ABSTRACT
The living conditions for marine organisms in oceans and seas depend on a large number of physical and chemical factors of the habitat. Fish migrate to the spawning ground, where they find proper conditions of salinity, temperature, and depth necessary for the development of the fertilized eggs. Spawning concentrations of shoaling fish are suitable to be fished with large efficient gear. Different species of commercially exploited fish vary in regard to abundance, availability for catch, distribution, size, and biological characteristics. The body of most fish species is covered with scales that are deep-seated in the skin. The scales are covered with epidermis containing many mucous cells which excrete a slime forming a protective layer on the fish body. The skeleton of bony fish consists of the backbone, the head skeleton, and the bones supporting the fins. Some fish also have a large number of thin intramuscular bones which are a nuisance for the processor and consumer.
