Freshwater fishes may now be the most threatened group of vertebrates, based on more than 5,000 species assessed, to date, by the IUCN (Reid et al., 2013). Major threats to freshwater fishes and other freshwater biodiversity, include: habitat modification, fragmentation, and destruction; invasive species; overfishing; environmental pollution; forestry practise; and climate change. Often species, or biodiversity, declines in response to more than one category of threat, and the real "threat" is the combined or synergistic impact of changes brought about by human activities.
Habitat modification, fragmentation and destruction
The amount of water impounded behind dams has quadrupled since 1960, and three to six times more water is held in reservoirs than in natural rivers. Water withdrawals from rivers and lakes have doubled since 1960, and 70% of all water used worldwide is for agriculture. This has taken a heavy toll on the world"s freshwater biota in general causing many species of freshwater fish to become endangered. General habitat loss is, for example, closely associated with the decline of Asiatic arowana (Scleropages formosus) in Thailand. One of the consequences of damming is the prevention fish migrations for reproduction, feeding or other purposes. Agricultural irrigation schemes can lead to salination of habitat and the decline of salt intolerant species.
A large amount of freshwater fish species are also being threatened by degradation, reduction or even loss of floodplains by damming, agriculture practices, urban development, rivers dredging and geomorphological modifications. Floodplains are of paramount relevance for fish life cycles, supporting most river production and, ultimately, fisheries sustainability.
Image: Nam Ngum 1 Hydropower Project – first dam constructed in Lao PDR. Photo: Shaara Ainsley.
After habitat destruction, the introduction of exotic or "alien" invasive species is believed to be one of the greatest global threats to native fish communities and their freshwater ecosystems. Many introductions are intentional ‚Äì to provide new fisheries (Nile perch in Lake Victoria), for commercial or cultural purpose (tilapia), as ornamental species (koi carp) or to control mosquito larvae as a biomedical measure against malaria (mosquito fish). Other introductions are unintentional, often migration from aquaculture (rainbow trout) or anglers" livebait (ruffe). Whatever the source, many of these aliens have caused problems worldwide, including the destruction of habitat, introduction of disease and parasites, and predation on or competition with native species of fish.
In many developing countries human population growth is driving a greater need for protein sources and an alternative income. Freshwater fishes are often the most accessible resource. Overexploitation (unsustainable fishing) poses a serious threat to fish and aquatic biodiversity and also to the livelihoods of people in riverine and lake communities. Major causes are excessive food demands, market pressures, fishing gear technology development, weak or lack of appropriate management approaches and policies, accidental by-catches, and an unregulated aquarium trade in wild species, such as Siamese tiger perch, Arowanas, many barbs, characins and other tropical fish.
Several taxa are facing a major decline in population, especially high value food fish: sturgeons, salmons, Mekong giant catfish. Another example is the European eel whose numbers have declined by approximately 90%-98% since the 1970s. Overfishing is a key cause, as eels have been an important source of food, both as adults and as elvers. The drop in the population of European eels became serious in and after 1990, when the catch of fry for export to China began to increase. The European eel is now listed as Critically Endangered on the IUCN Red List.
Sustainable fishing practice and management, based on local participation, are urgently needed. Knowledge of the indigenous fish is important for raising awareness of the need for conservation and for practical management plans.
Image: Mekong giant catfish (Pangasianodon gigas) at a fish market in Vientiane, Loa PDR. ¬© FISH-BIO
The outcome of pollution on freshwaters, whether as a result of industrial, domestic or agricultural activities, is often catastrophic and can result in the elimination of fish species, and dead rivers and lakes. Modern technology is able to deal with most types of pollutants and many point-source problems have been dealt with, resulting in cleaner rivers in some catchments. Diffuse source pollution from agriculture and other forms of land use is proving more difficult to tackle, but progress is being made in some countries.
In the face of environmental pollution, there is one source which does not immediately spring to people"s minds, but which does have a major impact on freshwater fish populations: the pharmaceutical products, such as antidepressants and birth-control pills that end up in human waste and eventually in freshwaters. In the USA alone, physicians issue more than 60 million prescriptions annually for antidepressants. Hence, evidence of drugs were found in 80% of the streams tested in 30 American states. This can cause significant damage in fishes, e.g. sex mutation in female and male gametogenesis, caused by the disruption of complex neuro-endocrine, endocrine and paracrine regulators.
