Learn about Freshwater fish

Freshwater fish are fish that spend some or all of their lives in freshwater, such as rivers and lakes, with a salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations in order to keep the ion concentration of their bodies balanced.

41% of all known species of fish are found in freshwater. This is primarily due to the rapid speciation that the scattered habitats make possible. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying island biogeography.

Physiology

Fresh water fish differ physiologically from salt water fish in several aspects. Their gills must be able to diffuse water while simultaneously keeping the salts of the bodily fluids inside. The scales of the fish also plays a part in the scientific process; freshwater fish that have lost too many scales get a surplus of water diffused in through the skin, causing the fish to die.

Another trait characteristic of fresh water fish are the well developed kidneys. These have to be large because a lot of water passes through them. .

Migrating fish

Sturgeons are found both in anadromous and fresh water stationary forms

Many species of fish do reproduce in freshwater, but spend most of their adult lives in the sea. These are known as anadromous fish, and include, for instance, salmon, trout and three-spined stickleback. Some other kinds of fish are, on the contrary, born in salt water, but live most of or parts of their adult lives in fresh water; for instance the eels.

Species migrating between marine and fresh waters need adaptations for both environments; when in salt water they need to keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Many species solve this problem by associating different habitats with different stages of life. Both eels, anadromous salmoniform fish and the sea lamprey have different tolerances in salinity in different stages of their lives.

Status
North America

About four in ten North American freshwater fish are endangered, according to a pan-North American study. The main cause being human pollution. The number of fish species and subspecies to become endangered has risen from 40 to 61, since 1989.

Eleotris fusca - (Puwakbadilla)

Family:	Eleotridae (Sleepers) , subfamily: Eleotrinae
Order:	Perciformes (perch-likes)
Class:	Actinopterygii (ray-finned fishes)
FishBase name:	Dusky sleeper
Max. size:	26.0 cm TL (male/unsexed; Ref. 5493)
Environment:	demersal; amphidromous
Climate:	tropical; 25; 45°N - 27°S
Global Importance:	fisheries: minor commercial; aquarium: commercial
Resilience:	Medium, minimum population doubling time 1.4 - 4.4 years (Preliminary K or Fecundity.)
Distribution:	Indo-West Pacific: East Africa to French Polynesia.
Diagnosis:	Dorsal spines (total): 7-7; Dorsal soft rays (total): 7-9; Anal spines: 1-1; Anal soft rays: 8-8. Diagnosis: (57)59-68 longitudinal line scales; preopercular spine present; (8)9-10 gillrakers on lower part of first branchial arch (Ref. 79840). Dark brown to black in color; horizontal lines on body; young with spotted fins (Ref. 5493).
Biology:	Adults inhabit lagoons, estuaries and freshwater (Ref. 6028, 48637, 79840). They occur in the lower reaches of freshwater streams, usually on mud bottoms (Ref. 44894). Juveniles are found mainly among mangrove roots in the more saline areas of lagoons and estuaries (Ref. 6028, 79840). Carnivorous (Ref. 79840). Marketed fresh (Ref. 12693)
Threatened:	Not Evaluated, see IUCN Red List , (Ref. 36508)
Dangerous:	harmless
Coordinator:

Sri Lanka country information
Common names:	Brown gudgeon, Puwakbadilla

Devario pathirana - (Barred danio)

Family:	Cyprinidae (Minnows or carps)
Order:	Cypriniformes (carps)
Class:	Actinopterygii (ray-finned fishes)
FishBase name:	Barred danio
Max. size:	6.0 cm TL (male/unsexed; Ref. 6028)
Environment:	benthopelagic; non-migratory
Climate:	tropical; 22 - 25°C; 7°N - 6°N
Global Importance:	fisheries: of no interest; aquaculture: commercial; aquarium: commercial
Resilience:	High, minimum population doubling time less than 15 months (Preliminary K or Fecundity.)
Distribution:	Asia: Nilwala River basin, Opatha, Sri Lanka.
Diagnosis:	Conspicuous color pattern consisting of some 7-11 irregular, parallel, dark blue vertical bars on the anterior part of the sides and a short longitudinal stripe on caudal peduncle, continued in the median caudal rays (Ref. 6874). Description: Dorsal fin with 3 simple and 11-12 branched rays; anal fin with 3 simple and 13-15 branched rays; pectoral fin with one simple and 11-13 branched rays; and pelvic fin with one simple and 7 branched rays (Ref. 6874).
Biology:	Occurs in still pools to swift flowing streams with pebble or boulder substrates, seldom over sand. Has not been found in silted rivulets flowing into mainstream. More sedentary than D. malabaricus. Forms groups of 3-5 individuals slowly moving near the surface. Probably feeds on insects. Aquarium fish exporters voluntarily refrained from collecting this species until it was better understood and bred commercially. Collected in moderate numbers
Threatened:	Critically Endangered, see IUCN Red List (B1+2c) , (Ref. 36508)
Dangerous:	harmless
Coordinator:

