Fundamentals of Ichthyology by S. P. BISWAS

Author:S. P. BISWAS [S.P. Biswas]
Language: eng
Format: epub
Publisher: Narendra Publishing House

Chapter 15 : Enemies and Diseases

15.1 NATURAL ENEMIES OF FISHES

Fishes are found in all types of water, thus displaying a wonderful power of adapting themselves to a wide range of ecological conditions. At the same time, fishes belong to one of the weakest aquatic animals and esteemed food for many organisms. Probably due to these reasons fishes have countless enemies. Some of the well-known enemies are mentioned below.

Among invertebrates, the carnivorous coelenterates do a considerable harm to fishes. A number of marine medusae consume fish larvae and even attack adult fishes by their tentacles and nematocysts. Arthropods, particularly aquatic insects are also responsible for heavy loss of carp eggs and larvae. The back swimmer (Notonecta), water scorpion, dragon flies are some of the notorious aquatic insects which make a heavy toll of fish spawn in both cultivated and wild waters. Certain molluscs, for instance, cuttle fishes consume fishes in fairly large quantities. There are enemies of fishes among echinoderms too. Notable among them are sea-stars and sea-urchins.

Most of the aquatic vertebrates are fond of fishes. There are predatory fishes which feed on other small sized fishes. Shark, Cod, Wallago, Bagarius are some of the well known predator fishes. More interesting is that in certain cases even the parents devour their own spawn (eg. Clarias batrachus). Among other vertebrates, frogs and water snake are destructive of fry and fingerlings. Tortoise and turtles too, are harmful to young fishes. Crocodiles and alligators are highly destructive to fishes of all age groups. Similarly, a good number of aquatic birds such as king fisher, herons, storks etc. capture fishes for their livelihood. Among mammals, otter is a voracious fish eater and it makes a heavy toll in both capture and culture fishery. River dolphin too, an exculsively piscivorous mammal. Other mammals viz., wolf and even tiger are fond of fishes. However, human being is undoubtedly, the greatest enemy to fishes. Millions of tons of fishes are consumed by over growing human population. Man-made pollution is also responsible for killing a large number of fishes every year.

15.2 COMMON TYPES OF FISH DISEASES

Fish as an animal being suffer to various ailments. Like other organisms, fishes are also subjected to attack by various pathogens. In fact, the parasites that infest fishes, there is almost no end. In the wild conditions, fishes are widely dispersed and diseases are not common as the risk of contamination are fewer. However, diseases may occur in virulent form in open water systems. For instance, the EUS (Epizootic Ulcerative Syndrome) was responsible for large scale fish mortalities in the Brahmputra and Barak river systems of N.E. India in 1987 - 88 and also elsewhere in the country in early nineties.

Fish diseases usually act in conjugation with other environmental factors. If the quality of water deteriotes for e.g.. significant fluctuations in the level of dissolved oxygen, temperature, pH, ammonia, salinity, suspended solids etc. which may be stressful enough to make the fish more susceptible to the pathogens. Thus, poor husbandry techniques can soon lead to severe outbreak, which if untreated, can cause a havoc to culture as well as capture fishery.

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