Sunday, January 19, 2020

Natural food and feeding Catfish

Although numerous studies on the food composition of C. gariepinus have been carried out, a consistent pattern has not emerged and they are generally classified as omnivores or predators. Micha (1973) examined catfishes from the river Ubangui (Central African Republic) and found that C. lazera (= C. gariepinus) fed mainly on aquatic insects, fish and debris of higher plants. They also feed on terrestrial insects, mollusc and fruits. 

Similarly, Bruton (1979b) found that catfish in Lake Sibaya (South Africa) fed mainly on fish or crustacea, and that terrestrial and aquatic insects were an important part of the diet of juvenile and adult fish which inhabit shallow areas. However, molluscs, diatoms, arachnids, plant debris were the minor food items consumed in this lake.

Munro (1967) studied the feeding habits of C. gariepinus in Lake McIIwaine (Zimbabwe) and found that feed composition changes as fish became larger. Diptera, particularly chironomid pupae, predominate in the diet of the smallest group but become progressively less important with increasing size. Zooplankton became more important with increasing size and predominates in the diet of the largest fish. Most of the minor food groups also showed a progressive increase or decrease in importance in relation to increasing size (Figure 5). The greater importance of zooplankton in the diet of large fish was believed to be due to the increased gape and number of gill rakers of the larger fish (Jubb, 1961; Groenewald, 1964); presumably resulting in a more efficient filter feeding.
Natural food and feeding Catfish
Apparent changes in the composition of the
mean daily ration of C. gariepinus in relation to increasing size. Source: Munro, 1967

Spataru et al. (1987) studied the feeding habits of C. gariepinus in Lake Kinneret (Israel) and found that preyed fish were the most abundant food component (81%) and constituted the highest biomass.

In conclusion, we can consider C. gariepinus as a slow moving omnivorous predatory fish which feeds on a variety of foods items from minute zooplankton to fish half of it's own length or 10% of its own body weight. 

In order to feed on this wide variety of organisms in different situations C. gariepinus is equipped with a wide array of anatomical adaptations for feeding under low visibility (Bruton, 1979b) including;
  • A wide mouth capable of considerable vertical displacement for engulfing large prey or large volumes of water during filter feeding.
  • A broad band of recurved teeth on the jaws and pharyngeal teeth preventing prey from escaping.
  • An abundant network of sensory organs on the
  • body, head, lips and circumoral barbels. These barbels are extensively used for prey detection and fixation. Hecht and Applebaum (1988) found that C. gariepinus with barbels were 22.6% more efficient at catching prey than those without. This could indicate that tactile behaviour is important in the prey catching processes.
  • A wide, rounded caudal fin, typical for fish which ambush their prey.
  • Long gill rakers on the five branchial arches.
  • A short and dilatable oesophagus which opens
  • into a distinct muscular stomach (mechanical digestion) and a simple thin walled intestine.

Slow, methodical searching is the normal predatory tactic of C. gariepinus, with catfish grasping their prey by suction; a negative pressure (suction) being created by a sudden increase of the bucco-pharyngeal chamber.

An important aspect of predation by C. gariepinus is their ability to switch feeding from one type of prey to another. In Lake Sibaya (South Africa), catfish ignore (or cannot catch) fish prey during daylight and feed mainly on invertebrates, which are abundant and relatively easy to catch. By contrast, at night, when fish prey become more vulnerable, they switch their feeding habits to fish prey (Bruton, 1979b). In general, fish prey provides far more energy per unit weight than other prey items. However, switching feeding habits relies on the existence of at least two  alternate abundant preys.