Fish Farm Cage Product

Overview

Open net pens are the dominant production platform for farmed Atlantic salmon and European sea bass. A typical cage holds 50,000–100,000 fish stocked at 15–20 kg/m³ density, consuming 5–10% of body weight in pellets daily. Unlike closed systems (RAS or land tanks), net cages allow unlimited water exchange, leveraging natural tidal and current flush for oxygen replenishment and waste dilution.

A single farm site comprises 8–16 cages arranged in lines or clusters, anchored in sheltered coastal waters 20–100 m depth. The Float Collar buoyancy ring maintains the net perimeter above water; the Netting Envelope envelope (knotless polyamide, 20 mm mesh) contains the stock while allowing water flow; and the Mooring Grid anchors the cage against currents, wind, and waves.

Flotation and Structural Design

The Float Collar is assembled from 12 high-density polyethylene tubes (300 mm OD, 275 kg buoyancy each), connected by stainless clamps. Total buoyancy is 3200 kg submerged, supporting the weight of a 50,000-fish biomass (100 tonne live) plus netting (~50 tonne wet weight). The collar is overdesigned for freeboard stability: typically 1.0–1.5 m of collar sits above the waterline, providing margin for wave action and wind-induced heeling.

Distributed lifting lugs (M24 eyes, 100 tonne SWL each) permit craning the cage for net changes, transport, or maintenance without center-of-mass collapse.

Netting and Mesh Engineering

The Netting Envelope envelope is fabricated from seamless knotless polyamide (3 mm nominal diameter, 20 mm diamond aperture). Knotless construction reduces abrasion on fish fins—a critical health factor because damaged fish become susceptible to sea lice and bacterial infection. The mesh size is a compromise: large enough (20 mm) to permit high water flow and fish respiration, small enough to contain juvenile smolt (~10 cm).

Netting lifespan is 3–4 years in exposed Atlantic waters; biofouling (mussels, kelp, sea squirts) reduces net permeability by 30–40% over time, eventually requiring replacement. Seasonal depth changes (lifting/lowering the net to avoid harmful algal blooms or temperature extremes) accelerate wear. The Net Weight Rope (lead-cored rope around the bottom perimeter) maintains 0.5 m penetration, preventing fish jumping out.

Mooring and Anchoring

The Mooring Grid is a star-pattern layout: a central steel spreader frame (30 m perimeter) with 6–8 chain legs (22 mm grade 70, 80 m reach) extending to concrete dead-men or natural rockbed anchors. Each chain is rated 500 tonne minimum break load, providing 4–5× safety factor under 1.5 m/s sustained current.

Swivel joints (ball-race type) at each anchor leg reduce twist stress on the cage perimeter; without swivels, tidal current reversals induce cyclical torsion, breaking welds and cracking the HDPE collar.

A 1.5 m/s current exerts roughly 45 kN lateral drag on a 30 m diameter cage; the 6-leg mooring spreads this across 7.5 kN per chain, within safe working limits. Peak winter storms (2.0 m/s) require brief submersion of the cage or partial destocking to reduce buoyancy.

Containment and Predator Control

The Bird Net Canopy (5 mm polyethylene mesh) is suspended above the water surface on a stainless frame, preventing cormorants and herons from diving into the cage. In Atlantic waters, birds pose moderate risk; in Mediterranean and Asian coastal waters, predation pressure is extreme, and electrified meshes are now standard.

The Gravity Net (Secondary) (secondary containment) is deployed as a backup during net-change operations, when the primary netting is removed for cleaning or replacement. A quick-disconnect system permits deployment in <30 minutes, preventing escapement of valuable stock.

Feed Distribution and Aeration

The Feeding Skirt is a perimeter feed ring (150 mm HDPE pipe, 30 m circumference) with internal baffles that distribute pellet feed uniformly. An automated feeder (not shown in BOM) delivers 2–5 tonne of pellets daily; underwater cameras monitor feed ring saturation to optimize appetite response.

Integrated aeration stones provide supplemental dissolved oxygen during low-current periods (slack tides). Six sintered corundum diffusers, each fed 1 m³/min compressed air, inject micro-bubbles, raising oxygen concentration 1–2 ppm under the cage.

Sump and Drainage

A subsurface Sump Basket collects organic debris, uneaten pellets, and dead fish. Weekly manual drainage via the Drain Cock prevents anaerobic decomposition and hydrogen sulfide buildup, which stresses fish health. The Air Release Valve prevents vacuum during sump draining.

Maintenance and Operations

Weekly tasks include:

• Visual inspection for net damage (holes, abrasion, tear initiation).
• Pellet feed adjustment based on appetite feedback.
• Mooring chain visual check for corrosion or slack.

Quarterly tasks include:

• Sump drainage and cleaning.
• Collar bolt torque check (saltwater corrosion weakens fasteners).
• Net biofouling assessment (measure light transmission through mesh).

Net changes (replacement) occur every 3–4 years. The procedure: deploy secondary gravity-net, remove primary netting, install clean net, retrieve secondary net. Total downtime is 1–2 days; fish remain contained and fed throughout.

Environmental Impact and Regulation

Modern cage farms are heavily regulated (EU Aquaculture Directive, Norwegian Aquaculture Regulations). Key metrics tracked:

• Fish welfare: oxygen, temperature, parasites, mortality.
• Nutrient loading: nitrogen/phosphorus discharge downstream.
• Escape monitoring: sensors on mooring grid detect cage breach.

Dissolved oxygen monitoring (continuous sensors) is now mandatory, with automated destocking triggers if oxygen falls below 6 mg/L for >4 hours. The Mooring Grid provides structural margin to prevent catastrophic net collapse, which would release tens of thousands of farmed fish into wild ecosystems—a major ecological and economic loss.