Mortality Collection System Product

Overview

Mortality collection systems automatically gather dead and moribund fish from tank bottoms, a critical welfare and biosecurity measure. Dead fish decompose rapidly in warm, oxygen-poor sediment, releasing pathogens and consuming dissolved oxygen. Manual scouring (divers or dip-nets) recovers ~30% of mortalities; automated systems capture 80–95%, improving farm health metrics and reducing disease pressure.

The principle is simple: perforated pipes on the tank floor draw water and dead fish to a surface collection chamber via airlift or vacuum pump. Water drains through a screen; mortalities are concentrated for immediate disposal (composting, rendering, or incineration).

How it works

Dead fish settle on the tank bottom due to gravity. The Submerged Collection Pipe Array network of perforated tubes runs across the tank floor. The Main Collection Pipe (DN40–DN75) connects to the Airlift or Vacuum Pump.

In airlift mode, compressed air from the Air Compressor Supply (3–7 kW) flows through a Air Filter Unit, emerges at 1.5–3 bar from the Air Pressure Regulator, and is injected at the base of the Airlift Riser (vertical pipe extending above water). The air bubbles rise, entraining surrounding water and any dead fish near intake perforations. The resulting two-phase flow (air + water + solids) rises rapidly through the riser, exiting into the Surface Collection Chamber.

The Surface Collection Chamber is a conical or V-shaped tank with a Screening Deck (perforated plate or mesh). Dead fish and heavy sediment accumulate on the screen; water drains through perforations back to the tank via the Chamber Drain Valve. An Overflow Weir prevents overfill if too much material arrives too fast.

The Control Valve Manifold controls operation: a Intake Selector Valve solenoid valve selects tank intake (normal collection) or closed (stop). A Discharge Diverter Valve routes system discharge to collection chamber or back to tank (priming mode, when starting the pump). The Control & Automation System (timer or PLC) triggers collection cycles 1–4 times daily (e.g., 06:00, 12:00, 18:00 UTC). Each cycle typically runs 10–30 minutes, then stops. The chamber is manually emptied 1–2× daily into a disposal bin.

The System Pressure Gauge on the manifold allows the operator to verify system pressure (should be 1.5–3 bar); abnormal low pressure indicates a blockage in intake lines or screen fouling.

Design considerations

Airlift vs. vacuum trade-offs. Airlift (pneumatic) is simpler and more robust: if the compressor fails, it's immediately obvious (no air bubbles visible in riser). Vacuum (centrifugal or rotary pump) is more compact but requires mechanical maintenance (impeller wear, seal leaks). Most farms use airlift for rearing tanks (large surface area, frequent stocking/sorting) and vacuum for smaller holding tanks.

Intake velocity and fish damage. Perforated pipes reduce intake velocity to <0.5 m/s to avoid sucking up live fish or damaging them. A 10 mm perforation in a DN20 pipe at 0.5 m/s velocity causes <0.1 kg/s flow per perforation. For a tank collecting ~5 kg/hour mortalities, 50–100 perforations are needed, spaced every 100–200 mm along the pipe length.

Sediment and silt handling. Tank sediment (uneaten feed, feces) is heavier than water and accumulates on the intake screen, reducing flow. The Sediment Settling Section (horizontal section before riser) allows sand and gravel to settle, preventing riser blockage. Weekly flushing with high-pressure water cleaner is typical maintenance.

Collection cycle timing. Mortalities are concentrated at dawn and evening (fish stress peaks); running collection at 06:00 and 18:00 UTC captures most deaths. Some farms add a 22:00 UTC cycle during high-stress periods (transport, vaccination). Running collection continuously would interfere with feeding and monitoring, so discrete cycles are preferred.

Chamber overflow and back-pressure. If the collection chamber fills faster than it drains, back-pressure can develop, slowing or stopping the airlift. The Overflow Weir lip should be set 10–20 cm above maximum sediment accumulation level, allowing excess water to spill back to tank while mortalities remain in the screen.

Integration with farm operations

The collection system is typically installed on:

1. Rearing tanks: collecting juveniles and grading-induced mortalities (1–3% daily mort rate expected)
2. Grow-out net-pens: trawling or dredging alternative in open-water farms (collection via submersible pump)
3. Pre-slaughter holding tanks: reducing ammonia spike from decomposition during 12–48 hour live-holding

A farm with four 100 m³ tanks running 1–4% daily mortality collects 4–16 kg fish/day. At one collection cycle, ~8 kg per cycle × 3 cycles/day = 24 kg handled, usually <1 hour of operator time total.

High-risk periods (summer heat, post-vaccination, post-transport) may trigger additional manual checks: diver inspection of tank floor, or portable vacuum unit skimming deeper dead-fish accumulation not reached by fixed intake lines.

Regulatory and biosecurity impact

EU regulation 2016/1139 (disease control) requires regular mortality monitoring and removal to prevent disease amplification. Documented daily mortality counts (logged during collection cycle) satisfy compliance. Removing mortalities within 4–6 hours post-death significantly reduces bacterial (Vibrio, Aeromonas) and parasitic multiplication, improving population health.

Rendering or composting dead fish must comply with animal-by-product regulations (EU 1069/2009): dead fish cannot be fed to other animals, but can be composted (>55°C, 2 weeks) or incinerated if rendering unavailable.