9.4.1 Competition for Water

People require large quantities of freshwater, which is why we build our cities on rivers, lakes and estuaries, create thousands of dams and diversions, and irrigate millions of acres of farmland. According to the United Nations (WWAP, 2019, p. 13):

Water left in rivers to support fishes and a wide variety of other life is often regarded as water ‘wasting to the sea’. The Aral Sea of Kazakhstan is a sad reminder of what can result from competition for water between people and fishes, when people win. Once the fourth largest lake in the world, the Aral Sea has mostly dried up because inflowing rivers are diverted to supply water to grow cotton, with no reductions of water diversion during drought (Figure 9.3). Thirty-two species of native fishes, as well as the fisheries they supported, have been extirpated as a result. At present, a small part of the northern Aral Sea is being restored to a lake-like environment, to conserve remaining endemic fishes and fisheries (Micklin, Reference Micklin2016).

Figure 9.3 The Aral Sea was once the fourth largest freshwater lake in the world. Water diversions for the expansion of agriculture have greatly reduced its size. A 2018 NASA image of the Aral Sea shows the drastic contraction of the sea since 1960 (yellow lines depict 1960 shoreline).

Source: NASA Earth Observatory Explorer 2021.

While the Aral Sea disaster is visible from satellites, most habitats of disappearing fishes are small, such as the spring pool systems spread across arid southwestern North America. The endemic pupfishes and similar desert fishes disappear one by one as the water from the aquifers feeding these springs is pumped away for urban and agricultural use. Many of the remaining pools contain aggressive non-native fish species.

9.4.2 Habitat Alteration

Most of the world’s major rivers are severely altered from their historical conditions by levees, dams, diversions, roads and other human constructions (Figure 9.4). Our cities and farms typically cover the floodplains and wetlands that have only recently been appreciated for their importance to fish and other components of riverine ecosystems, as well as for flood management (Opperman et al., Reference Opperman, Moyle, Larsen, Florsheim and Manfree2017). Habitat alteration is often continuous from the valley floor lakes and rivers to mountain tributaries, as people attempt to control the movement of water to send it where it is desired, not necessarily where it would flow naturally (Williams et al., Reference Williams, Moyle, Kondolf and Webb2019). In many areas, intensive use of lands in watersheds for a myriad of purposes, such as timber production, agriculture and urban expansion, results in watersheds with degraded streams and lakes that have lost much of their native fauna.

(a) The 2000 photo depicts river conditions during construction of the dam.

(b) The 2006 photo shows the dam following completion.

Source: NASA Visible Earth 2021.

Figure 9.4 The Three Gorges Dam constructed on the Yangtze River is the largest hydroelectric dam in the world. It has severely affected the fish fauna of the river.

Increasingly, native fishes are isolated in the least disturbed aquatic habitats or in protected streams because they cannot adapt to new, extreme conditions. This general observation was the basis for the Index of Biotic Integrity (IBI) developed by James Karr in the 1980s, for streams in the Midwestern USA, and widely used as the basis for rapid bioassessment by resource management agencies (Karr, Reference Karr1991). A fundamental assumption for IBIs is that the abundance and diversity of native fishes is an excellent indicator of ‘healthy’ habitat. However, in some highly disturbed waters, native fishes are integrated into fish assemblages that also contain many non-native species, forming novel ecosystems shaped by human presence (Moyle, Reference Moyle2014). Most aquatic habitats of the world today are disturbed to some degree, and thus are increasingly divergent from historical conditions (Figure 9.5). Climate change is accelerating this process, which is leading to increased homogenisation of fish faunas around the world. See Closs et al. (Reference Closs, Angermeier, Darwell, Balcombe, Closs, Krkosek and Olden2015) for further discussion of these habitat alteration issues.

Figure 9.5 Gold mining pits in the Amazon rainforest of eastern Peru result in forest clearing, sedimentation, and the discharge of toxic substances into rivers and wetlands with devastating impacts to freshwater fishes.

Source: NASA Earth Observatory 2021.

9.4.3 Pollution

Streams, lakes, ponds and estuaries are sumps for the waste and other by-products of human activity: all water flows downhill, carrying dissolved materials with it. Pollution, from coarse sediment to sewage to ‘contaminants of emerging concern’ such as residuals of human medications, is ubiquitous. Traces of pesticides and other materials can be found even in the most remote and ‘pristine’ waters. The vast literature on pollution effects on fishes documents such things as:

• fish kills over broad areas

• reduced survival of eggs and larvae

• reduced growth rates

• altered behaviour, such as reduced ability to migrate

• diminished ecosystem health as the result of the combined effects of multiple pollutants and other elements (Matthaei and Lange, Reference Matthaei, Lange, Closs, Krkosek and Olden2016).

