The dynamics of destructive grazing of kelp beds by sea urchins in Nova Scotia

The dynamics of high-density moving aggregations (or fronts) of sea urchins (Strongylocentrotus droebachiensis ) were studied as they destructively grazed kelp beds (Laminaria spp.) in two locations along the Atlantic coast of Nova Scotia: a wave-exposed site off a headland and a more sheltered site in a semi-enclosed embayment. At both sites, the fronts advanced in an inshore direction from deeper (10 m) to shallower (7 m) water. The average density and rate of advance of the urchin front was greater at the exposed site (226 urchins m⁻² ; 1.3 m mo⁻¹ ) than at the sheltered site (75 urchins m⁻² ; 0.3 m mo⁻¹ ), resulting in a greater rate of consumption of kelp biomass (wet weight) per linear meter of front at the exposed site ( 5.6 kg mo⁻² ) than at the sheltered site ( 1.5 kg mo⁻¹). At both sites, the rate of advance of the front was highest in the summer 1992 and declined in the fall as the urchins became more dispersed. A dense front reformed in the winter at the exposed but not at the sheltered site. Sea urchins in the fronts were significantly larger and had a higher gonad index at the peak of their reproductive cycle than those found in the recently grazed barren grounds in the wake of the fronts.