Examination of assumptions critical to body component indices: application to the giant scallop Placopecten magellanicus

Abstract

Allometric relations of gonadal mass to shell height for the scallop Placopecten magellanicus from Baie des Chaleurs, Gulf of St. Lawrence, varied between maturing and fully mature individuals. We describe a technique for separating the two groups. We calculated gonosomatic indices for fully mature scallops and found that the three assumptions critical to body component indices, that (i) allometric relationships for the body component being studied and the reference component do not change over the size range of animals studied, (ii) slopes in the allometric growth equation for the studied and reference components are similar, and (iii) mass of the reference component does not change over time, were violated. We examined two alternative methods for calculating body component indices. The first applied a correction factor to the gonosomatic index and the second directly scaled the component studied to maximum shell height. We recommend the latter. We identified spawning (drops in gonadal mass) between successive dates using a modified Jonckheere rank test, which is appropriate for time-series data.