Impact of fisheries on coastal systems
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Aquaculture has grown and developed enormously over the past four decades, however this development is not without cost. There are environmental implications that need to be considered including the increase of nutrients into the exploited area that can lead to eutrophication. The impact of fisheries on coastal systems can be negative through habitat destruction leading to the alteration of the ecosystem. Marine mammals can be affected through incidental death as a result of becoming caught as by-catch or becoming entangled in nets through ‘ghost fishing’. Fishing can also have an effect on the target species as a result of over-fishing, which can lead to changes in the ecosystem structure. In order to reduce these impacts there is a need for more research into the long-term impacts of fisheries as well as an integrated coastal management strategy. Especially as humans are becoming increasingly dependant on resources from the sea, particularly coastal areas.
Mariculture – Aquaculture
Aquaculture is an attempt, through inputs of labour and energy, to improve the yield of useful aquatic organisms by deliberate manipulation of their rates of growth, mortality and reproduction. Most often the word mariculture is used to describe extensive cultivations of marine animals and plants where the input of energy (mostly food) is minimal. Fish farming, collecting invertebrates from the shore and mollusc cultivation are typical examples of mariculture. Farming and other marine aquaculture practices influence coastal systems, utilizing land, wetlands and the sea (fish cages or artificial reefs). While most of the finfish production is in freshwater, a growing proportion originates from coastal brackish and marine farming systems, including seaweed, shellfish, and crustaceans. Aquaculture has grown substantially over the past four decades and, in many parts of the world, aquaculture in coastal and marine waters is the only growth sector within marine fisheries.
With the growth of industrial aquaculture, environmental problems have arisen. However, major achievements have been made in the past decade to cope with many of these environmental problems, including improved site selection criteria, improved husbandry techniques (e.g. better stress management and improved disease control). As there is often a high prevalence of pathogens and diseases in mariculture compared to wild stocks, chemicals may be used to control diseases and pests. Toxic and persistent chemicals, antibiotics and antifouling substances (copper, for instance) are often used routinely. Nevertheless, the use of antimicrobials in modern farming systems can now be avoided because of the development of appropriate vaccines for some key diseases.
All types of mariculture are liable to produce excessive amounts of nutrients and deposit organic material in the exploited area. Such inputs contribute to eutrophication. Many of the environmental implications are well known: pre-emption of critical fishery habitats (e.g. mangroves); pollution of estuarine and lagoon waters; excessive exploitation of natural stocks of larvae and juveniles; imports of inferior, sick or non-compatible seed stock; and introduction of exotic species.
Fishing is an important economical activity in the coastal zone. However, the potentially high benefits come with equally high risks of ecological deterioration through over fishing, killing of unwanted by-catch and habitat destruction. The most obvious effect of fishing is the removal of fish and shellfish from the ecosystem, but fisheries affect coastal marine ecosystems in many other ways while fish stocks are in turn affected by environmental factors. In general, fisheries can induce different selective pressures, either directly, i.e., through elevated mortality (which is often highly selective) or through ecosystem-level responses, as exploitation affects food availability and predation risk in both target and non-target species. Responses to selection can be observed at two levels. Firstly, at the community level, some species may suffer more from effects of fishing than others; some may even increase in abundance. Responses by species to exploitation are associated with their life histories. In particular, species with late maturation at large size and with low population growth rate tend to undergo more pronounced declines than early-maturing species with rapid growth. Secondly, the phenotypic composition within species may also change. If phenotypic variability has a genetic basis, then fisheries-induced selection can result in evolutionary change in life-history traits influencing sustainable yields, behavioural traits (e.g., gear-avoidance behaviour), and morphological traits.
Impact on fish populations
The quantitative impact of fisheries on fish populations is far from being negligible as, for instance, up to 1/3 of the biomass (from 3.5 to 2.5 million tones/year) is taken out from the North Sea, 1/3 being consumed by fish, 1/3 going back to the ecosystem to other predators, through diseases, etc. The removal of fish and shellfish biomass as food for humans may lead to stocks being depleted, some stocks even being on the verge of collapse, i.e. North Sea herring, mackerel and cod. Presently, 50% of the haddock present at the start of a year is taken in the same year, but not all species are affected in the same way. Declining species also include slow reproducing animals such as elasmobranches (cartilageneous fish, i.e. Dogfish Scyliorhinus canicula, Rays, Conger eel Conger conger, Allis shad Alosa alosa).
The activity of fishing itself might impact living marine organisms in many ways. The removal of target species leads to changed abundance and altered structure of these populations. Shifts in population structure affect the composition of the ecosystem with a decrease in abundance of larger individuals, a shift in population age as older individuals are removed. Life-history changes, increased growth rates and a lowered age at maturity have been reported in Plaice Pleuronectes platesssa and Cod Gadus morhua in the North Sea. Through selection of animals adapted to modern fishing practices it has been demonstrated that whole populations have changed their genetic composition in response to increase human pressure.
Impact of discard
Because of the influence of market forces (undersized fish, over quota catch not being appropriate for landing) or just because they are not fit for human consumption, whole organisms (fish & benthos) are thrown back into the sea. It even happens that for high-grading the catch in order to maximise the value of a certain species at a certain time, marketable fish is thrown back overboard to the sea by fishers. Indirect death or injury takes place when fish are in contact with the gear or escape through the mesh. Such post-escapement mortality seems significant but further work is required for assessing its full impact on populations. Such discard of unwanted catch is a food-source made available at an unusual place and time to scavengers and opportunist predators. The impact on ecosystems is comparable to that from the discharge of offal, tissues rejected after fish is cleaned and gutted at sea. Seabirds, notably, are affected through access to a new source of food.
Impact on seabed
In addition to the potential for over-fishing, fishery operations can have a destructive physical impact on the seabed (Fig. 1), and affect population levels of non-target species through incidental catch, such problems being of particular significance for cetaceans, sea turtles and seabirds such as the albatross. All commercial bottom fishing disturbs sea-floor organisms and the seabed, with impacts on both habitats and species. The removal of non-target organisms (by-catch) may impinge on commercial species or be of no value to fisheries. The mechanical effects on the sea-floor habitat of trawlers, for example, include changes in the size composition of invertebrates with repercussions on the overall trophic structure of the ecosystem. Modification of the substrate include a possible increase in shell debris and the silt / clay content through resuspension of fine particles of sediment, changing in turn geophysical parameters such as penetration depth and surface roughness. Changes in benthic community composition consist of shifts from larger long-lived species to smaller opportunistic species, depending on the intensity of trawling, the gear used and the background level of disturbance, including pollution.
Impact on marine mammals
Unfortunately, effects of fisheries on marine mammals are not well documented. By-catch of small cetaceans, caught in bottom-set gill-nets, have been reported. Litter dumped or lost from fishing vessels includes fishing gear leading to ‘ghost fishing’ where disposed or lost gear continues fishing for year, entrapping marine mammals and birds. It is important and urgent to integrate fisheries into coastal zone management, with the aim of ensuring sustainable yields, while protecting vulnerable areas and species of birds and mammals. More research is needed into the long-term impacts of fisheries, and their effect on populations of target and non-target species – indeed, on the entire ecosystem of the coastal zone. Changes in the climate, such as the North Atlantic oscillation (NAO) appear to be affecting fish distributions and their spawning patterns These changes are coupled with pollution effects on the fish and bioaccumulation of pollutants.
- ICES, unpublished reports 1999
- Dethlefsen, 1989; Elliott et al., 1988
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