World Aquaculture - September 2023

16 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG testing two sizes of urchins, juveniles and adults (Figure 2), with two urchin stocking densities of relatively low (10 urchins) and high (20 urchins) numbers of grazers. The experiment also included control cages where no urchins were added, simulating traditional oyster monoculture in the area. Preliminary results show positive control of biofouling by the urchins (Figures 3, 4 and 5), although a slight tradeoff between the condition index of oysters and biofouling control provided by urchins may exist. Gonads of urchins improved in condition, measured as the ratio between the weight of the soft tissue of interest and the whole body during summer but decreased in condition during winter (Table 1). The decreased condition in winter suggests food limitation, as also noted in previous investigations (Hill and Lawrence 2003). Conditions of oysters were consistent and considered good in both preliminary experiments (Table 1). A similar project with the common periwinkle (Littorina littorea) was also deployed near VIMS Gloucester Point campus (Figure 6) as an evaluation of a grazer species that tolerates a lower salinity than the urchins used in the first trial. Despite preliminary evidence that both urchins and periwinkles help keep the farming gear cleaner, one of the main limitations observed to date is the mortality of the added grazers, particularly the urchin species. While survival rates of the oysters were higher than 90 percent in all treatments, the winter experiment was terminated after a 100 percent urchin mortality event following a winter storm. Atlantic purple urchins appear to be extremely susceptible to stress. Additionally, it is likely that there was food limitation, at least for the adults, at high density. In a pilot test, periwinkles were preyed upon by crabs when deployed in intertidal racks. The current subtidal deployment is ongoing and the periwinkles seem to be thriving, despite not being in their natural habitat on grass on shore. Sustainable Solutions and Economic Effects While technological innovation abounds recently in aquaculture, with “disruptive” highend innovations, many shellfish growers may find it difficult to afford or incorporate new technologies. This coculture approach has the potential to bring environmental and societal benefits, adhering to the International Union for Conservation of Nature’s standards for Nature-based Solutions (NbS; IUCN 2020). Firstly, it addresses the provenance of nutritious seafood with less expensive management, potentially adding a new production crop in the same leased farm area and with the same equipment. Secondly, it can be scaled out and adaptatively managed based on frequent monitoring and evaluation of the efficacy of the coculture, therefore assuring intended results. It can also increase ecosystem integrity by using another native species to control biofouling, and restore native urchins to areas where they have disappeared. Alternatively, FIGURE 3. Example of quadrats showing biofouling material differences of treatments inside farming bags; a) juveniles - high density, and b) control with no urchins. FIGURE 4. Comparison of biofouling mesh plate samples showing basket with and without urchins (Attribution disclosure: This graph has been designed using resources from ‘iamwildan’ found at FIGURE 5. a), Atlantic purple sea urchin spawning, and b), details of its Bordeauxcolored roe: a possible new seafood item? (Photo credits: urchin spawning, William Walton; detail of roe, Ricardo Cruz). a b a b