The University of Maine – Center for Cooperative Aquaculture Research has been providing sea urchin juveniles Strongylocentrotus droebachiensis to growers along the Gulf of Maine. The farmers are developing systems combining their main crops (oysters, scallops, and seaweed) with them which provides an extra income using the infrastructure already established and/or experimenting with the sea urchins to mitigate and control biofouling, especially in the oysters and scallops’ gear. Another way the farmers are growing green sea urchin is in modified lobster cages, known as bottom cages.
We designed a project to evaluate the survival and growth of green sea urchin Strongylocentrotus droebachiensis juveniles in two leased sites in Maine: LPA/lease in the Jordan river in Frenchman Bay and in an oyster farm in the New Meadows River in West Bath.
We evaluated the effectiveness of two settlement substrates (bio-barrels and wavy polycarbonate plates), the survival and growth of the juvenile in bottom cages at three different stocking densities, and the survival and growth of the juvenile sea urchins using lab seeded sugar kelp and wild-set fouling organisms (macro-alga and invertebrates) in an oyster farm.
Ocean resources implemented the use of bottom cages with three stocking densities (100, 150, and 200 juveniles per cage), and Winnegance Oyster Farm was seeking to add urchins and their feed into an existing oyster farm and test two readily available food sources for suitability as urchin feed: lab seeded sugar kelp and Wild-set fouling organisms (macro-algae and invertebrates).
The survivorship per density at 150 and 200 sea urchin per cage in the Jordan River site was 100%, and 87% for the 100 sea urchin per cage. We attributed this finding to eventual escapes due to the size of the sea urchins at out planting or when the cage was open to add food. The correlations observed in all the cages sampled in the Jordan River site were negative independent of the density. This finding suggested that there was no difference in growth rate amount the different densities, leading to the conclusion that 34-sea urchin per cubic foot could yield a better growth and a more efficient use of the space in the cage. This project in the Jordan River site confirmed that the initial minimum test diameter at out planting is 15 mm.