Body composition in fishes: body size matters

• Accounting for fish body size as water mass reveals two strong relationships.
• Water mass is very strongly related to protein mass and to ash mass.
• Ratios of water:protein and water:ash decrease in larger fish.
• Fish body composition can be predicted from wet weight and percent water.
• Water mass predicts protein & ash; subtraction from body mass yields lipid mass.

Fish proximate body composition is of great interest in aquaculture because it affects fish appetite, growth and the efficiency of food utilization. Proximate body composition also affects other aspects of fish biology and ecology, including 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. Results from this approach suggest 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.