Seasonal variations in the energy density of fishes in the North Sea

The energy density ( E _D, kJ g^-1 wet mass) of saithe Pollachius virens , haddock Melanogrammus aeglefinus , whiting Merlangius merlangus , Norway pout Trisopterus esmarki , herring Clupea harengus , sprat Sprattus sprattus , sandeel Ammodytes marinus and pearlsides Maurolicus Muelleri , from the North Sea, increased with total length, L _T. However, there was not always a significant ( P> 0·05) linear relationship between L _T and E _D. Seasonal differences in E _D were obvious in mature fish, while geographical differences were insignificant. For all species there was a highly significant correlation ( P< 0·0001) between the percent dry mass of the fish ( D S) and E _D. A general relationship was established for gadoids and sandeel E _D=–3·1492+0·3459 D _S and herring E _D=–4·6395+0·4170 D _S. Thus seasonal and size-specific data on E _D needed for bioenergetics and gastric evacuation models can be determined simply from D _S, which is considerably less costly and time consuming than calorimetry or proximate analysis.