9.4.4 Invasions of Non-Native Species

Most waterways around the world support non-native species of fishes. Species with global distributions due to human intervention include brown and rainbow trout (cold waters), common carp, goldfish and mosquitofish (temperate waters), and Mozambique tilapia and guppy (tropical waters). In highly altered habitats (e.g. reservoirs), alien fishes suppress native fish populations through predation, competition or disease. In California, there are over 50 species of alien fishes and about 130 species of native fishes (Moyle and Marchetti, Reference Moyle and Marchetti2006). Generally favoured in altered habitats, non-native species show an increase in abundance as climate change accelerates habitat change and increases competition for water between people and fishes (Moyle et al., Reference Moyle, Kiernan, Crain and Quiñones2013). This is a pattern occurring throughout the world. The result will be increased dominance of local fish faunas by a few highly tolerant species, many non-natives (Figure 9.6).

Figure 9.6 Bighead carp (Hypophthalmichthys nobilis) are a non-native species established in rivers in the midwestern USA, but have not yet invaded (as of 2021) the Great Lakes. They are plankton-feeding fish that can become extremely abundant, altering food webs and displacing native fishes.

Source: U.S. Geological Survey 2021.

9.4.5 Commercial Exploitation

Commercial fisheries for wild fish are a significant protein source in much of the world, especially fisheries in tropical rivers (Funge-Smith and Bennett, Reference Funge-Smith and Bennett2019). In the floodplains of the lower Mekong River, for example, over one million tons of wild fish are harvested each year (Ziv et al., Reference Ziv, Baran, Nam, Rodríguez-Iturbe and Levin2012). Not surprisingly, overexploitation of many, if not most, stocks of wild freshwater fishes contributes to the overall decline of freshwater fishes (Allan et al., Reference Allan, Abell and Hogan2005). Such fisheries are usually not very selective because they employ a wide variety of gear, capturing fishes of all sizes. Allan et al. (Reference Allan, Abell and Hogan2005) noted that ‘assemblage overfishing’ results in significant, often undesirable, changes in fish faunas in many large waterways, from the Laurentian Great Lakes to major tropical rivers such as the Mekong (Figure 9.7).

Figure 9.7 Fishing with a cast net in lower Mekong River basin. Fisheries in the Mekong are an important source of protein for millions of people.

Source: U.S. Geological Survey 2021.

9.4.6 Global Climate Change

Rising sea levels, prolonged droughts, massive floods, warmer waters (see Barbarossa et al., Reference Barbarossa, Bosmans and Wanders2021) and increasingly distressed human populations are all predictions of global climate change. All of these events will adversely impact freshwater fish abundance and diversity, although a few hardy, widely introduced alien species (e.g. common carp) will no doubt thrive (Moyle et al., Reference Moyle, Kiernan, Crain and Quiñones2013) (Figure 9.8). A significant factor affecting native fishes is the predictable human reaction to take actions to fight the changing conditions, which further decreases habitat for fishes. Such actions include construction of additional hardened infrastructure (dams, levees, sea walls, etc.), increased exploitation of remaining fishes, increased diversion of freshwater, invasion of salt water into historically freshwater areas and general degradation of freshwater habitats. Much depends on how and when we let such seemingly inevitable changes take place and how conservation fits into the planning. However, the ever-present desire of most governments for such big projects does not bode well for the fishes.

Figure 9.8 Threatened bull trout (Salvelinus confluentus) are apex predators in the cold streams and rivers of the western United States. Climate change threatens these fish by decreasing snowpack and stream baseflows, increasing summer water temperatures, and inducing more frequent flooding from rain-on-snow precipitation events.

Source: U.S. Fish and Wildlife Service.

