Fly fisherman above Ojo Calenti Bend on the Firehole River, Wyoming, USA, October 2007. Photo: Mike Cline.
North America has a broad array of freshwater ecosystems because of the continent’s complex geography and geological history. Within a multitude of habitats—that include streams, large rivers, natural lakes, springs, and wetlands—rich assemblages of fishes reside, representing diverse taxonomic groups with unique ecological requirements. They face an unprecedented conservation crisis.1 In the last few decades, the proportion of inland fishes of North America, which are considered imperiled or extinct, increased from 20 to 40%.2 Although extinctions have occurred, many species and populations are declining in range size and abundance. The fish biota of the continent as a whole remains diverse; however, we can take action to stem any further declines.
Globally, fishes outnumber all other vertebrates combined and have the highest rate of discovery of new species.3 Fishes exhibit a remarkable diversity of morphological attributes and biological adaptations and occur in most aquatic habitats on Earth. Even in North America, where scientific knowledge of the fauna is advanced, new species are described every year. These discoveries are the combination of applied technologies, such as gene sequencing, which increase recognition of biodiversity at all levels, and the documentation of new, morphologically distinct forms and populations. Biological taxonomy is the discipline of classifying and naming organisms using an internationally accepted system or code. Because of the dynamic nature of fish taxonomy, and the extent of unexplored areas of the planet with potentially many undescribed species, statements or conclusions about numbers and percentages of species occurring in particular habitats or geographical regions are but rough approximations. Published information reveals the following:
A conservative estimate is that as many as 32,500 extant (living) fish species may exist in the world.3 This number may eventually prove greater, however, with approximately 30,000 currently recognized as valid and over 300 new species described each year.4
Fresh water constitutes only about 1% of the Earth’s surface area and less than 0.01% of its water by volume.
About 12,000 species, or approximately 43% of all currently named fishes, occur exclusively in fresh waters. A small number are diadromous, regularly living part of their lives in rivers, streams, or lakes, and part in the oceans.
North America has the greatest taxonomic richness of freshwater fishes among temperate regions of the world,1 although it is greatly surpassed in number of species by less documented areas of the tropics—especially the biological hotspots of South America, Africa, and southeast Asia.5,6 Currently, there are approximately 1,200 recognized fish species that occur in inland waters of the continental United States, Canada, and Mexico.
Collectively, the fish fauna (ichthyofauna) of North America’s freshwater ecosystems has at least 435 imperiled species, another 72 species with distinct populations in trouble, and 36 species that are extinct from the wild.2
Worldwide, historical, and emerging trends in the conservation of fishes and fishery stocks portray continued or even accelerated population decreases; yet, there are reasons for optimism, as well as potential for recovery and protection of these declining resources given societal resolve.7
Threats to freshwater fishes and habitats
Threats to freshwater ecosystems are so widespread that many endemic species—those naturally restricted to a single drainage or ecoregion—are imperiled simply because restricted geographic distribution makes them more vulnerable to human modification of landscapes. The most important documented threats to fishes in freshwater habitats include:1,7,8,9
Destruction or modification of habitat resulting in reduced range size and/or loss of populations. Examples include dam construction, channelization, mining, clearing of natural forests for agriculture, urban development, and other intensive land-use practices.
Water depletion. Some desert fishes have become extinct because of human exploitation of limited groundwater resources.
Pollution from point and non-point source contaminants. Runoff from urban areas, and the compound effects of multiple pollutants, often reduces water quality to the point that only the most tolerant species remain in receiving water bodies.
Erosion and sedimentation. Fine sediments can smother bottom substrates, to the detriment of many bottom-dwelling (benthic) species, whose prey and reproductive success are dependent on clean substrates and good water quality.
Overexploitation for commercial, recreational, scientific, or educational purposes. Examples of fishes that have been overharvested include salmons, whitefishes, trouts, striped bass, and sturgeons.
Disease or parasitism. For instance, whirling disease, a microscopic parasite introduced from Europe, has ravaged many wild and hatchery populations of trouts and salmons in the U.S. and Canada.
Other anthropogenic factors—including introduction of non-native species—which may result in hybridization, competition, and predation. Numerous introductions of fishes and other aquatic organisms, both from outside of North America and intracontinental transplants, have had severe negative impacts on native species,including some that have caused extinction.10
Climate change. Regional variation in rainfall patterns, storm events, and droughts can affect habitats and potentially have negative consequences for rare species.
