If you measure directly on the lateral line of the fish, the measurements will have more variance when compared with results from measuring above the lateral line. The reason is that if you measure directly under the skin on the lateral line, there is a strip of flesh which tends to contain more fat. The amount of fat present in this area varies significantly due to seasonal changes. If you measure above the lateral line, then you are measuring directly into the fish flesh & muscle area. This is much more repeatable (stable) and representative of the fat distribution in the flesh & muscle. We recommend this approach since it'll give more consistent results with low variance (changes less) over time as seasons change.