Underwater work at open-water fish farms

Published 22 September 2026 · 9 min read · 1,743 words

Aquaculture has for years been the professional diving sector people talk about least and where the most dive hours are logged. And when the farm is in open water too — exposed to swell, moored in deep water and far from the coast —, the work stops resembling that of a sheltered bay and becomes something with a logic of its own, closer to offshore operations than its reputation suggests.

The shift of cages towards exposed waters isn't accidental. Sheltered bays are saturated, environmental pressure over the impact on shallow seabeds has grown, and the sector has found better water exchange, fewer health problems and room to grow out in open water. The result is a generation of installations with bigger structures, far more serious moorings and a critical reliance on underwater maintenance.

What's actually under a cage

An open-water fish farm isn't a net hanging from a ring. It's a distributed mooring system, made up of anchors or dead weights on the seabed, lines running up at different angles, chains, shackles, swivels, intermediate buoyancy buoys, and a surface frame — a grid of tensioned ropes — from which the cages themselves hang. Each cage has its own culture net, ballast weight at the bottom edge to hold its shape against the current, and often an outer predator net.

The whole assembly works under constant tension in an environment that degrades it: abrasion between chain and seabed, corrosion, fatigue at connection points, biofouling that increases drag and therefore load. The system is designed with redundancy, but redundancy only works if someone regularly checks which elements are still doing their job. That someone is the diver.

Mooring inspection is, for that reason, the bread-and-butter task of the trade. The lines are worked along, each shackle and connection point is checked, chain wear is measured at the contact sections, intermediate buoys are checked for buoyancy, and anchors are checked for dragging. It's the same discipline as mooring line inspection applied at floating energy installations, with a smaller budget and higher frequency.

Nets: the most frequent task, and the most dangerous

The net accounts for a large share of the dive hours, and almost all of the risk. It's inspected for tears, wear points from rubbing against weights or structure, deformed mesh and areas where biofouling has added weight and drag. An undetected tear means escaped biomass, which for the farm is a serious financial loss and an environmental and regulatory problem.

The problem is that working inside or next to a net means working surrounded by a trap. A torn panel floats, folds and moves with the current. A poorly tensioned predator net creates pockets. A loose rope wraps around the umbilical. And unlike other environments, the danger here isn't in a rigid structure you can see and avoid, but in a flexible material that closes around whatever it touches. The correct technique — always approaching from outside, umbilical controlled by the tender, a knife accessible with either hand, never passing through a panel — isn't a recommendation, it's the difference between coming back and not.

For that reason, diving in aquaculture tends to be done with surface-supplied diving and permanent voice communication, even at depths a recreational diver would handle on open-circuit. It's not bureaucracy: it's that a diver tangled in a net with no communication has no way to call for help.

Mortality and suction systems

Removing dead fish from the bottom of the cage is a routine and health-critical task. In modern installations it's done with suction systems — a hose connected to a surface pump — that the diver operates from inside the lower cone of the net. In simpler installations, with a scoop net or by hand.

Suction systems deserve specific attention because they introduce a risk that isn't always managed well. An active suction mouth can catch gloves, suit or umbilical; the suction effect isn't obvious until you're right on top of it; and the stop control is rarely somewhere the diver can reach. The operating rule is simple and has to be enforced: positive control from the surface, verbal order to start and stop, and no one touches the mouth while the pump is running without explicit agreement. It's the same logic as isolation and lockout applied to any energised equipment.

What changes in open water

Compared with a bay farm, an ocean installation brings three major operational differences.

Current and swell

The current is constant and sometimes strong, which means planning the dive around the tide state and accepting the work will be done holding onto the structure. Swell, more than making the dive itself harder, complicates entry, exit and handling the umbilical from a boat that's moving. Many days are lost not because diving isn't possible, but because it isn't safe to work from the deck.

Depth and timing

Ocean cages sit deeper and the moorings go to depths not seen in a bay. That puts the operation squarely into managing tables and bottom times, with repetitive dives through the day. The decompression sickness isn't a theoretical risk in aquaculture: it's one of the sector's real medical problems, precisely because the working culture tends to see these installations as shallow and to relax the controls.

Isolation

An installation miles from the coast with a small team is far from any assistance. That means carrying real response capability on board — oxygen, a written evacuation plan, communication that actually works, staff trained in diving first aid— not the nominal version signed off for the paperwork. The distance to the nearest hyperbaric chamber needs to be a fact known to the whole team before the first dive, not something looked up when there's already a problem.

A sector with steady demand

For someone building a career, ocean aquaculture has a virtue that's hard to find: continuity. The cages always need checking, the moorings always degrade, mortality always turns up. It isn't campaign work that comes and goes, but perpetual maintenance with predictable cycles. That lets you build up real hours, learn to work with rope and chain until it's second nature, and build the reputation for reliability that later serves for everything else.

Rates are below offshore and often below port work, and the setting — remote coastal areas, small boats, long seasons — isn't for everyone. But it's one of the few doors that's still open to someone who's just finished training. Getting in requires the relevant qualification, a current medical certificate and, above all, showing up in person at local service companies and at the farms themselves, which tend to hire through direct contact rather than adverts. The company directory and jobs in the sector channels help identify who's operating in each area; the rest is decided in conversation.

Anyone using aquaculture as a stepping stone needs to look after two things from day one: keeping the dive login good order, because those hours count, and never accepting a poorly set-up operation. In a sector with tight margins, the pressure to cut corners exists, and a diver who gets used to working without a supervisor, without communication or without a plan loses exactly what they need to make the jump: judgement.

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Published 11 September 2026 · By Arthur More, professional saturation diver

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