Discarding takes place because, in the course of fishing, many species other than the target species are often caught. This by-catch is usually discarded at sea unless it is worth keeping or dumping at sea is expressly forbidden.
When the by-catch consists of a small proportion of mature specimens from healthy stocks, relatively little damage is done, but when it consists of juveniles of commercial species it may be quite damaging. Catching large numbers of juveniles is likely to reduce the future number of mature fish. This will have a direct impact on the fishery taking the by-catch, or on other fisheries if the juveniles belong to their target species.
By-catch discards by commercial fisheries have long been regarded as wasteful and are the subject of both international and national action. Apart from the loss of a massive amount of potentially valuable food, the incidental capture of dolphins in tuna purse seine nets, turtles in shrimp trawls and marine mammals, birds, turtles and fish in high-seas squid driftnets has led to widespread public concern. Unfortunately, by-catches are an inevitable consequence of an industry that depends upon the capture of species that live alongside other creatures in an opaque medium and as a result can seldom be directly observed and targeted.
While the by-catch arises primarily because fishing gears and practices do not selectively target the desired size and species, the reason for discarding part of the catch is generally economic. If in an unregulated fishery the cost of bringing fish to market is greater than its market value, there is every incentive to dump it at sea. Similarly, where a fishing vessel has limited holding capacity, low-value species are discarded in favour of the high-value ones, a practice known as 'high-grading'.
In managed fisheries, setting minimum landing sizes to reduce the capture of juvenile fish has led to the discarding of undersized fish. Similarly, output controls such as the use of catch quotas can encourage several things - discarding fish to stay within the quota, high-grading, and price dumping of all or part of the catch when market prices fall in order to save the quota for when prices improve.