Population biology of green sea urchins on rocky barrens

Abstract

Populations of the sea urchm Strongylocentrotus droebachensis in lughly grazed rocky barrens in eastern Newfoundland were sampled quantitatively in summer 1968. Comparisons among populations at an extremely exposed, a moderately exposed and a protected site show Mferences in recruitment and/or juve~ule survival, mortality and growth which may be caused by dfferences in temperature, wave exposure and food availability. Dense populations have an unusual growth pattern. Juveniles grow very slowly and at a certain point growth can virtually stop causing the formation of a stationary mode composed of several year classes of juveniles. The position of this conglomerate mode varies from 8 to 11 mm in test diameter at different locahons or depths. Urchins larger than 15 to 20 mm have a much lugher growth rate. I hypothesize that the slow growth of juvenlle urchins in dense populations is due to a severe food shortage coupled to their low mobhty. Urchins larger than 15 to 20 mm move actively about searching for macroalgal food; it is postulated that their increased growth rate is due to their mobllity and resultant greater access to food. If any of the larger actively foraging urchins were to die they would be replaced by the ever abundant juveniles, thus assuring the persistence of urchin-dominated barrens.