25 Fish – an introduction

Many species of fish are kept in captivity, varying from small tropical species housed in indoor heated aquaria to larger temperate species in outdoor ponds. Enclosures are often expensive to build and maintain, and ornamental fish can themselves be expensive to purchase. Unlike some other captive animals, it is often imperative to treat the aquarium or pond inhabitants medically as a group, although there will be instances when individuals present for therapy.

Taxonomy

There are two superclasses of fish. Although some aquarists keep cartilaginous fishes such as small sharks and rays, most fish species kept as pets belong to the last class in Table 25.1, the ray-finned fishes (Actinopterygii). This group includes (among many others) carps, flatfishes, perch-like fishes and seahorses.

Table 25.1 Taxonomic classification of fish

The most common captive cold freshwater fishes are the goldfish (Carassius auratus) and koi carp (Cyprinus carpio), both of which are freshwater fish and belong to the Cyprinidae family. Guppies (Poecilia reticulata) and tetras (such as the neon tetra, Paracheirodon innesi) are other freshwater fish popular with novice aquarists, although the latter species requires a tropical tank compared with cold water for the former. Brackish-water species such as the glassfish (Chanda ranga), mudskippers (Periophthalmus spp) and gobies (such as the spotted goby, Stigmatogobius sadanundio) are more difficult to maintain. Common tropical marine fish include angelfishes (such as the blue-ringed angelfish, Pomacanthus annularis), surgeons and tangs (such as the regal tang, Paracanthurus hepatus, and yellow tang, Zebrasoma flavescens), triggerfish (such as clown triggerfish, Balistoides conspicillum), wrasses (such as the cleaner wrasse, Labroides dimidiatus), and seahorses (such as the yellow seahorse, Hippocampus kuda). Coldwater marine fish include rainbow wrasse (Coris julis) and the tompot blenny (Parablennius gattorugine). Aquaria commonly hold a mixed collection (a ‘community’ tank) of fish species with the same environmental requirements, although some species can be kept only as single specimens.

Living plants are useful in aquaria for a number of reasons. They will provide shelter for the inhabitants, are a source of food, and improve water quality by absorbing nitrates.

Biology

Most fish species have protective scales covered by a layer of mucus; damage to these leads to problems with osmotic balance. The main site for gaseous exchange is the gills; these are also involved in osmoregulation and excretion of nitrogenous waste. These delicate organs are often the site of pathology. Internal organs are similar to mammals, although there are no lungs or diaphragm, and the kidneys lie dorsally just beneath the spine. Some species have a swim-bladder to assist with buoyancy. Fish are poikilothermic, and thus water temperature is important for immune function and metabolism of drugs (Fig. 25.1).

Figure 25.1 (a,b) External and internal anatomy of a fish, typified by the koi carp (Cyprinus carpio).

Enclosures for fish in captivity vary enormously. Many small fish are maintained in little tanks, while bigger groups or larger fish require more substantial enclosures (often ponds). Whatever the size, water quality should be maintained (monitoring regularly to ensure it is optimal). Filtration is required to control levels of ammonia and nitrite (the main toxic products excreted from fish), usually involving mechanical, biological, chemical or ultraviolet components. Fish may require freshwater, brackish-water, or marine water. Tropical or marine species may require various salts to be added, and heating may be needed.

As a general rule, fish are fed commercial complete diets, sometimes supplemented with live foods. Thankfully, this means that nutritional deficiencies are infrequent.

Goldfish (Carassius auratus)

Free-ranging goldfish originate from China (Fig. 25.2). They grow up to 25 cm in length. Captive breeding has produced many variations in colour, finnage and body form. This is a coldwater species (10–20°C), requiring a water pH of 6.5–7.5 and dH (hardness) to 15°. They produce a large amount of waste, necessitating efficient filtration and regular water changes (25% weekly). As with most fish, they are omnivorous; captive animals are usually fed on flake or pellet foods, but may also take vegetable matter and invertebrates. Eggs, when produced, are scattered in the tank.

Figure 25.2 The oranda is a variety of goldfish (Carassius auratus).

Guppy (Poecilia reticulata)

This live bearer comes from Central America, measuring 3–5 cm in length (although cultivated specimens are larger). Captive fish require a water temperature of 22–28°C. Their diet is similar to that of the goldfish. Many colour forms and fin varieties have been developed. Other Poecilia species are also popular, including the mollies.

Neon tetra (Paracheirodon innesi)

These are the most popular aquarium fish. They originate from South America, grow to 4 cm, and can live over 10 years. They are omnivorous, being fed on small live foods (such as insect larvae) as well as frozen, flake, and pellet foods, and supplemented with green foods. Water temperature should be 22–26°C, with a pH of 5.0–7.0, and dH to 10°.

Mudskippers (Periophthalmus spp)

These species originate in mangrove swamps, distributed from Africa through South-east Asia to Australia. They need a beach area to crawl out onto, along with rocks and roots for climbing out of the water. Air above the water should be warm and humid. Water temperature should be 25–30°C, with a pH of 8.0–8.5, dH to 15° and SG (salinity) 1.002–1.007. They feed on invertebrates (e.g. worms, crickets and flies), and may accept frozen and flake foods.

Cleaner wrasse (Labroides dimidiatus)

These Indo-Pacific fish grow to 10 cm in the wild. They are carnivorous, and feed on finely chopped fish and shellfish in captivity, supplementing their natural diet of ectoparasites from other fish. Water should be 24–26°C, with a pH of 8.3–8.4 and SG 1.023–1.027.

Tompot blenny (Parablennius gattorugine)

These originate from the eastern Atlantic coast from the Mediterranean to northern Scotland, and grow up to 20 cm in length. As a carnivorous species, they should be fed meaty foods (e.g. fish, shrimp and mussels). Water temperature should be 12–15°C, with a pH of 8.0–8.4 and SG 1.024–1.025.

History

The environment of the fish is important in maintenance of health (Box 25.1). The veterinary clinician should obtain details of the aquarium or pond set-up, including monitoring protocols and records employed by the aquarist. For large or complicated enclosures, it is often easier to visit the owner’s home to assess husbandry conditions visually.

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Sep 3, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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Fish – an introduction