METABOLIZABLE ENERGY OF FISH WHEN FED TO CAPTIVE GREAT BLUE HERONS (ARDEA-HERODIAS)

The efficiency with which the gross energy content of herring (Clupea harengus) mackerel (Scomber scombrus), and trout (Oncorhynchus mykiss) is metabolized was determined for 11 captive Great Blue Herons (Ardea herodias). There was a linear relationship between apparent metabolized energy and gross energy intake for the mackerel and trout. This relationship was lower and more variable for herring. Estimates of the apparent metabolizable energy coefficient for mackerel and trout were affected by the level of energy intake. Correcting for endogenous energy losses in the excreta yielded estimates of true metabolizable energy coefficients that were independent of gross energy intake. The true metabolizable energy coefficient of mackerel and trout did not differ and averaged 0.866 (SD = 0.014, n = 3 diets). Correcting for nitrogen retention did not improve the estimate of the metabolizable energy coefficient. The metabolizable energy coefficient of herring was highly variable and showed no consistent pattern in relation to energy intake.