The potential of culturing wild-collected sea urchins Lytechinus variegatus with eastern oysters Crassostrea virginica was demonstrated at farms on Florida’s Gulf of Mexico coast in a previous pilot study. Preliminary results showed urchins reduced fouling on oysters and culture bags at one site, resulting in interest by growers as biofouling control is a major expense and effort in off-bottom culture. To move this proof of concept forward, research was needed to reevaluate the co-culture of hatchery-produced urchins and oysters over a crop cycle at commercial densities.
Field trials were conducted at commercial farms over a nine-month period to assess oyster performance and biofouling on oysters and gear cultured with and without urchins. Due to hatchery problems, urchins were collected from natural populations. In the nursery trial, 12 subadult urchins (30 mm test diameter) were stocked with triploid oyster seed (1200/bag, 16 mm shell height) and reared in 4 mm mesh floating bags. Urchins did not survive at farms where salinities were less than 25 psu. At a higher salinity farm site, urchin survival after 2.5 months ranged from 8.3 to 66.6%. In the intermediate growout trial at the latter farm site, oysters were restocked at 400 per 9 mm mesh bag. Urchin density (0, 10, 15, 20/bag) and placement of floats varied. After 3.5 months, urchin survival was highest in bags with floats placed on top (87-98%) versus sides (53%). Growth and survival of oysters in all treatments were similar, suggesting urchins did not influence oyster performance. There was a 64% reduction of barnacles on oysters in the treatment with 15 urchins. The biggest difference observed in the biofouling assessment was the matrix of tube-building amphipods and accumulated sediment and detritus on oysters without urchins compared to those with urchins (Figure 1).
For the final growout trial, oysters were reduced to 150 per 14 mm mesh bag and stocked with subadult and adult (49 mm test) urchins. After 3.5 months, growth rates were higher for subadult urchins versus adults and survival (96%) was higher for urchins in bags with floats placed on top. As in the intermediate trial, oyster growth and survival did not differ. Results of biofouling on oysters were also similar; however, fouling weights on bags were lowest in the treatment with 20 adult urchins. Overall, results did not clearly demonstrate the potential of using urchins for biofouling control of oysters and limit the potential for oyster and urchin co-culture in Florida