Onboard Fish Processing: Weighing, Grading and Labelling at Sea

What onboard fish processing involves

Onboard fish processing is the chain of handling steps between the moment the catch comes aboard and the moment it is stowed in the hold: sorting by species, bleeding and gutting, washing, grading by size, batching into boxes or blocks, labelling, and chilling or freezing. Weighing is involved at almost every step, because weight determines value, quota consumption, pack contents and the quantities reported to authorities and buyers.

How much processing happens at sea depends on the vessel. A day-boat may only sort and ice whole fish in boxes. A trawler on a multi-day trip grades, boxes and labels fresh fish. A freezer trawler or factory vessel fillets, packs and freezes product ready for the cold store. The weighing and data tasks scale accordingly, but the underlying questions are the same: how much of which species, from where, in which pack, and can that be proven later?

Why accurate weighing matters on board

Accurate weighing on board matters because weight is simultaneously the basis of revenue, of fisheries control and of traceability, and errors in any of the three cost money or create compliance risk.

• Revenue. Fish is sold by weight. Every gram packed above the declared weight is product given away; every box below a contracted minimum risks a claim. On a trip of 5,000 boxes of 20 kg, a systematic overweight of 2 % adds up to 2 tonnes of product.
• Fisheries control. Under Article 14 of Regulation (EC) No 1224/2009 as amended by Regulation (EU) 2023/2842, logbook estimates of catches retained on board must fall within a margin of tolerance of 10 % per species compared with landed or inspected quantities, or 20 % for species retained in quantities not exceeding 100 kg. Weighed data makes those estimates reliable.
• Traceability. Article 58(5) requires net weight in kilograms per lot as part of the minimum traceability information passed digitally to the next operator.
• Grading value. Price per kilogram usually rises with size category. Misgrading large fish into a smaller class sells them below value.

Weighing on a moving deck

A scale on a vessel measures force, and vessel motion changes that force, so a scale that works perfectly ashore can show errors of several percent at sea unless motion is dealt with.

When the deck accelerates upwards, everything on the scale appears heavier; when it drops, lighter. A vertical acceleration of 0.5 m/s² already changes the reading by about 5 % (0.5 / 9.81). Roll and pitch add tilt, which changes the component of gravity acting on the load cell. Waiting for the display to settle does not help much, because the vessel never stops moving.

Motion-compensated marine scales detect the disturbance and correct the reading for it. The principles are explained in How Motion Compensation Works and on the weighing at sea hub, and the legal context of scales used for trade on the legal metrology hub. For processing equipment such as belt graders, where each piece is weighed in a fraction of a second, the question of how motion is handled should always be put to the supplier explicitly.

Grading by weight

Weight grading sorts fish into weight classes by weighing every piece on a moving belt and diverting it to the matching outlet, replacing visual size judgements with measured values.

A belt grader consists of an infeed belt that spaces the fish, a weigh belt on a load cell and a sorting belt with outlets. Its practical capacity depends on product length and spacing as much as on belt speed, because each piece must be alone and fully on the weigh belt during the measurement. In the EU, Council Regulation (EC) No 2406/96 defines size categories by weight for many species, for example cod in five categories from 0.3–1 kg up to 7 kg and over. Buyers often specify narrower commercial classes.

Fresh and frozen products behave differently: fresh fish leaves slime and water on the belt, frozen fish bounces and builds ice on it, and glaze on frozen product is excluded from the declared net weight. The full explanation, including a worked throughput calculation and a table of EU size categories, is in Weight Grading Fish: How Graders Sort by Weight Class.

Batching and giveaway

Batching builds packs that reach a target weight with minimal overweight, and it matters because filling a box piece by piece overshoots the target by roughly half the weight of an average fish.

The overshoot grows with the average piece size and with the spread between small and large pieces. For 450 g fish with a 120 g spread it is about 240 g per 5 kg box, close to 5 %. Batching algorithms reduce this by keeping several packs or buffered pieces open and choosing where each piece goes, or which combination of pieces closes a pack. In an illustrative simulation with those hypothetical inputs, choosing the best combination from ten buffered pieces reduced average overweight to around 13 g per box.

Targets must also respect the rules that apply to the product: contractual minimum weights, or the average system and tolerable negative errors for e-marked prepackages. The algorithms, the legal background and the trade-offs are explained in Reducing Giveaway: Batching Algorithms Explained.

Catch registration and traceability

Catch registration records, for every lot, the species, catch area, gear, catch date and net weight, linked to the fishing trip, so that each box can be traced from haul to harbour and beyond.

