Location: Warmwater Aquaculture Research Unit
Title: Effects of fish barrier screening material on water flow in split-pond aquaculture systems Authors
Submitted to: NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter
Publication Type: Abstract Only
Publication Acceptance Date: November 4, 2013
Publication Date: N/A
Technical Abstract: Ponds serve two functions as fish-culture units. They hold water and fish, much like the walls of an aquarium, and they produce oxygen and treat wastes produced during culture. Split-ponds separate those two functions to make management easier. A large lagoon that provides the ecological services is connected to a much smaller basin that confines the fish. The operative word in that last phrase is “confines” because if fish escape from the small confinement area, they will be very difficult to capture and the advantages of the split pond disappear. Fish escape is prevented by using screens or barriers across the two conduits connecting the fish-holding basin and the waste treatment lagoon. Fish barriers reduce channel cross-sectional area and restrict water flow to some degree depending on the type of screen and opening size. This is important because water flow transports oxygenated water into the fish holding basin and transports wastes out. Restricting water flow might therefore affect performance of split ponds. So, we evaluated changes in water flow caused by two different types of fish barriers in the open channels. We conducted this study in the prototype 0.7-acre split-pond system at the Thad Cochran National Warmwater Aquaculture Center. One set of barriers (one for each channel) was constructed from polymer-coated steel-mesh wire (1.0-inch x 1.0-inch openings) mounted to a square aluminum tubing frame (1.25-square inch, wall thickness of about 1/8 inch). The second set of barriers tested used the same aluminum frame as the first barrier type, but the mesh material was expanded metal with ¼- inch x 1-inch openings. Rotational speeds (1.0, 2.0, 3.0, and 4.0 rpm) of a slow-rotating paddlewheel pump were tested and water flow rate was measured with and without fish barriers in the open channels to determine changes in water flow caused by friction as water flowed through the screens. Fish barriers reduced water flow compared to that in channels without barriers, and flow reductions increased as paddlewheel rotational speed increased (Figure 1). Fish barriers constructed out of polymer-coated steel-mesh wire reduced water flow rate less than barriers constructed out of expanded metal. For example, at 4.0 rpm, flow rate was reduced from 19,330 gallons/minute (gpm) to 17,320 gpm for channels with polymer-coated steel-mesh wire and to 14,847 gpm for channels with expanded metal barriers. This trend was expected based on differences in mesh open area in the steel-mesh wire and expanded metal. The polymer-coated steel-mesh wire has about 80% open area whereas the expanded metal has about 58% open area. The frame of the fish barriers further restricted the open surface area of the channel by approximately 11%. The total cross-sectional area of the open channel was 39.6 square feet and the combination of frame and barrier material reduced the open surface area to 28.0 square feet (71% of the open channel area) for the polymer-coated steel-mesh wire and 20.4 square feet (52% of the open channel area) for expanded metal. Reduced water flow has been observed in similar production systems such as in-pond raceways after installing smaller mesh (1/2-inch x 1-in) fish barriers before stocking fish. In conclusion, fish barriers installed in conduits connecting split-pond sections should have the maximum mesh size possible to retain fish so that reduction in water flow caused by frictional losses across the barrier are minimized. Biofouling or fouling with grass or other debris will further reduce open channel area. Flow reductions and labor needed for cleaning the fish barriers can be reduced by using screens with large open areas to remove larger debris before water flows through the fish barriers. Various types of bar screens have been developed for use at the headworks of wastewater treatment plants to pre-filter the waste stream before entering the treatment plant. Similar devices could be developed for split-ponds to collect floating debris which would allow for easier maintenance of fish barriers.