species-specific, and as such, be diverse. Not only can nociception differ between species, it may also differ within a species. This further complicates the determination of nociception in fish. Genes involved in fish nociception are the same genes that regulate nociception in mammals, and that other different genes were also regulated.