Freshwater fish
[3] Among fishers in the United States, freshwater fish species are usually classified by the water temperature in which they survive.
[4] In 2021, a group of conservation organizations estimated that one-third of the world's freshwater fish species were at risk of extinction.
[8] There is an increasing trend in freshwater fish for local taxonomic, functional, and phylogenetic richness in more than half of the world's rivers.
[8] This increase in local diversity is primarily explained by anthropogenic species introductions that compensate for or even exceed extinctions in most rivers.
[12] For example, the Bigmouth Buffalo is now the oldest age-validated freshwater fish in the world, and its status urgently needs reevaluation in parts of its endemic range.
Dams not only interrupt linear water flow and cause major geological channel shifts, but also limit the amount of water available to fishes in lakes, streams and rivers[16] and have the potential to change the trophic structure because of these alterations of the habitat and the limitations to movement and connectivity.
[17][18] Unnatural water flow below dams causes immense habitat degradation, reducing viable options for aquatic organisms.
Upstream migration is hindered by the dam structure and can cause population declines as fishes don't have access to normal feeding and/or spawning grounds.
[16] Additionally, dams can cause the isolation of fish populations, and the lack of connectivity creates possible problems for inbreeding and low genetic diversity.
The loss of connectivity impacts the structure of community assemblies and increases the fragmentation of habitats, which can compound existing problems for vulnerable species.
Temperature can cause changes in fish behavior and distribution habits as well by increasing their metabolic rates and thus their drive to spawn and feed.
Fertilizer runoffs can create excess nitrogen and phosphorus which feed massive algae blooms that block sunlight, limit water oxygenation, and make the habitat functionally unsustainable for aquatic species.
Freshwater fish face extinction due to habitat loss, overfishing, and "forever chemicals."
Conservation efforts, sustainable practices, and awareness are crucial in maintaining fish populations and species diversity.
One such example is the destruction of the endemic cichlid population in Lake Victoria via the introduction of the predatory Nile perch (Lates niloticus).
This could irreversibly compromise the genetic identity of one or both of the parent species and even drive them to extinction if their range is limited.
[28][26] Additionally, the cichlids in Lake Victoria evolved over 700 unique species in only 150,000 years[29] and are theorized to have done so via ancient hybridization events which led to speciation.