Media coverage of conservation issues about the world’s animals is often greatest for species that are particularly endearing to humans, such as mammals and birds, whereas perils to aquatic faunas, including fishes, are often much less publicized. This disparity is due, in part, to taxonomic bias in research and funding in the field of conservation biology.11 Based on published scientific literature, fishes, amphibians, reptiles, and invertebrates are greatly underrepresented in conservation studies relative to the proportion of species of each group known worldwide,11 and organisms in the freshwater and marine realms receive much less coverage than those in terrestrial environments.12 Among invertebrates, however, it is notable that mollusks and crustaceans, of which many species are aquatic, are generally better studied than the vast diversity of arthropods—especially insects, with the exception of butterflies and moths. Even within freshwater fishes, game, and commercial species often receive greater attention than non-game species.
Major deficits in funding for faunal surveys, monitoring, basic research, and the general lack of public awareness about the conservation status of fishes across taxonomic groups and ecosystems is a significant problem. Given the myriad of threats to aquatic habitats throughout the world; the degree to which degradation of these habitats is accelerating; and the overall proportion of biodiversity represented, exceptional natural resources are at risk of being severely diminished or lost. In particular, freshwater habitats are some of the most threatened in the world.8,9,13 Moreover, aquatic systems are inextricably linked to terrestrial habitats, and pollutants and sediments from perturbed landscapes flow into lakes, streams, and rivers.
The Endangered Species Committee of the American Fisheries Society (AFS-ESC) has tracked the plight of imperiled fishes in North America for over 30 years, with the explicit aim of providing objective and unbiased status assessments independent of the influence of policy or regulatory considerations. Recently, the AFS-ESC—represented by 16 scientists from the United States, Canada, and Mexico, with the assistance of numerous colleagues— evaluated the conservation status of the entire continental fish fauna.2 In the latest assessment, approximately 40% of described North American freshwater and diadromous fish species are documented as imperiled or extinct, representing a substantial increase over previous assessments. Past conservation assessments by the AFS were limited to determining the status of distinct species and subspecies. Undescribed forms, or those not named in the scientific literature using classic Linnaean binomial nomenclature, were included where sufficient data were available to document taxonomic distinctiveness, as evidenced by unique morphological, genetic, or other attributes.
In the most recent assessment, additional infraspecific taxa were included in the form of distinctive populations, or what are sometimes referred to in the scientific community as evolutionarily significant units (ESUs) or distinct population segments (DPSs, although this term has certain legal connotations under the Endangered Species Act within the U.S.).14 A taxon (taxa, plural) is a unit used in biological classification and is defined based on a natural relationship, formally recognized as one or more lineages (= clades) of descendants sharing a common ancestry.
Seven hundred fish taxa are considered imperiled in North America’s inland waters, currently, which represent 133 genera in 36 families (see Figure 1).2 The majority of taxa are named species (63%), followed by named subspecies (13%), populations (12%), undescribed species (7%), and undescribed subspecies (5%). Previous lists from 1979 and 1989 also had about 63% listed species and 37% infraspecific taxa. Of the total taxa currently listed,
- 280 taxa are endangered (E), i.e., in imminent (fewer than 50 years) danger of extinction, or extirpation (loss of populations) throughout most portions of a taxon’s range.
- 190 are threatened (T), or in imminent danger of becoming endangered.
- 230 are vulnerable (V), that is, in imminent danger of becoming threatened, which is comparable to a designation of “Special Concern” by many agencies and conservation organizations.
- 61 are presumed extinct (X), meaning a taxon that has not been observed for over 50 years. Two subcategories are included: Possibly Extinct (Xp), a taxon suspected to be extinct, as evidenced by more than 20 but less than 50 years since living representatives were observed, and Extirpated in Nature (Xn), where all populations in natural habitats are presumed eliminated but surviving individuals are maintained in captivity.
Chronological increase in the number of fish taxa imperiled in North American freshwater ecosystems, as assessed by the American Fisheries Society Endangered Species Committee.2 Assessments were conducted in 1979, 1989, and 2008. Delisted taxa are those that appeared on a previous list but were removed due to improved status, taxonomic invalidity, or extralimital distributions.
The number of imperiled fishes represents a 92% increase over a nearly 20-year period dating to 1989. The list of imperiled taxa encompasses fishes that span a remarkable diversity of lineages, morphologies, life histories, and habitats. A taxonomic breakdown of the list and comparison to known, described species reveals disparities by family.