Image: Domestic waste in Sanya River, Hainan Island, China in 2006. Photo: Lu Gang@KFBG
The impact of forestry practice – both afforestation and deforestation – on freshwater habitats and fishes is a cause of much concern. The effects of each stage of the forestry cycle – ground preparation, tree planting to canopy closure, the maturing crop and felling – may have an impact on local freshwater. Massive plantations designed to produce biofuel are a recent aspect of this.
The physical aspects affect the hydrology of streams (this is shown by increased loss of water through interception and evaporation from the forest canopy; and a tendency to higher flood peaks and lower water levels during droughts). The release of sediments to streams due to erosion following deforestation and weathering of exposed soils. Deforested streams also reduce woody debris in-channel input, thus simplifying channel morphological and hydrological complexity. The lack of the debris reduces rearing and food habitat availability.
There can be reduced water temperatures in afforested streams where the channel is shaded and increased water temperatures where the catchment has been deforested. The leaves dropping from forest canopies will affect stream chemistry. Such physical and chemical effects combine in various ways to affect the plants, invertebrates and freshwater fishes in areas impacted by forestry. Changes in hydrology, ambient water temperatures and extreme fluctuations in chemistry tend to make conditions in streams too extreme for most biota.
Turbidity decreases plant growth through reduced light penetration and physical siltation. Increased nutrients alter plant communities and cause problem blooms of algae in streams and lakes. Acidification of water courses affects the composition of their plant and invertebrate communities, completely eliminating fishes in some cases. Other aquatic vertebrates which eat fishes (mainly reptiles, amphibians and birds) may also disappear.
Image: Preparing land for cultivation after the forest was burnt. This practice in northeastern India leads to soil erosion and siltation in headwaters of major rivers, thus destroying fish habitats. Photo: Waikhom Vishwanath.
Climate change is generally causing the world"s waters to warm as well as bringing changes to rainfall patterns, water levels, river flow and water chemistry. Fish cannot control their body temperature. Therefore increasing or decreasing water temperatures will have an impact on growth and reproduction, as will changes of flow and chemistry. Characteristic impacts on fish include stunting, reduced numbers of offspring and even a failure to reproduce at all. Some fish such as salmon, catfish and sturgeon cannot spawn if winter temperatures do not drop below a crucial level. By contrast, higher temperatures will reduce oxygen levels, making waters uninhabitable for many fishes. Unlike marine fish, which may be able to migrate to cooler waters, freshwater fish often have no means of migrating in this way, being confined to river catchments or lakes. Reduced water flow down rivers, as predicted in numerous regions, means that many (often commercially valuable) species will no longer have enough water to survive or to migrate and breed.
Many geographically-isolated freshwater fishes are already living near their thermal tolerance limits and now face potential extinction as waters warm through climate change. One important recent issue with regard to climate change is the status of wet peatland habitats. Approximately 60% of the world"s wetlands are peat, consequently peatlands cover 3% of the global land mass. Under natural circumstances these habitats act as "carbon sinks" because they store organic materials. Indeed peatlands store 30% of all soil carbon and are therefore of high importance in climate change management. Peatlands are also home to many rare and specialised fishes that are found nowhere else in the world. Owing to human activity, including mining and drainage, their habitat is destroyed and a huge amount of greenhouse gas carbon dioxide is released. The remaining dry peat is in contact with the air and starts oxidizing, decomposing, and emitting cabon dioxide; or sometimes the land is burned for agriculture. The average annual emission from peatlands is 2,000 million tonnes of carbon dioxide, almost three times the annual emission by, for example, Germany. It is, therefore, very important to protect and restore the degraded peatlands to decrease the carbon dioxide emission and to secure the survival of many fascinating species of freshwater fishes.
Reid, G. McG., Contreras MacBeath, T. and Csatadi, K. 2013. Global challenges in freshwater fish conservation related to public aquariums and the aquarium industry. International Zoo Yearbook 47(1): 6-45. Available here