9.5.1 Cyprinidae: Global Distribution, Local Threats

Carps and minnows are one of the most species-rich, abundant and widespread freshwater fish families. Over three thousand species are recognised (Fricke et al., Reference Fricke, Eschmeyer and Fong2022), with the number of taxa being described rapidly increasing. Cyprinids range in size from the giant barb of the Mekong, which reached over 2 m in length with weights of 300 kg (Figure 9.9), to the dwarf minnow (Paedocypris progenetica), the world’s smallest freshwater vertebrate, from Sumatra, which only reaches 8–12 mm in length. Cyprinid fishes are often the most abundant fishes in river systems and support important fisheries. For example, migratory cyprinids are harvested literally by the billions in the Mekong River and are a principal source of protein and income for residents of the Mekong Basin (Baird et al., Reference Baird, Flaherty and Phylavanh2003).

Figure 9.9 The giant barb or Siamese carp, Catlocarpio siamensis, is the largest carp/barb (Cypriniformes) in the world and is endangered by dams and overfishing. This giant barb was caught on the Mekong River.

Photo credit and permission: Zeb Hogan, University of Nevada, Reno.

Cyprinid fishes are native to river systems in tropical and temperate climates of every continent, except Australia and Antarctica. They have adapted to environments from giant floodplain rivers to tiny desert springs and are significant elements of many regional fish faunas, often showing high endemism. Thus, Xing et al. (Reference Xing, Zhang, Fan and Zhao2016) found that the 654 species of Cyprinidae known in China made up almost half the total fish species (n = 1323) of the country and that 67% were endemic to China. In contrast, some species, such as the common carp and grass carp, have been widely introduced outside their native range, adjusting quickly to new environments, often with harmful effects on native fishes.

It should not be surprising that with such a high diversity of freshwater-dependent species, extinction threatens many cyprinids. Leidy and Moyle (Reference Leidy, Moyle, Fiedler and Karieva1997) found that 157 species, 8% of the known cyprinids at that time, were threatened. Thirty-five years later, 312 species were listed as threatened by IUCN, which includes 24% of the assessed species (IUCN, 2022). Within China alone, 108 cyprinid species (16.5% of the family) fit the IUCN threatened species criteria (Xing et al., Reference Xing, Zhang, Fan and Zhao2016). While such a rapid decline reflects better information, it is also highly likely to be real, led by the loss of species in isolated or highly modified habitats, especially big rivers.

All the factors listed in this chapter as proximate causes of decline affect cyprinids. For example, the Twee River redfin is endemic to a small watershed in South Africa. It is facing extinction through removal of water from the stream by farmers, riparian habitat alteration, pollution from agricultural wastewater and contaminants, and invasions of non-native predators and competitors (Impson et al., Reference Impson, Marriott, Bills and Skelton2007), all factors exacerbated by climate change. In the Mekong River, the fisheries, among the most productive in the world, are showing signs of overfishing: they focus more and more on small fishes (usually cyprinids) with wide population fluctuations. Many of the larger species are threatened with extinction, including some of the largest of all freshwater fishes (Hogan et al., Reference Hogan, Moyle, May, Vander Zander and Baird2004; Fengzhi et al., Reference Fengzhi, Zarfl and Bremerich2019). Even these productive fisheries, however, are threatened by the rapid construction of dozens of large hydropower dams, which change flow timing and volume and capture sediment. New dams proposed for the river, if built as planned, will likely eliminate the vast fish migrations and the fisheries that depend on them (Baran, Reference Baran2010). They are also likely to decimate the many unexploited endemic fishes that are part of the highly diverse fauna (781 species). Unfortunately, only after dam completions will we find out which cyprinid species have declined or are extinct.

9.5.2 Cichlidae: Food for People, Food for Thought

The Cichlidae are deep-bodied, spiny-rayed fishes adapted for living in lakes, rivers, estuaries and backwaters of tropical Asia, Africa and the Americas. Their complex behaviour, most notably parental behaviour, and adaptability has led to rapid speciation in tropical regions, but especially in the rift lakes of Africa. A handful of tilapia species (Nile, Mozambique and blue tilapias, plus their hybrids) are important aquaculture species throughout the tropics (Figure 9.10). This use has resulted in their becoming widespread in natural environments as well. And, not to be dismissed, the popularity of small brightly coloured cichlids as aquarium fishes has led to the introduction of diverse species into warm waters worldwide. No doubt aquaculture and aquarium species will assure the continued presence of Cichlidae in waters of the world, even if many ‘wild’ species have disappeared.

Figure 9.10 Map showing location of African Great Lakes, which support an astonishing diversity of cichlid fishes, with a different group of hundreds of species in each lake. Unfortunately, the predatory Nile perch was introduced into Lake Victoria to establish a fishery. One of its impacts has been to drive many of the lake’s cichlid species to extinction through its voracious predation. Pastel drawing created by and copyright held by Chris Mari van Dyck.

