Changes in growth and reproduction of green sea urchins, Strongylocentrotus droebachiensis (Müller), during repopulation of the shallow subtidal zone after mass mortality

Following a disease outbreak that caused mass mortality of green sea urchins, Strongylocentrotus droebachiensis, along the Atlantic coast of Nova Scotia in September 1999, changes in growth and reproduction were monitored over 3.75 years as surviving individuals migrated from deep water to repopulate the shallow subtidal zone at a wave-exposed site. Urchins were sampled at 4 depth strata: at 24 m on a boulder field where the population was unaffected by the disease, at 12 and 16 m on a steeply sloping bedrock ramp, and at 8-10 m along the lower margin of a kelp bed (Laminaria digitata) where urchins formed a grazing front by January 2002. Urchins migrating shoreward from the deep-water refuge responded rapidly to increased algal productivity in the shallows through increased growth and reproduction. Measures of annual increments of skeletal elements (rotules) from urchins across the depth gradient indicated that the fastest growing individuals from the source population formed the grazing front. Urchins in the front reached a larger asymptotic size and produced larger gonads than urchins lower on the ramp. The annual cycle in gonad index showed a pronounced spring spawning period across all depths; a secondary fall spawning was evident at the front and 12 m. The presence of mature, fertilizable ova and short response time to spawning induction in both spring and fall supported the occurrence of two spawning periods.