A Low Cost Bar Grader for the Harvest of Hybrid Striped Bass (Morone chrysops x M. saxatilis)

Four bar graders were designed and built to separate a mixed-size population of market-size reciprocal cross hybrid striped bass Marone chrysops x M. saxatilis in rectangular culture tanks in an indoor, recirculating aquaculture system. Grader frames were constructed of 5.1 cm (2 inch) PVC pipe and fittings. PVC electrical conduit ( 1.27 cm, 0.5 inch) was used to form a series of parallel, equally spaced vertical bars within the frame. Bar slot spaces were 1.90, 2.54, 3.20, or 3.80 cm (0.75, 1.0, 1.25, 1.5 inches). A strip of flexible vinyl siding was attached to the outer edges of the bar grader to permit a tight fit between the grader and tank walls, and prevent fish from swimming around the grader. The graders were placed sequentially into one end of the tank, largest slot size first, and maneuvered to the opposite end. Each grader remained in the tank for 60 min. Fish too large to pass through a grader were netted and measured for weight, length and width. Regression analysis was performed for average weight retained at each bar spacing (Y = 7.13619 + 0.070716 X; r2 = .9987). By rearrangement, an equation was derived which allows a culturist to select a bar spacing that retains fish of a predetermined weight: Grader bar slot size (mm)= Fish weight (g) -177.9 / 31.7). Construction of each grader required approximately 2 h, and materials cost $20.