Used with permission, from Moyle (Reference Moyle1993).

The total number of cichlid species is an open question, given that many (if not most) species are undescribed. Fricke et al. (Reference Fricke, Eschmeyer and Fong2022) counted 1749 species in the taxonomic literature, but estimates of the number of species in the rift lakes are around 500 for Lake Victoria, 500–1000 for Lake Malawi and 1800 in Lake Tanganyika. Even if these estimates are widely off, they demonstrate three of the most astonishing radiations of fishes on the planet (Hastings et al., Reference Hastings, Walker and Galland2014) and indicate that conservation efforts need to concentrate on the lakes and their watersheds.

The African rift lake of most concern for cichlid conservation now is Lake Victoria, the world’s second largest freshwater lake. The introduction of the Nile perch, a voracious piscivore that grows to 2 m and devours small cichlids of all sorts, threatens the entire endemic cichlid radiation. An additional threat is competition from introduced Nile tilapia, which thrive in the more turbid water. Increased human populations in the watershed have resulted in deforestation and expanded agriculture, which in turn led to more sediment going into the lake and increased pollution and subsequent eutrophication. Fisheries have also grown, while native cichlid populations have crashed in most areas of the lake. Fifty-five percent of the assessed species have been found to be in danger of extinction; many species may already be extinct (Sayer et al., Reference Sayer, Máiz-Tomé and Darwall2018).

In summary, the Cichlidae contains some of the most abundant and widespread (through introductions) freshwater fishes in the world. But it also includes many endemic species threatened with extinction, with numerous taxa not yet described. The future of most cichlid species depends on effective conservation and management of their native waters.

9.5.3 Bedotiidae: Madagascar Endemics

The Indian Ocean island of Madagascar, located off the east coast of Africa, is a freshwater biodiversity hotspot (Ravelomanana et al., Reference Ravelomanana, Maiz-Tome, Darwell, Máiz-Tomé, Sayer and Darwall2018). Due to long isolation, Madagascar supports a highly endemic (>50%) freshwater fish fauna of approximately 183 species, with many new endemic species descriptions anticipated (Stiassny and Raminosoa, Reference Stiassny, Raminosoa, Teugels, Guégan and Albaret1994). Most of these endemic fishes are in decline, as exemplified by the endemic family Bedotiidae, the Madagascar rainbowfishes. The Bedotiidae includes 28 species of small (<10 cm) fishes found in small- to medium-sized rivers and lakes, and in seasonal brackish swamps draining from Madagascar’s eastern mountain slopes (IUCN, 2022). Every species is threatened, including six species that are considered Critically Endangered, nine as Endangered and eight as Vulnerable (IUCN, 2022).

Madagascar rainbowfish are declining for multiple reasons. Chronic, widespread clearing of forests for wood and agriculture have decimated aquatic habitats for almost all of Madagascar’s native freshwater fishes (Ravelomanana et al., Reference Ravelomanana, Maiz-Tome, Darwell, Máiz-Tomé, Sayer and Darwall2018). The IUCN, (2022) identifies deforestation and non-native fishes as the greatest threat to rainbowfish. Deforestation results in loss of fish spawning habitats due to increased sedimentation, increased water temperatures from the reduction of streamside forests, and water quality degradation. The introduction of non-native fish species to support fisheries and aquaculture, including the common carp (Cyprinidae), largemouth bass (Centrarchidae), trout (Salmonidae), several species of tilapia and the Asian snakehead pose severe threats to rainbowfish through direct competition, predation, and disturbance of nesting and spawning sites (Ravelomanana et al., Reference Ravelomanana, Maiz-Tome, Darwell, Máiz-Tomé, Sayer and Darwall2018).

Other human activities are also detrimental to rainbowfish (Ravelomanana et al., Reference Ravelomanana, Maiz-Tome, Darwell, Máiz-Tomé, Sayer and Darwall2018; IUCN, 2022).They include wetland conversion, subsistence fisheries, farming, grazing and ranching, and mining and forestry operations that exacerbate soil erosion and sedimentation. Increased occurrence of extreme events related to climate change, such as prolonged droughts, and increased flood frequency and intensity also contribute to habitat degradation for these fishes (Ravelomanana et al., Reference Ravelomanana, Maiz-Tome, Darwell, Máiz-Tomé, Sayer and Darwall2018).

