Applications

Weighing Solutions for Trawlers

Updated September 14, 20264 min readBy WPL Industries Engineering

Short answer

Trawlers weigh sorted catch per species for logbook estimates, box and pack weights for sale, and samples of pumped catch for species and size composition. A practical set-up combines a motion-compensated bench scale at the sorting line, a small sample scale and a deck platform scale, with weights logged per species and haul.

What trawlers weigh and why

Trawlers weigh sorted catch by species to support logbook estimates, box and pack weights for sale and labelling, and samples of mixed or pumped catches to determine species and size composition. On a moving deck, those weights are only useful if the scale compensates for motion and survives daily washdown.

The weighing pattern differs between the main trawl fisheries:

Demersal trawlers sort catch on deck or on a sorting belt, gut and wash fish, and pack them in boxes with ice. Weighing takes place per box or per species batch.
Pelagic trawlers, using midwater gear of the kind described in the FAO fishing gear fact sheets, typically pump catch into refrigerated or chilled seawater tanks. The FAO/Torry advisory note on stowage of fish in chilled sea water distinguishes CSW, cooled with ice, from RSW, cooled by mechanical refrigeration. Here, weighing concentrates on samples taken for species composition and size grading.
Freezer trawlers add processing, block freezing and carton packing; see Weighing and Grading on Factory Vessels.

Typical weighing tasks on board

Most trawler weighing tasks fall into five groups, each with its own load range and accuracy needs.

Task	Where on board	Typical load	Purpose
Weighing sorted catch per species	Sorting belt, boxing station	Boxes and tubs, tens of kilograms	Logbook estimates, stock of catch on board
Recording catch below minimum conservation reference size	Sorting area	Baskets, a few to tens of kilograms	Separate logbook entry under the landing obligation
Sampling pumped or unsorted catch	Pump outlet, sampling point	Sample buckets, a few kilograms	Species composition, size grades
Box or pack weights for sale	Fish hold loading, packing station	Fixed-weight boxes	Consistent box weights, labels
Bulk items on deck	Deck, fish room	Tubs, big bags, pallets up to hundreds of kilograms	Bulk catch, frozen product, stores

Why on-board weights matter: logbooks and the landing obligation

In the EU, the master's logbook estimate of each species retained on board must fall within a legal margin of tolerance, which makes weighing at sea the most direct way to stay within it. Under Regulation (EC) No 1224/2009 as amended by Regulation (EU) 2023/2842:

catching vessels keep an electronic fishing logbook with estimated quantities in kilograms live weight per species, and vessels of 12 metres or more record these per fishing operation;
quantities below the minimum conservation reference size are recorded as a separate entry;
the permitted margin of tolerance is 10 % per species, or 20 % for species retained on board in quantities not exceeding 100 kg live-weight equivalent (applicable from 10 July 2024).

Undersized catches of species subject to catch limits must, in general, be landed under the landing obligation of Article 15 of Regulation (EU) No 1380/2013, so they also need to be quantified.

These on-board weights are distinct from the official weighing for landing declarations. The amended Article 60 requires fishery products to be weighed per species after landing on weighing systems approved by the competent authorities; weighing on board instead is a derogation for sorted products, subject to Commission approval, a sampling plan and approved on-board weighing systems. The Legal Metrology hub explains the difference.

Recommended WPL set-up for trawlers

A typical trawler set-up combines a motion-compensated bench scale at the sorting line, a smaller scale for samples and a platform scale for bulk loads, all feeding one data system.

Position	Task	Suggested equipment
Sorting belt or boxing station	Boxes and species batches	M2-L or M2-XL with 60 kg (10–20 g) or 100 kg (20–50 g) range; wall mount standard, column mount or hopper optional
Sampling point	Buckets from pumped catch, small species	M2-S with 6 kg (1–2 g) or M2-M with 15 kg (2–5 g)
Deck or fish room	Tubs, big bags, pallets of frozen product	M5 flip-up platform or M6 with optional ramp, 600 kg or 1,500 kg range
Registration and labels	Catch location, gear, weights and lengths on labels	R50 Catch Label System, with optional GPS and NFC sensors
Data	Logging, statistics, export	WeightControl: automatic log of every weighing, statistics per preset, CSV export and JSON API

The M2 connects over USB, RS232 or Ethernet to printers, labelling equipment, catch management software and spreadsheets. The M5 compensates for motion, tilt and gravity changes; the M6 compensates for motion and tilt. None of these on-board weights replaces approved weighing at landing unless the competent authority has granted the on-board derogation.

Installation and operating tips

Mount rigidly. Fix bench scales to a bulkhead or column close to the sorting belt, preferably low and near midships to limit vertical accelerations.
Keep water away from the load path. Make sure washdown water drains off the platform and boxes do not rest against the frame.
Tare consistently. Use presets with stored tare values for each box type, and re-tare when boxes are wet or iced differently.
Check daily. Weigh a known test weight at the start of each trip day and after heavy weather or cleaning.
Use presets per species. Presets make totals per species and per haul available directly for the logbook estimate.

Background on why motion compensation is needed is in the Weighing at Sea hub, and materials and cleaning are covered in Hygiene and Materials.

Frequently asked questions

Can a trawler use its own scale for the landing declaration?

Only where the competent authority allows it. In the EU, fishery products are normally weighed after landing on approved weighing systems. Weighing on board is a derogation for sorted products that requires Commission approval, a sampling plan and approved on-board systems. Check with the flag and landing Member State.

Why weigh on board if the catch is weighed again at landing?

Because the logbook estimate is made at sea and must fall within the margin of tolerance, typically 10 % per species. Weighing boxes or batches as they are packed gives a far better estimate than counting boxes and assuming a nominal weight, and it also supports consistent box weights for buyers.

Which scale capacity suits a sorting line?

Choose a capacity comfortably above the heaviest filled box or batch, including ice and the box itself. For typical fish boxes a 60 kg or 100 kg range works well; for small species or samples a separate lower-capacity scale gives finer readability.