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Salt has been scientifically proven to have a number of benefits for freshwater fish.
The Use
of Salt in Aquaculture
UF/IFAS
Adding salt properly in holding and transportation tanks will help treat
health problems and water quality problems. Salt reduces the effects of
stress,
especially during transport and in conserving energy for
physiological emergency purposes, and in countering the
electrolytes disturbances associated with stress-elevated adrenaline and
cortisol levels.
The presence of salt does not repress stress hormones, it softens the
side effects. By adding salt you are increase the salinity of the water to
somewhere between fresh water and sea water (which on average is 3.5%) which
is called brackish.
After the stress of being caught and handled it is best to let shad recover
in brackish water for 1 - 2 days.
Salt helps shad maintain their proper blood chemistry while under stress. Sodium chloride (NaCl) is approved for aquaculture use as an "osmoregulatory enhancer." Salt can change the osmoregulatory balance (water balance) of aquatic organisms. It can sometimes be used effectively to control external parasitic protozoans by placing them in a condition of severe osmoregulatory shock.
Salt is used as a 0.5% to 1.0% concentration in water as an indefinite (long-term) treatment or as a 3% concentration in water for 1-10 minutes. Care must be exercised to avoid overtreatment which will place the fish in the same condition of osmoregulatory shock.
SALT CONCENTRATION
The effects of salt on fish are determined both by salt concentration and duration of exposure. Seawater contains 3 percent salt by weight; this is equivalent to 30 parts per thousand (ppt) or 30,000 parts per million (ppm). Some parasitic infestations of freshwater fish may be effectively eliminated by dipping fish in a seawater solution for 30 seconds to 10 minutes, depending on the species. Weaker solutions containing 0.5 to 1.0 percent salt may be used as a bath for several hours to eliminate some freshwater parasites. Concentrations of 0.1 to 0.3 percent may be used to enhance mucus production and osmoregulation in freshwater fish during handling and transport. Very weak salt treatments, measured in ppm, may be used to control methemoglobinemia in some freshwater fish species.
Freshwater fish use cells in their gills to take in the salts that they lose to the water, which helps keep their bodies salty. Adding salt to the water when fish are stressed means that they don't lose as much salt from their bodies. The principle is much the same as a saline drip for hospital patients.
Some fish keepers and salt manufacturers recommend adding
salt as a routine permanent treatment. I believe while salt has advantages,
there is no need to dose with salt on a permanent basis in your holding
tanks.
Using pure salt (sodium chloride 99.9% or refined salt which contain only
0.1 - 0.5% other elements vs Sea Salt which is "(98% NaCl ) +
80 other elements (2%) is not as good.
Stress in Fish
Fish stress out when caught, handled or being chased or
transported. Stress is adrenaline released into the bloodstream. Adrenaline
is followed closely by other steroids such as cortisol, which prepare the
fish for its reaction. The result is raised blood glucose levels, red blood
cell counts, heart and ventilation rates. The digestive processes may
cease temporarily.
e.g., escape (Mazeaud and Mazeaud, 1981).
Adrenaline disturbs ion transport at the gill membrane, and both adrenaline
and cortisol cause temporary changes in gill permeability which, in fresh
water, results in dilution of the blood by excessive gain of water, and vice
versa in normal seawater.
(Mazeaud et al., 1977; Folmar and Dickhoff, 1980).
Blood levels of calcium, magnesium, sodium, and other vital
electrolytes are pushed out of normal operating ranges for as much as 24
hours after stress such as being caught in a casting net.
(Wedemeyer, 1972).
Trying to restore physiological and metabolic order diverts precious energy,
which leaves the shad less capable of fighting pathogens. Cortisol elevation
itself suppresses immune system function.
(Barton et al., 1987; Maule et al., 1987).
A general recipe may be found in Spotte,
(1970).
Alternately, concentrations of single ions or salts reflecting their
occurrence in natural seawater may serve (for sodium chloride, this will be
of the order 0.5 to one percent in aqueous solution, to achieve a final
salinity of about half that of normal seawater).
The Bulk density of salt approximate (dry, ASTM D 632
gradation) 1.154 (72 lb/ft3)
Since 1 ft3 = 7.48 gallons, the Bulk density of salt converts to 72 lb /
7.48 gal = 9.6 lb/gal.
And 1 gallon = 16 cups, so salt weighs 9.6 lb/gal X gal/16 cup = 0.6 lb/cup
gallon of fresh water is about 8.36 lb
1.0 percent of a gallon of fresh water is 0.0836 lb.
For 50 gal: 50 X 0.139 cups = 6.95 cups
For 30 gal: 30 X 0.139 cups = 4.17
Osmoregulation is the technical term for the physiological mechanism fish use to control the amount of salt and water in their bodily fluids. As the name suggests, it's based on osmosis which is the movement of dissolved stuff through a semi-permeable membrane from a strong concentration to a weaker one.
Freshwater fish are saltier than the water they live in, and their skin is semi-permeable. Since there's a big difference between the amount of salt on the inside of the fish and the amount of salt in the freshwater they live in, freshwater fish leak bodily salts and take in water.
This presents two problems for a freshwater fish:
Taking in salt and getting rid of water.
Osmoregulation process of the fish does not work well when the fish are stressed or diseased. This "osmoregulatory dysfunction" means that fish find it difficult to get sufficient salts from the water and may have problems getting rid of excess water.
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