The biological impact of discards varies according to the vulnerability of the species affected. Some can be of major concern, but others are much less of a problem.
The following list, based on Hall (1996), attempts to cover the different levels encountered
It is a generally recognised that the discarding of fish at sea may have detrimental biological effects and may have effects on the stocks of commercially important species. Discards of undersized target species in the Gulf of Maine groundfish fisheries have been identified as contributing to a decline in the populations of fish in the region. (Saville 1980). In the Irish Sea the decline in abundance of the common skate (Raja batis) has been attributed to excessive quantities being taken as bycatch (Brander 1981). In the North Atlantic minimum landing sizes for cod, haddock and whiting lead to the capture of undersized specimens which are in turn discarded.
Shrimp fisheries in tropical and subtropical regions are also known to harvest large quantities of incidental catch. Studies of the impact of shrimp fishery discards in the Gulf of Mexico have demonstrated reductions in the population levels of non-target species. A reduction of 40% in Atlantic croaker populations since the 1970s has been cited as being caused by over-fishing as bycatch in other fishing operations. It has been noted that in this fishery the catch of croakers now consists of almost entirely one year class in contrast to previous years when several year classes of much larger individuals were present.
The Norwegian mackerel fishery particularly targets fish of more than 600 gms as these specimens are much more valuable than smaller ones. The non-selective nature of the fishing gear meant that large numbers of smaller mackerel were being discarded and only larger ones being used to make up quotas - the fish were being high graded. It was thought that this practice was having adverse effects on the balance of year classes within the population. Since no discard bans are difficult to enforce at sea and the quota system was modified so that quotas specify the landings reflect a balance between fish over 600 gms and those less than 600 gms based on the year classes in the populations being fished. Thus a balance of the population is removed rather than a distinct group which could cause imbalances in the natural stock.
Apart from changes in single species stocks caused by discarding the balance between species within a multi-species population may change. Prior to the introduction of shrimp trawling on the east coast of peninsular Malaysia the families of Leiognathidae, Ariidae, Carangidae, Nemipteridae and Pomadasyidae dominated the marine community. After the introduction of shrimp trawling the population of Leiognathidae, dropped sharply (Chan and Liew 1986). A similar situation was noted in the populations of Leiognathidae and Dasayatidae in the Gulf of Thailand over ten years of shrimp trawling. (Pauly and Neal 1985) It has also been noted that in the Gulf of Carpentaria (Australia) the balance between demersal and pelagic fish species has changed over a twenty year period with demersals declining and pelagics increasing in abundance. (Harris and Poiner 1990)
Apart from impacts on fish populations there may also be environmental impacts or effects on non-fish species caused by discarding at sea. The raising of benthic organisms to the surface which are then discarded dead back into the sea can cause habitat modification. However the destruction of habitat caused in this way and the damage caused by the gear itself (particularly bottom trawling) are difficult to separate. The oxygen depletion of water bodies caused by the decomposition of discards on the sea bottom may cause effects on the benthic community. Where discards are low or the vessel is moving through the water whilst discarding is occurring, the density of the detritus reaching the bottom and decomposing may not have a serious effect.
For scavengers, discarding may have positive effects on populations, with birds, sharks and dolphins having been noted to commonly scavenge for fish discarded during fishing. It has been documented that the rise in sea bird populations in the North Sea can be attributed to the ready availability of food from fish discarded by fishing vessels.
Thus not all impacts of discards are negative as far as populations of individual species are concerned and it can be important to consider both positive and negative impacts when considering alleviation of the problem.
In many cases the effect of discarding is unknown, but public perception of the problem is that it must be wasteful and causing biological/environmental damage and if the fishing industry is to maintain its image as a responsible industry it needs to be seen to be tackling the problem.
Economic losses associated with discards can be divided into a number of differing categories. Discard mortalities associated with a fishery which discards fish of economic importance to another fishery is often the most cited example. This situation can be compounded by quota systems which permit individual fishers to only land specific species. Studies of the Bering Sea multi-species ground fish fishery cited by Alverson et al demonstrated that if fishermen were allowed to seek out the optimum mix of catch and bycatch known as the Acceptable Biological Catch (ABC) there could be a significant improvement in the discard picture compared with that imposed by a quota system based on specific species.
A second economic loss occurs where discard induced mortalities affect immature individuals or non-legal sexes of the target species. Studies of the Texas shrimp fishery determined that the value of the harvest increased by some $9.4 million when closure of the fishery allowed juvenile shrimp to grow to more marketable sizes. The Bering Sea crab fishery has been cited as loosing between $40 and $50 million per year through the discarding of illegal crab. Queirolo et al (1995) discuss and attempt to measure the opportunity costs of fishing discards in the Alaska groundfish fisheries recognising that not all discards represent losses of catches for alternative fisheries. These include catches of fish that have no economic value at present and those where the present quotas are not fully utilised and therefore the discards can be regarded as surplus to the needs of the fishery.
A third economic consequence of capture and subsequent discarding is associated with the capture of non-target species of little, if any, commercial value. The losses are not in this case associated with the loss of commercial value per se but with the extra costs associated with catching, sorting and throwing the unwanted catch over board, as well as the opportunity cost of the better utilisation of the discards. Costs can be associated with higher fuel consumption in active fishing operations (such as trawling), longer on-deck times for target species whilst the catch is sorted leading to a reduction of quality and therefore value of the fish, employment of extra crew members required to sort and remove the unwanted catch from the target catch, greater "wear and tear" on the fishing gear and vessels employed and other such examples.
A fourth stream of costs associated with discards is the money that is spent each year on monitoring surveillance and control. In global terms the cost of monitoring and surveillance operations associated with fishing activities are very large although only a portion of those costs are associated with reducing discards, Alverson et al give a conservative estimate of $4.5 billion a year.7.3 Socio-cultural Impacts
The most difficult to quantify and describe are the social and cultural impacts that arise because of discarding fish. People in different circumstances have different perceptions of the value or otherwise of incidental catches. For instance the environmental groups in the US have instigated an embargo on shrimp imports where the method of harvesting may be harmful to sea turtles. However, this will affect the livelihood of Indonesian shrimp fishermen, for instance, whose principal source of income is from shrimp. Dolphin safe tuna and the "walls of death" have been a subject of hot debate in recent years and yet some drift net fisheries have been shown to have relatively low discard rates and some fishermen have been encouraged to use them on conservation grounds. In most debates of this sort there are at least two sides to the story and it is often that side with the highest political profile that will win the day. This is often based on emotional and heightened public awareness rather than on hard evidence that a conservation, ecological or economic problem exists. This can be particularly so in the difference between protein rich developed nations and less developed nations where fishermen are dependent on fishing for their very existence and the national economy may rely on exports, to developed nations, of high value marine products such as shrimp.
In the debate concerning conserving the biological equilibrium and maintaining stocks of commercially important fish stocks, the human dimension for some of the poorest members of the world community must not be forgotten. The possible banning of the use of set bag nets in Bangladesh, because they are thought to catch unacceptably high numbers of juveniles of commercially important species, is one example impacting on small scale fisherfolk. The search for alternative income generating activities for these people is a priority if they are not to be displaced by such a ban. In the same region and in other parts of the developing world a large number of people rely on bycatch from the shrimp fisheries which they sort, selecting out fish for sale for human consumption and then drying the remainder for eventual sale as crude animal feed. The reduction in the amount of bycatch caught by these fisheries in such a situation would not only affect the livelihood of such people but would also reduce the amount of animal protein available for feeding livestock. These problems and the balance between them are not easy to solve, but they must not be forgotten in the search for solutions.