It is generally recognised that fisheries statistics upon which fisheries management decisions are based and estimates of optimum fishing practises are made do not include the quantities of organisms discarded at sea and which contribute substantially to the mortality caused by fishing operations. This is specifically recognised in the FAO fish landing statistics, for instance. (See Figure 1) The non-recording of discards is felt particularly, therefore, by those wishing to estimate the out-take of fish and other creatures from a fishery and, from those estimates, derive measures to control fishing intensity and sustainability of the fishery and biological system. Species which are caught incidentally to the main species of interest are less likely to be recorded accurately than those targeted for sale. Indeed, where restrictions are made on the species and sizes of fish and where and when they can be caught, the catchers of "illegal" fish may deliberately hide the fact that this fish has been caught resulting in under-recording of the impacts of fishing and, if sold, distortion of the trade in fish products. It has been found that relying on fishing crews to record accurately the nature and extent of discards is unreliable and produces inaccurate data. Direct observation and recording by unbiased observers on board vessels of discards is rare and costly, for enforcement or scientific agencies, but is now mandatory in some fisheries such as Namibia and Canada.
The discarding of fish at sea is often associated with the reasons for wastage listed above. This would not be a major biological problem if, when returned to the sea, the fish were alive and able to survive. Most fish species when discarded, however, are already dead or in such a moribund state that they will not survive. Their survival depends on the species, the method of capture and probably the physiological condition of the fish. Estimates of 100% mortality are common and reported in Alverson et al. One study of mortalities of halibut (NRC 1990 cited in Alverson et al) gives figures ranging from 2 - 14 % for long line vessels operating in the North Atlantic to 90 - 100% for ground fish trawl operating in the same waters. The same NRC study gives mortalities for Chinook salmon in the North Pacific of 2 - 28% for fish caught by gillnet in the Washington area to 50 - 90% for the same species caught by purse seine. There is general agreement that round fish with large swim/air bladders (such as cod, hake, pollock, croakers, groupers and snappers) die as a result of distended bladders when being lifted from the sea, whereas some flat fish, sharks and invertebrates may survive the trauma of rapid depth changes and temperature. The survival rate therefore will depend on species, the depth at which they are caught, gear type, time taken in fishing and a number of other factors. Some species, if returned to the sea quickly, will survive.