9.5.4 Sturgeons and Paddlefishes: Big, Valuable River Fishes

The sturgeons (Acipenseridae, 25 species, 2 subspecies, and 42 subpopulations) and paddlefishes (Polyodontidae, two species) are the only living representatives of the ancient order Acipenseriformes (Figure 9.11). They are among the most imperilled lineages of freshwater fishes in the world, in large part because they are the only source of ‘true’ caviar. Sturgeon and paddlefish are adapted for living in large river systems that include inland seas, large continental rivers and estuaries. All undergo migrations in or to freshwater for spawning. Found in North America and Eurasia, they include the Critically Endangered beluga sturgeon, which is the largest freshwater fish in the world, reaching lengths of 8 m and weights of 1300 kg.

Figure 9.11 The world’s largest fishes are some of the most endangered. The American paddlefish (Polydon spathula) is native to the Mississippi River drainage, USA, and its conservation status is vulnerable to pollution, overfishing and other factors. Its closest relative, the Chinese paddlefish, recently was declared extinct.

Photo credit: Ryan Hagerty, U.S. Fish and Wildlife Service.

Ninety-four percent of sturgeon species, subspecies or subpopulations listed by the IUCN (2022) are threatened (n = 65) with extinction; 36 have experienced steep decreasing population trends such that they face an extremely high risk of extinction in the wild. The American paddlefish is extirpated from much of its historical range, and remaining populations are in steep decline; the Chinese paddlefish is apparently extinct.

The cumulative effects of all major causes of fish decline discussed in this chapter have severely affected sturgeon and paddlefish populations (Haxton and Cano, Reference Haxton and Cano2016). However, global sturgeon populations have suffered most from two factors: illegal overexploitation and the fragmentation of large, free-flowing rivers by dams. Harvest is a problem for these fishes because of the high value of caviar, which only increases in value as wild sturgeon and paddlefish become rare. The caviar fishery for all species focuses on large mature females, which are likely to be 25–100 years old, depending on size and species. Replacement rates are slow, despite high fecundities.

Eurasian sturgeons. Construction of the Volgorad Dam in 1958 on the lower Volga River blocked spawning migrations of beluga, Russian and stellate sturgeons (Ruban et al., Reference Ruban, Khodorevskaya and Shatunovskii2019). Altered flow regimes below the dam further adversely affected the quality of remaining spawning, juvenile rearing and foraging habitats in the Volga River and the Caspian Sea. Illegal fishing and pollution in the Volga River Delta and the Caspian Sea have also adversely affected reproduction, growth and survival (Ruban et al., Reference Ruban, Khodorevskaya and Shatunovskii2019). For similar reasons, five of six sturgeon species found in the Danube River and the Black Sea are Critically Endangered (Friedrich, Reference Friedrich, Schmutz and Sendzimir2018).

Alabama sturgeon. The small (to 0.78 m) Alabama sturgeon is North America’s most endangered sturgeon. Dams have reduced its river habitat in the lower reaches of just two rivers in southern Alabama (Kuhajda and Rider, Reference Kuhajda and Rider2016). There are no reliable population estimates for the Alabama sturgeon. Without restored access to historical spawning grounds through the removal of several dams, the Alabama sturgeon has a high likelihood of extinction.

Chinese sturgeon and paddlefish. In the Yangtze River, China, there are several imperilled sturgeon species, as well as the Chinese paddlefish (likely extinct) (Figure 9.12). They suffer from the same human activities that have adversely affected sturgeon elsewhere (Xing et al., Reference Xing, Zhang, Fan and Zhao2016), i.e. continued adverse dam operation, new dams and overexploitation.

Figure 9.12 The pallid sturgeon (Scaphirhynchus albus) is native to the Mississippi and Missouri River systems, USA, and is considered endangered. Dam construction and dredging have altered its habitat, preventing the sturgeon from reproducing.

Photo credit: Ryan Hagerty, U.S. Fish and Wildlife Service.

Because sturgeons and paddlefishes are so valuable, large aquaculture operations exist for some species, mainly to provide caviar. Their value and adaptability to captive conditions create strong incentives to rear species in captivity, despite the high costs of producing mature fish. However, aquaculture is also a tool to restore sturgeon and paddlefish populations to the wild.

