Inconsistencies in measurement of fish condition: a comparison of four indices of fat reserves for Atlantic herring (Clupea harengus)

Condition is often equated to the magnitude of fat reserves, which can be represented by biochemical, bioenergetic, or morphometric indices. Biochemical indices are technically more demanding, but are direct measures of fat, in contrast to morphometric indices which are easier to measure but are only surrogates of physiological status or condition. This study compared Fulton's K (K), a common morphometric index, with two bioenergetic indices, mesenteric fat and muscle fat content, measured by a commercial fatmeter in Atlantic herring. Fatmeter values and K were also compared with a biochemical measurement of muscle fat. The relationship between K and mesenteric fat was inconsistent and often non-existent. The relationship between K and fatmeter values was stronger for fish with inactive gonads, and the biochemical measure displayed a much stronger correlation with fatmeter values than with K. Studies need explicitly to define condition and ideally validate it against a benchmark, such as a biochemical index.