Sri Lanka country information
Common names:	Barred danio

Devario malabaricus - (Rath kailaya)

Family:	Cyprinidae (Minnows or carps)
Order:	Cypriniformes (carps)
Class:	Actinopterygii (ray-finned fishes)
FishBase name:	Malabar danio
Max. size:	12.0 cm TL (male/unsexed; Ref. 41236)
Environment:	benthopelagic
Climate:	tropical; 18 - 25°C; 22°N - 6°N
Global Importance:	fisheries: of no interest; aquarium: highly commercial
Resilience:	High, minimum population doubling time less than 15 months (tm<1; fec="200)
Distribution:	Asia: west coast of India and Sri Lanka. Has been widely transported around the world through the aquarium fish trade industry.
Diagnosis:	Dorsal spines (total): 3-3; Dorsal soft rays (total): 9-9; Anal spines: 2-2; Anal soft rays: 5-5
Biology:	Found in a variety of habitats from boulder-strewn mountain torrents to small pools in dry zone streams. Most common in flowing water than in reservoirs and tanks. Forms medium sized shoals and prefers flowing water. Feeds on terrestrial insects and detritus. Spawns in shallow water, among marginal weeds and roots usually after heavy rains. Eggs are light orange and sticky; more than 200 eggs are laid and hatch in 1-2 days. Fry are free-swimming on fifth day. Exhibits cannibalism on eggs (Ref. 6028). Rarely reaches 12 cm (Ref. 41236)
Threatened:	Not Evaluated, see IUCN Red List , (Ref. 36508)
Dangerous:	harmless
Coordinator:

Sri Lanka country information
Common names:	Giant danio, Rath kailaya

Ctenopharyngodon idella - (Grass carp)

Family:	Cyprinidae (Minnows or carps)
Order:	Cypriniformes (carps)
Class:	Actinopterygii (ray-finned fishes)
FishBase name:	Grass carp
Max. size:	150 cm TL (male/unsexed; Ref. 30578); max. published weight: 45.0 kg (Ref. 7248); max. reported age: 21 years
Environment:	demersal; potamodromous ; depth range 0 - 30 m
Climate:	subtropical; 0 - 35°C; 65°N - 25°N
Global Importance:	fisheries: minor commercial; aquaculture: commercial
Resilience:	Low, minimum population doubling time 4.5 - 14 years (K=0.13-0.19; tm=2-10; tmax=21; Fec=50,000)
Distribution:	Asia: China to eastern Siberia (Amur River system, Ref. 1441). Widely transported around the world (Ref. 7248). Introductions often brought with it the parasitic tapeworm Bothriocephalus opsarichthydis (synonym of B. acheilognathi) (Ref. 12217). Several countries report adverse ecological impact after introduction.
Diagnosis:	Dorsal spines (total): 3-3; Dorsal soft rays (total): 7-8; Anal spines: 3-3; Anal soft rays: 7-11. No barbels. Snout very short, its length less than or equal to eye diameter. Postorbital length more than half head length (Ref. 4967). 18 soft rays for caudal fin (Ref. 40476).
Biology:	Occurs in lakes, ponds, pools and backwaters of large rivers (Ref. 5723), preferring large, slow-flowing or standing water bodies with vegetation. Tolerant of a wide range of temperatures from 0° to 38°C, and salinities to as much as 10 ppt and oxygen levels down to 0.5 ppm. Feeds on higher aquatic plants and submerged grasses; takes also detritus, insects and other invertebrates. One of the world's most important aquaculture species and also used for weed control in rivers, fish ponds and reservoirs (Ref. 9987). Spawns on riverbeds with very strong current (Ref. 30578). Utilized also fresh and eaten steamed, pan-fried, broiled and baked (Ref. 9987). Considered as a pest in most countries because of the damages made to submerged vegetation (Ref. 43281)
Threatened:	Not Evaluated, see IUCN Red List , (Ref. 36508)
Dangerous:	potential pest
Coordinator:

Sri Lanka country information
Common names:	Grass carp