Physiology of Finfish and Shellfish (2+1)

2.4.1. Introduction

The living organism can be described as an aqueous solution contained within a membrane, the body surface. Both the volume of the organism and the concentration of solutes should be maintained within rather narrow limits. Optimal function of an animal required a well-defined, relatively constant composition of its body fluids, and substantial deviations are usually incompatible with life.

Water obtained from sources contains dissolved substances – salts, gases, minor amounts of organic compounds, various pollutants – and the temperature of the water is of the greatest physiological importance.

Sea water contains about 3.5% salts (i.e., 1 liter sea water contains 35g salts). The major ions are sodium and chloride, with magnesium, sulfate, and calcium present in substantial amounts.

Composition of sea water. In addition to the ions listed, sea water contains small amounts of virtually all elements found on earth. (Potts and Parry 1964)

Ion	Amount per 1 liter sea water		Amount per 1 kg watera
	mmol	g	Mmol	g
Sodium	470.2	10.813	475.4	10.933
Magnesium	53.57	1.303	54.17	1.317
Calcium	10.23	0.410	10.34	0.414
Potassium	9.96	0.386	10.07	0.394
Chlorine	548.3	19.440	554.4	19.658
Sulfate	28.25	2.713	28.56	2.744
Bicarbonate	2.34	0.143	2.37	0.145
“In thermodynamics concentrations are calculated per kilogram water and are referred to as the molality of the solution. A brief synopsis of the physics of solutions is found in Appendix E.

Fresh water, in contrast to sea water, has a highly variable content of solutes. Minute amounts of salts are present already in rainwater, but its composition is greatly modified as is runs over and through the surface of the earth.

The total salt content in fresh water may vary from less than 0.1 mmol per liter to more than 10 mmol per liter, and the relative amounts of different ions can vary over a tremendous range.

Typical composition of soft water, hard water, and inland saline water, given in millimoles per kilogram water and listed in the same order as in Table 1. [Recalculated from Livingstone 1963 (a, b, c, and d) and from steinhorn et. al. 1979 (e)]

Ion	Soft lake water a	River water b	Hard river water c	Saline water d	Dead Sea e
Sodium	0.17	0.39	6.13	640	1955
Magnesium	0.15	0.21	0.66	6	2028
Calcium	0.22	0.52	5.01	32	481
Potassium	--	0.04	0.11	16	219
Chloride	0.03	0.23	13.44	630	7112
Sulfate	0.09	0.21	1.40	54	5.3
Bicarbonate	0.43	1.11	1.39	3	3.7
“Lake Nipissing, Ontario”. Mean composition of North American rivers. “Tuscarawas River, Ohio. “Bad Water, Death Valley, California. “Dean Sea, Israel. This water also contains 74 mmol per Kg H2O of bromide.

Natural rain is slightly acid. This is because carbon dioxide from the atmospheric air dissolves in the rainwater, which then can be expected to attain a pH of about 5.6. However, much of the rain that falls in the northeastern United States has a pH as low as 4. It has been suggested that this is because sulfur dioxide and nitrous oxides from the combustion of fossil fuels form strong acids that lower the pH of the precipitation.

The degree of acidity that can be tolerated by a given fish is not related to pH alone; the amount of dissolved salts is extremely important. The effect of acid is probably an inhibition of the active uptake of sodium, for sodium uptake has in experiments been shown to be inhibited when the water acidity is high (Maetz et al. 1976). This is in accord with the observation that the fish disappear first from lakes with extremely small amounts of dissolved salts and that fish kills have been particularly severe during the snow melt in spring when the runoff of melting snow carries a sudden surge of acid in virtually unbuffered water.

Brackish water occurs in coastal regions where sea water is mixed with fresh water. At the mouth of a large river fresh water dilutes the ocean water for a considerable distance, and if the tides are large the estuary at the mouth may extend far up the river. In this area the salinity varies rapidly undiluted sea water.

As a commonly accepted definition we can say that brackish water refers to salinities between 20-25 pt. Brackish water is physiologically extremely important. It forms a barrier to the distribution of many marine animals on one hand as well as fresh-water animals on the other, and it also forms an interesting transition between marine and fresh-water habitats. In geographical extent, however, brackish water covers less than 1% of the earth’s surface.