The revised EU Control Regulation sets the minimum data per lot in Article 58(5) of Regulation 1224/2009, including the lot number, the unique fishing trip identification number, the FAO alpha-3 species code and scientific name, the relevant geographical area, the fishing gear category, catch dates and net quantities, with separate quantities for undersized fish. Article 58(6) requires this information to be kept and passed on in a digital way. Article 56a limits a lot to a single species, product presentation, area and vessel, with a derogation for mixed quantities under 30 kg per vessel per day. These rules, introduced by Regulation (EU) 2023/2842, apply from 10 January 2026 for fresh, chilled and frozen products.

The details, including how trip, haul, lot and box records connect and how paper compares with digital registration, are in Catch Registration and Traceability from Haul to Harbour.

Labelling boxes and crates at sea

Labels turn a box of fish into an identifiable lot: they carry the species, catch area, gear, net weight, dates and a lot number, usually with a barcode, and they must stay readable through ice, water and frozen storage.

For products offered to consumers and mass caterers, Article 35 of Regulation (EU) No 1379/2013 requires the commercial and scientific name, production method, catch area and gear category, whether the product has been defrosted, and the date of minimum durability where appropriate. Regulation (EU) No 1169/2011 adds general food information such as net quantity, storage conditions, the responsible operator and, for frozen unprocessed fishery products, the date of freezing.

On the hardware side, thermal transfer printing on synthetic labels with adhesive rated for cold, wet surfaces is the robust choice. GS1-128 barcodes encode GTIN, net weight, lot and dates in a form any scanner in the supply chain can read. Label templates are commonly written in ZPL. The complete guide is Labelling Fish Boxes and Crates at Sea.

Fresh versus frozen processing

Fresh and frozen processing lines weigh the same fish under different conditions, and the differences affect equipment choice, recipes and labelling.

The temperature requirements for fishing and freezer vessels are laid down in Regulation (EC) No 853/2004, Annex III, Section VIII. Guidance on the science of chilling and freezing fish is available from FAO, for example the technical paper Quality and quality changes in fresh fish.

Hygiene and materials

Equipment in a fish processing area must survive daily wash-down, salt water and cleaning chemicals, which is why stainless steel, sealed enclosures and smooth, drainable surfaces are the norm.

Regulation 853/2004 requires work areas on factory vessels to be easy to clean and disinfect and designed to prevent contamination. For weighing equipment that translates into AISI 316 stainless steel for parts exposed to sea water and product, no crevices where residue collects, and enclosures rated for high-pressure water. Ingress protection is expressed as an IP code under IEC 60529; IP69K, used for high-pressure, high-temperature wash-down, comes from road vehicle standards. The hygiene and materials hub compares steel grades and protection ratings in detail.

From deck to data

Processing data only creates value when it leaves the scale: in logs that can be audited, in statistics per product, in exports for buyers and in transmissions to shore during the trip.

A practical onboard data chain records each weighing with timestamp, preset, net weight and lot, stores it locally so nothing is lost when connectivity drops, and forwards it to catch registration, office software or a shore system. The interfaces, data formats, APIs and satellite or mobile links involved are covered in the data integration hub, including Weighing Data Logging at Sea.

Choosing equipment for onboard processing

The right equipment follows from the task at each step: static scales for boxes and bulk, graders for high piece volumes, registration systems for lot data and printers for labels.

Help with sizing and selection is on the buying guides hub.

How WPL approaches this

WPL Industries designs weighing systems for fishing, offshore and research use and has delivered to fishing and research fleets across Europe, the Americas and beyond. The processing and traceability range combines motion-compensated scales with the R50 catch registration and label system and the WeightControl and R10 Configuration Panel software, so that weighing, labelling and data export can be set up as one chain, with room to bring in data from other processing equipment on the line.

Implementation checklist

1. Map each processing step and decide where weight is captured and in which unit (piece, box, block, pallet).
2. Define species lists with FAO alpha-3 codes, product forms and conversion factors.
3. Set grading classes and packing targets per product and buyer, and store them as presets.
4. Agree a lot numbering scheme that respects the lot composition rules.
5. Design label templates with the required items and a GS1 barcode; verify scanning.
6. Configure logging so every weighing is stored with timestamp, preset, lot and operator.
7. Decide how and when data is exported or sent ashore, and test the fallback when connectivity is lost.
8. Review giveaway, misgrades and logbook differences after each trip and adjust.