Fish Grader Product

Overview

Fish graders are mechanical sorting machines sorting live fish by body size, a critical step in post-harvest processing. In land-based aquaculture or wild capture fisheries, fish of mixed sizes enter the system; grading separates them into uniform lots for market sale, filleting optimization, and stocking uniformity. A grader can process 50–500 fish/min depending on species (smaller fish like smelt faster, larger fish like salmon slower) and gap setting.

Two primary grading principles exist: bar-bed grading (fish smaller than the bar spacing fall through; larger fish roll over) and roller-deck grading (gentler, using rotating cylinders). Modern farms use a combination: primary bar sorting followed by secondary roller refinement to reduce injury and achieve tighter size distribution (±5–10 mm).

How it works

Fish arrive at the Intake Hopper via gravity or pump. The Slide Gate Valve throttles flow into the Flume Channel, a water-filled inclined or horizontal channel. A centrifugal Water Jet Pump propels fish smoothly forward at 0.5–1 m/s without turbulent shock.

Fish encounter the Grading Bar Bed: parallel stainless steel rods, 10–100 mm apart, slowly rotating at 2–8 rpm if powered, or stationary. Fish smaller than the gap width fall through the bars into the Primary Discharge Chute; larger fish roll over the bars onto the Roller Sorting Deck.

The roller deck consists of 100–200 mm diameter rubber-coated cylinders (10–50 mm gaps), driven by a Roller Drive Motor at 10–30 rpm. This gentler sorting method reduces fish fin and scale damage vs. stationary bars, and allows a third sorting stage if needed (small, medium, large grades).

Sorted fish exit separate Collection & Discharge Chutes with smooth curves (0.5–1 m radius) preventing impact shock, draining into live-fish bins or direct-to-processing lines.

The Control Panel & Interface allows the operator to adjust Grading Bar Bed spacing (10–100 mm in seconds via screw drive), motor speed (0–100% via VFD), and monitor throughput (fish/min) on the Touchscreen Display. The Flow/Throughput Sensor detects blockages; the Pressure Safety Switch triggers auto-shutdown on overpressure.

Design considerations

Bar vs. roller vs. hybrid. Static bars are simplest and lowest cost, but cause visible fin/scale abrasion on high-friction contact. Rotating bars (slow 2–8 rpm) reduce friction and fish stress. Roller decks are gentlest but more complex and require maintenance. Most modern installations use bar primary + roller secondary (hybrid).

Size accuracy and bimodal distributions. If the wild fish population spans 200–800 g, bar sorting at 500 mm spacing creates two grades: <500 g undersized, >500 g market. But if the population is tight (400–450 g), no meaningful sort occurs. Roller decks shine here: they can achieve ±5 mm tightness within a size band using stepped roller gaps (10, 20, 30 mm) in tandem.

Water flow dynamics. Insufficient flow (flume velocity <0.3 m/s) causes fish jamming and stacking; too fast (>1.5 m/s) causes injury and uneven entry into the bar bed. Optimal is 0.5–1 m/s. The Flume Channel must have low turbulence and smooth corners to prevent fish striking walls.

Throughput vs. damage trade-off. At maximum speed (500 fish/min), fish impact bars hard, causing 3–5% scale loss and fin tears. Slowing to 100–150 fish/min reduces damage to <1%. Most operations accept 1–2% loss as acceptable given the economic benefit of consistent-sized product.

Multi-stage sorting. High-value species (sea bass, sea bream, salmon) often use a three-stage approach: primary bar (coarse separation), secondary roller (finer grade), tertiary roller (premium grade). Rejected fish (deformed, injured) are diverted at each stage, reducing costly processing time downstream.

Maintenance and sanitation

Bar and roller beds accumulate slime, scales, and organic debris, requiring daily cleaning with freshwater spray and weekly caustic soak (1% NaOH). Bearing housings and seals need monthly inspection for water ingress. The Gearbox Reducer and Drive Coupling require annual oil analysis and replacement if bearing wear detected (ISO 4406 cleanliness ≥18/16/13 minimum).

Chutes and observation ports should be inspected weekly for cracks; UV exposure degrades acrylic windows, requiring periodic replacement every 2–3 years in outdoor installations.

Integration with processing

Graded fish exit into stainless steel V-shaped or conical live-fish bins (100–500 L each), kept aerated via diffuser tubes. Bins are sized for 30 min holding at peak throughput, then transferred via air-blast stunning system, evisceration line, or filleting machine depending on end product. Premium whole fish are packed directly into ice; gutted fish proceed to fillet automation.