- Nearly one quarter of all imperiled taxa belong to the most species-rich family of North American freshwater fishes, the Cyprinidae, represented by the minnows and their allies.
The Candy Darter is a vulnerable species found in the New River system of Virginia and West Virginia. Darters are a colorful, species-rich group of small-bodied fishes limited in distribution to North America; their greatest diversity is in uplands of central and eastern U.S. Photograph by N.M. Burkhead.
Another 15% is represented by the second-most diverse family—the Percidae—that includes a large number of darters—small, colorful fishes that have their greatest diversity and abundance in clear-flowing streams of the central and eastern U.S.
The Salmonidae—trouts, salmons, ciscoes, and their allies—comprise nearly 12% of all imperiled taxa, but are disproportionally represented in comparison to other families by large numbers of infraspecific taxa.
There are distinct geographic trends evident for imperiled North American fishes based on distributions within natural hydrologic units or ecoregions (defined by a combination of physical drainage features and faunal similarity, see Figure 2).12 Concentrations of at-risk fishes occur in the southeastern U.S., the mid-Pacific coast, the lower Rio Grande, and coastal and south-central inland regions of Mexico.1 Of particular note is the distribution of imperiled fishes in North America within ecoregions; 80% of all taxa are confined (endemic) to a single ecoregion, and another 10% are limited to two ecoregions. Much of the imperilment of the inland continental fish fauna is attributed to a combination of limited range sizes of many species and broad-scale habitat degradation.
Numbers of imperiled freshwater and diadromous fishes by ecoregions, within North America, based on the most recent conservation assessment by the American Fisheries Society’s Endangered Species Committee.2
Why be concerned about the decline?
Loss of biodiversity on planet Earth is thought by some to be the greatest impending environmental crisis currently facing humanity.15 The decline of North American fish species and populations, as with elements of biodiversity throughout the world, directly or indirectly impacts other faunas, is detrimental to freshwater ecosystems in general, and affects humankind in a variety of ways. Freshwater fishes are important sentinels of environmental conditions and play a crucial role in the ecology and sustainability of natural ecosystems.
The natural balance of both aquatic and terrestrial communities, including birds, mammals, reptiles, and other fishes, is dependent on fish populations that provide critical functions, such as cycling nutrients and serving as prey to a large variety of carnivores. The larvae of native freshwater mussels, called glochidia, require fish hosts in order to complete their life cycles. Some migratory fishes—such as shads, smelts, chars, and salmons—serve as keystone species of entire ecosystems. For instance, a variety of predators and scavengers feed on adults of migrating and spawning salmon, their eggs, fry, and their decaying carcasses. The nutrients that are transferred from the sea and incorporated into the food chain contribute to the health of forests adjacent to streams in which salmon spawn, thereby illustrating the linkage between terrestrial and aquatic habitats.7,16,17
Herbivorous species provide important functions in terms of cropping algae and plants or disseminating seeds and fruits. Humans derive extensive recreational, commercial, and intangible benefits from fish and fishery resources; and our welfare is linked directly to their protection and sustainability. Conversely, degraded aquatic ecosystems—with simplified communities and altered food webs—can lead to an increase in vectors, such as mosquitoes and snails, water-related diseases like malaria, schistosomiasis, and cholera.18
These are but a few examples of the global importance of freshwater fishes, their habitats, and the need for the conservation of both. Overall, fish species, and communities in the inland streams, rivers, lakes, and wetlands of North America, provide countless ecosystem services. Beyond the economic value of fish, fisheries, and freshwater habitats, healthy natural aquatic ecosystems may have profound positive effects on the human spirit and well-being.
What can be done to ensure effective conservation?
Although the status and future prospects for many North American fishes may be discouraging, many actions can reverse declining trends, recover populations, and provide solutions. In a review of a number of studies, G.S. Helfman summarized the following solutions to environmental problems associated with the conservation of fishes and their habitats7:
Enforce existing laws that protect native assemblages and habitats.
Create reserves, protected land areas, and water bodies. Large reserves provide the greatest impact for protecting multiple species and communities.
Promote ecosystem-based management.
Be precautionary and proactive. Act despite uncertainty and without waiting for scientific consensus. Conservation crises may become irreversible, and management actions may be required before supporting scientific data are available.
Monitor results and manage adaptively. Make adjustments to improve conservation actions and implement successful ones for the long term.