In this paper the main factors that cause stress, injury and mortality of the fish after escaping from the fishing gears, mainly trawl nets, have been analysed. The main covered factors are: mesh size and shape, fish size, swimming exhaustion, weather conditions at sea, sea temperature, duration of the tow and the consequences of skin injuries. Factors that cause death of discarded fish (caught by the trawl and discarded from the deck of the vessel) have been explained, such as the presence of a swim bladder, exposure time on the deck, sea and air temperature, duration of the tow and total catch amount. Although the main focus of the paper is trawl induced mortality, the mortality related to other fishing gears (gillnets, pots, purse sines, hook and line gears) has also been described. In the last chapter, the standard research methods used to estimate survival of fish after the escape from the trawl codend are described, with basic steps of collecting 'escaped' individuals, transporting them to the observation cages and observing their mortality for a certain period of time.