Nursery-phase culture of green sea urchin Strongylocentrotus droebachiensis

Sea urchin hatchery techniques are well established, but cost effective grow-out strategies are still under development. Juvenile green sea urchins (Strongylocentrotus droebachiensis) with a test diameter of less than 15 mm are vulnerable to predation when released into the wild and mortality can be high; therefore a protected nursery phase is required. This study investigated the feasibility of a cost effective on-bottom nursery cage system to provide protection at this stage. Wild-caught juvenile urchins (average diameter 7.93 ± 0.65 mm) were held in specially designed high-density polyethylene (HDPE) mesh tubes, 50 per tube, at two lease sites in Penobscot Bay, ME. Replicate tubes were placed on 3 bottom types (mussel, cobble, and ledge) at the Sloop Island site and on cobble bottom type at the Job Island site. Groups of urchins were counted and measured 5 wk, 3 mo, and 6 mo after placement to gauge handling mortality, growth, and survival. Handling mortality after 5 wk was 5% with no significant difference between treatments. Final survival indicated that cobble bottom type supported the highest survival at Job Island (89%) and Sloop Island (71%), followed by Sloop mussel (59%) and Sloop ledge (56%). After 6 mo the average diameter reached 11.08 ± 1.49 mm. Final average test diameter was significantly larger at Sloop ledge (12.17 mm) and Sloop mussel (12.58 mm), than at Sloop cobble (9.83 mm) and Job cobble (9.66 mm). These results suggest on-bottom culture through the critical nursery phase is technically feasible and may represent an economical way to rear hatchery produced green sea urchin seed to the "planting out" size.