Utilizing an oyster upweller as a nursery system presented 2 problems.
First, by design, the flow came from the bottom of the silo, past the shellfish, into a center pipe, and then out of the upweller. This meant that because oysters don't move, this design is ideal. However, since bay scallops swim to find a better place to attach, they were prone to just swim directly out of the upweller. Mesh could be placed over the outflow pipe, but this just served to significantly lower flow to each silo.
Second, as oysters are perfectly fine growing in a large pile as long as there is sufficient flow, the single layer of mesh on the bottom and all oysters piled on top works excellent. However, bay scallops need to be on a single layer, with room to grow into, in addition to sufficient flow to support optimal growth.
Therefore in 2014, the scallops grew very well for the first month in the FLUPSY, so that by day 30 they needed to be sorted into 2 size classes.
Following the sort, the larger size class was stocked into 2 silos, which resulted in a low density with room for the scallops to grow. The smaller size class was also stocked into 2 silos, though this resulted in a higher stocking density with the scallops piled several deep.
The larger size class had surface area to spread out on, and therefore continued to grow at a fast rate (0.29 mm/day). The smaller size class did not have room to grow, and therefore their growth rate declined from 0.29 mm/day to 0.04 mm/day (Ward Aquafarms, unpub. data).
We then constructed 6 "silos" (3'W x 3'L x 4'H); 3 on each side of a central trough (1'W x 9'L x 5'H). Each silo has a 4" opening at the top to allow water to enter. Attached to the pipe is a 36" downpipe to pull water from below the waterline in order to avoid contaminants on the water surface. Each silo drains into the central trough through a 3"H x 36"L gap at the bottom of the silo. With a 3/4 hp pump attached in the central trough, this design can yield approximately 150 gal/min per silo. Each silo can accommodate up to 6, 3'W x 3'L x 4"H trays stocked with scallops. Since the flow is reversed and goes in at the top and down through the shellfish and out of the system, the scallops cannot swim out of the system at any time. Additionally, since each silo now has 6 trays instead of one mesh bottom, surface area in the system increased from 32ft² in a standard FLUPSY to 324ft² in the redesigned floating downweller.
The new design gave sufficient surface area for a large number of scallops to grow from the size from the hatchery (0.75 mm from Muscongus Bay Aquaculture (Brenmen, ME) in 2015), through 20 mm, when they are ready to be put into grow out gear. The downweller resulted in growth rates of 0.34 mm/day, 0.39 mm/day, and 0.29 mm/day (each two week period, respectively) for the six weeks preceding being moved out into growout gear. We know what results and survival we can get with a FLUPSY, and after 2015 we now know that we can get good growth through 20 mm in the downweller. However, there are many parts of the nursery system which remain to be optimized. We don't know optimal number of trays, stocking density, grading size, flow rate, mesh size, size at move to grow out site, etc. Preliminary data from the downweller demonstrates that it may be a viable nursery system, but it needs to be compared to other seed production methods in terms of efficiency and ability to produce seed at an economically viable cost.