Body Composition in Fishes: Body Size Matters

Fish proximate body composition affects many aspects of fish biology and ecology, including appetite, growth, reproduction, survival, and energy value to predators.  Two very strong relationships among body components are revealed by taking into account fish body size in terms of water mass.  There is a very strong relationship between water mass and protein mass, with the amount of water per unit protein decreasing in larger fish.  The strength of this relationship and its presence in a variety of fish species suggest a physiological or biochemical cause.  Similarly, there is a very strong relationship between water mass and ash mass, with the amount of water per unit ash decreasing in larger fish.  These two strong relationships enable fish body composition to be predicted from wet weight and percent water.  Calculated water mass is used to predict mass of protein and ash, then lipid mass is found by subtraction of water, protein, and ash from body mass.  The effectiveness of this approach emphasizes that there is virtually no functional relationship between body lipid and body water.  Fish energy density can be calculated from proximate composition.  These relationships should be useful in studying fish bioenergetics and other aspects of fish growth.