Effect of food availability and body size on out-of-season gonad yield in the green sea urchin, Strongylocentrotus droebachiensis

The out-of-season gonad yield of the green sea urchin, Strongylocentrotus droebachiensis, was low in barren ground habitats, intermediate in kelp forest habitats, and high in laboratory tanks supplied with fresh kelp? Laminaria hyperborea and L. digitata. In August, the laboratory population had already attained gonad sizes comparable with the prespawning maximum of sea urchins from kelp habitats. Analysis of covariance (ANCOVA) suggested that both absolute gonad mass and relative gonad mass (gonad index) were increasing functions of urchin size. However, both cubic polynomial regression and graphical analyses detected a curvilinear relationship with a tendency toward decreasing gonad mass in the largest animals in the laboratory population and in the barren ground population. The tendency toward decreased gonad yield in the largest animals is consistent with a hypothesis of size-dependent reproductive senility. The practical implication for future echiniculture operations is that there seems to be a physiologically determined optimum size where gonad yield is maximized. The present data suggest that this optimum size is located in the 55 to 60 mm size interval. In this size interval, the laboratory population had approximately twofold larger gonad mass than the kelp population and approximately threefold larger gonad mass than the barren ground population.