9.5.5 Salmonidae: Hidden Diversity in Cold Water Fishes

The Salmonidae are cold water fishes, adapted for thriving in the dynamic Pleistocene and post-Pleistocene fresh and salt waters of the Northern Hemisphere. Their oily flesh, abundance and high aesthetic qualities have made them the focus of sport and commercial fisheries, as well as aquaculture, worldwide. But their wide distribution and tendency to become easily isolated into small, distinct populations (often recognised as species) means that many species and populations are facing extinction in the modern world.

The most iconic species, the Pacific and Atlantic salmons, are anadromous; they spawn in freshwater but migrate vast distances to forage in northern oceans. Salmonids are masters at adapting to local conditions, so many isolated populations of ‘trout’ and ‘char’ are found throughout the family’s native range (Kershner et al., Reference Kershner, Williams, Greswell and Lobon-Cervia2019), as are species ‘flocks’ of whitefishes that partition the food resources of the large lakes scattered across the northern landscape. Local adaptation is rapid, so many of these isolated trout, char and whitefishes are now recognised as distinct taxa. Two hundred and fifty-seven known species, subspecies and subpopulations are now recognised by the IUCN (2022), with more being described regularly as new genetic techniques uncover additional taxa. When we wrote about this family in 1997, only 56 species were recognised.

Recognised taxa are only a small part of this family’s diversity. Thanks to the enormous interest in the family by the public, fishers and scientists, considerable efforts have been made to understand the evolutionary history and genetic structure of local populations of widespread species. Much of the recent research has been to catalogue diversity so distinctive forms can be protected. To demonstrate the value of this approach, Rand et al. (Reference Rand, Goslin and Gross2012) compiled information on populations of sockeye salmon throughout its range, in spawning streams along the Pacific Rim from eastern Russia to the western USA. They identified 98 ‘independent populations’. Using IUCN criteria, five of these populations were determined to be extinct, and 27% of the 63 populations evaluated were at risk of extinction. IUCN now includes this information as part of their database on threatened salmonids. In the USA, language of the 1973 Endangered Species Act allows protection of ‘distinct population segments’ (DPSs) of vertebrate species and today many such DPSs of fishes, especially salmon, are protected under the ESA. For example, in California, three of eight DPSs of Chinook salmon are listed as threatened or endangered species, although all eight segments could qualify (Moyle et al., Reference Moyle, Lusardi, Samuel and Katz2017). This demonstrates the importance of protecting distinct populations of species, both to protect the species as a whole and to make sure each species retains the ability to adapt to changing conditions.

The sockeye salmon discussed above is an excellent example of a widespread species that is likely to persist indefinitely, even with the extirpation of most subpopulations. This persistence will be in part due to its high value in fisheries. Similar species include rainbow trout (steelhead), Chinook salmon and Atlantic salmon. All will persist at some level, perhaps only through hatcheries that produce semi-domesticated juveniles for release into the wild. The North Pacific Ocean now has so many hatchery-origin chum and pink salmon that they are reducing the abundance of other species, even birds (Springer and van Vliet, Reference Springer and van Vliet2014).

In contrast, many inland species of salmonids are threatened with extinction. The IUCN (2022) lists 95 taxa as being threatened, with about 87% of species needing status updates. The causes are multiple and involve all the factors listed for fishes in general, as described eloquently by Behnke (Reference Behnke2002) for salmonids throughout North America and by Kershner et al. (Reference Kershner, Williams, Greswell and Lobon-Cervia2019) for the world. For many isolated populations of trout, char and whitefish, declines are tied to degradation of watersheds by domestic livestock grazing, logging, mining and other factors, often in combination with invasions of aggressive non-native species such as rainbow trout and brown trout (which now have worldwide distributions in cold water). Restoration or protection of endemic species of salmonids often requires both elimination of non-native species from the watershed and intensive watershed management to restore appropriate ecosystem functions. For example, Eagle Lake rainbow trout in California are now maintained by regular planting of hatchery-reared fish. Long-term restoration of a wild population requires eradication of alien brook trout from their principal spawning stream, as well as protection of the stream and lake from cattle grazing and other disturbances. If present trends continue, the Salmonidae will be represented in the future mainly by a few widely introduced species, remnant or hatchery populations of widespread anadromous species and a few highly managed populations of endemic trout, whitefish and grayling.

The obvious conclusion we reach from this section is that extinction does not respect taxonomic boundaries; no family or lineage is immune to anthropogenic changes. Large families such as the Cyprinidae and Salmonidae are more likely to have representatives in future fish faunas, although species richness will be much lower. Families with few species are candidates for total extinction.