Do Fish Sleep?: Fascinating Answers to Questions about Fishes

Six: Dangers and Defenses

SIX Dangers and Defenses Question 1: What diseases are common among fishes? Answer: Fish, like humans, are subject to all sorts of diseases. But since there are thousands of different species of fishes, they have many more diseases, most of which we know nothing about since they haven’t been studied. Fish disease is particularly important to aquarium hobbyists and to aquaculture operations, so diseases of those species have been the subject of considerable study. Gas bubble disease (GBD) results from supersaturation of gases in the water. It can occur in aquarium or aquaculture fishes in water that was pumped under pressure, or in wild fishes living in water below dams, where entrained air is dissolved in water under pressure from the deep plunge. Gas bubbles form mostly in the blood, gills, eyes, and under the skin. Bubbles within capillaries restrict blood flow and form hemorrhages and clots. The effects of GBD, analogous to “the bends” in human divers, can range from mild to fatal depending on level of gases, fish species, life-cycle stage, depth, and such. Fish may have bubbles visible under their skin or show signs of swimming upside down or vertically. Some diseases that have been investigated in wild fishes appear to be associated with water pollution. Tumors are found more frequently in highly polluted areas; for example, Dover sole, Microstomus pacificus, in urban estuaries have liver tumors. Tumor surveys can be useful in indicating the presence of R5367.indb 93 R5367.indb 93 10/26/10 7:14:33 AM 10/26/10 7:14:33 AM Common Infectious Diseases in Aquarium Fishes “Ich,” “Ick,” or White Spot (Ichthyophtirius): Small white spots caused by a protozoan, mainly on the fins. When the protozoa first attach themselves to the fish, they are not visible to the naked eye, but they become visible after feeding on the body fluids of the fish for a few days. Fin Rot: Rotting fins and loss of appetite due to bacteria that infects the fins, which may be susceptible due to fin nipping by other fish or to poor water quality. Fish Fungus: White or gray cotton-like growths on the body. Fish seem to be more likely to have fungal rather than bacterial infections. Of the many different types of fungi that infect fish, Saprolegnia, the most common, is a filamentous fungus that appears as gray or white patches. It establishes in a specific area then spreads rapidly over the body and gills. Fungi often infect fish secondarily after some other infection, stress, or injury. Fungi also affect fish eggs, initially unfertile ones, but fertile eggs as well. Pop-Eye: One or both eyes appear to be protruding abnormally due to a bacterial infection. The eye bulges out because of fluid buildup behind or in the eye itself. The bulging eye may have a thin layer of tissue around it that covers it, keeping it in the socket. The fish may also be less active and uninterested in food. Pop-eye can also be caused by gas bubble disease, resulting from excess levels of gas in the water. Swim Bladder Disease: Fish have a difficult time staying upright and may hang in the water (see chapter 3, question 5: “How do fishes prevent themselves from sinking?”). Dropsy: Fluids build up and cause the fish to bloat and scales to protrude due to a bacterial kidney infection, causing fluid accumulation or kidney failure. Velvet or Rust: A protozoan disease producing yellow to light brown “dust” on body, clamped fins, and respiratory distress . The fish may show signs of irritation, like rubbing against objects. The gills are usually the first part affected. It is highly contagious and can be fatal. R5367.indb 94 R5367.indb 94 10/26/10 7:14:34 AM 10/26/10 7:14:34 AM DANGERS AND DEFENSES 95 cancer-causing pollutants in the water or sediments. High frequencies of liver and skin cancers in brown bullheads, Ameiurus (Ictalurus) nebulosus, in the Black River, Ohio, and the Buffalo River, New York, were associated with high concentrations of polynuclear aromatic hydrocarbons (PAHs) and metals in the sediment. Similar pollutants and tumors are seen in English sole, Parophrys vetulus, in Puget Sound near Seattle, Washington. Anglers may note some disease in a fish and wonder if it is safe to eat. The safest response is, “When in doubt—don’t.” However, very few fish diseases can be...