World Aquaculture December 2019

WWW.WA S.ORG • WORLD AQUACULTURE • DECEMBER 2019 41 Recruitment success was measured by quantifying spat and oyster density after five months. Recruits were categorized as spat (< 1 cm) or oyster (> 1 cm). Oyster recruitment on oyster shells in baskets was more than 2.5 times greater than that on dowels and more than 37 times greater than that on bottles (Table 2). Smaller oysters appeared to be from a second recruitment. Spat recruitment per unit area on bottles was greater than that on recycled oyster shells in baskets, but not significantly so. Spat recruitment on dowels was least. Oysters on shell in baskets reached an average of 28 ± 3 mm (near market size for this area) after five months and were visually appealing for market. Oysters on dowels were smaller (23 ± 5 cm) and oysters collected on bottles were the smallest (14 ± 0.6 mm). Designs were evaluated based on material cost, construction and maintenance labor and oyster marketability (Table 2). Baskets were the most expensive to produce, although they were the least labor-intensive. We were able to successfully involve an oyster vendor and fisher in their construction. Grow-out structures cost US$320 per unit. Although initial installation was time-consuming, maintenance was minimal and recruitment structures were reusable. Dowels were time consuming to produce, oysters were not visually appealing and there was significant loss when they were removed from the dowel. The bottle structure fouled quickly and was not visually appealing in the mangroves, which are often visited by tourists. Mangrove Oyster Grow-out A six-month permit to allow placement of standing structures in navigable waters was obtained from the US Army Corps of Engineers after ( C O N T I N U E D O N P A G E 4 2 ) TABLE 1. Water quality from Rincon Lagoon, where spat were collected, and the Boquerón Bay farm site, where oysters were allowed to grow-out from June 2011 and June 2013. Rincon Lagoon Farm Site Temperature (oC) 27.4 ± 0.8 29.9 ± 6.1 Dissolved oxygen (mg/L) 4.1 ± 0.4 4.7 ± 0.4 Conductivity (µS) 43 ± 5 51 ± 2 Specific conductivity (µS) 41 ± 5 49 ± 1 Salinity (ppt) 27.1 ± 2.6 31.9 ± 0.7 Turbidity (NTU) 4.5 ± 1.0 9.9 ± 9.9 pH 7.99 ± 0.17 8.20 ± 0.11 Total suspended solids (mg/L) 48 ± 9 38 ± 12 Chlorophyll a (µg/L) 709 ± 51 594 ± 46 Nitrate (mg/L) 0.22 ± 0.16 0.24 ± 0.17 Nitrite (mg/L) 0.014 ± 0.010 0.010 ± 0.005 Phosphate (mg/L) 0.11 ± 0.09 0.10 ± 0.07 TABLE 2. Recruitment structure feasibility study compared cost, labor and recruitment success after five months. Cost Spat Oysters < 1 cm > 1 cm (number / m 2 ) Basket $5 / basket 416 ± 275 973 ± 164 Dowel $1 / dowel 146 ± 64 370 ± 219 Bottle $1 / structure 640 ± 82 26 ± 12 FIGURE 7. Oysters larva settle on natural and artificial substrate. Here bottles are used. Although an inexpensive and good recruitment method, few oysters survived to market size.