Introduction
Sea urchin gonad (uni) is regarded as a premium seafood product. High international demand for these products has, however, led to extensive exploitation and overfishing, and the consequent decline of natural stocks has stimulated research and commercial interest in echinoculture. Tripneustes gratilla is a tropical fast-growing shallow water echinoid that occurs along the east coast of South Africa and is one of the most commercially important sea urchin species in Asia, especially Japan . Our group has been investigating Tripneustes echinoculture since 2008 and we have successfully addressed all key areas of the life cycle t o facilitate commercial-scale production. However, technology for upscaling from the research phase to commercial-scale producti on needs to be developed further. As part of the EU funded ASTRAL project, we are taking echinoculture from the research phase to the commercial phase and aim to develop the first commercial echinoculture system, integrated with seaweed, in Southern Africa. Th is contribution will discuss research that is underway to enable commercial scale cultivati on of Tripneustes under local conditions.
Materials and Methods
E ffect of basket depth and feed type on spine loss and consumption was investigated using a 2×3 factorial experimental design with two-treatment levels, basket depth and food type. T hree feed types tested (in triplicate) included: formulated feed (pellets) containing 20% dried Ulva lacinulata (Cyrus et al., 2 015); fresh U. lacinulata; and fresh kelp (Ecklonia maxima). Both deep (40×15×35cm, L×W×D) and shallow (40×40×10cm, L×W×D) baskets had an internal surface area of 0.32m2 and stocking density in both treatments was ~6kg.m2 . T rials were conducted over 3 days, with b askets cleaned and animals fed ad libitum daily. The effect of basket removal on spine loss was also conducted. E ffects of stocking density on growth and gonad development of Tripneustes fed fresh Ulva ad libitum were assessed over 3 months . A separate gonad enhancement trial was conducted for 2 months, where animals were fed formulated feed containing 20% dried Ulva (1.5% BW.day-1). Initial stocking density of baskets (40×40×15cm, L×W×D) for both trials w as 4, 6, and 8 kg.m2 . Urchin growth, gonad development and gonad quality were assessed before and after treatment. E ffects of varying stocking densities on urchin parameters (consumption, faecal production, and spine loss) w ere also assessed. The first pilot commercial scale urchin-Ulva production and hatchery systems were designed and constructed at Buffeljags Abalone farm in South Africa . Several important functionality aspects were tested in the IMTA, including cycling of nutrients (N, P, NH3) , nutritional content and growth of Ulva and optimal harvesting rates of Ulva . Data from these studies was used to develop a farm-scale model to inform biotechnical feasibility of T. gratilla-Ulva IMTA systems and assist prospective farmers.
Results & Discussion
Literature suggests production of various urchin species is reduced when cultivated in deeper baskets. The present study confirmed these findings, with deeper baskets resulting in significantly lower consumption of various feed types (p <0.026). This is likely the consequence of lower feed accessibility, which in turn causes the observed reduced yield. Baskets of ~ 10cm depth are therefore recommended to enhance production of Tripneustes. Trials provided important management recommendations (e.g., removal of urchins from baskets for extended periods (>5s) or feeding rigid feeds (E. maxima ) increases spine loss (p<0.0001)). While higher stocking densities did significantly reduce mass SGR (p<0.044), mo rtality, cannibalism and gonad size/ quality were not influenced by stocking density. Difference in SGR are attributed to spine loss from negative behavioural interactions. From our data, the optimal stocking density for both g row-out and gonad enhancement of Tripneustes is ~20% coverage (surface area of urchins’ tests by surface area of basket). The farm-scale model suggests Ulva can remove 100% Total Ammonia Nitrogen (TAN) from urchin effluent in the IMTA , but TAN emissions from urchins is insufficient to sustain Ulva production. The implications of this study for Tripneustes echinoculture development will be discussed , and additional key findings from on-going commercial-scale hatchery and grow-out trials at Buffeljags Abalone will be shared.
Reference
Cyrus, M.D., Bolton, J.J., Macey, B.M. (2015) The role of the green seaweed Ulva as a dietary supplement for full life cycle grow-out of Tripneustes gratilla. Aquaculture. 446: 187-197.
This study received funding from the EU Horizon 2020 Research & Innovation Programme ASTRAL Project under Grant Agreement No. 863034