Applications

Weighing on Longliners and Small-Scale Fishing Boats

Updated September 14, 20263 min readBy WPL Industries Engineering

Short answer

Longliners weigh individual high-value fish and batches for grading, labels and catch accounting; small-scale boats weigh boxes and small catches for logbook estimates and direct sales. Both need compact, motion-compensated, corrosion-resistant scales, with battery power on small boats and large platforms and capacities of up to several hundred kilograms on longliners.

Weighing needs on longliners and small boats

Longliners and small-scale fishing boats mostly weigh individual high-value fish and small batches of catch, often in cramped, wet conditions with limited power. What they need is a compact, motion-compensated and corrosion-resistant scale that is quick to use, can run on battery where required, and records weights for logbooks and labels.

A longline, as described in the FAO fact sheet on set longlines, consists of a main line with snoods carrying baited hooks at regular intervals. Fish come on board one by one, which makes it natural to weigh, grade and label each fish or each small batch as it is processed. On small-scale vessels using lines, nets or pots, catches are smaller but the same pressures apply: space, spray, and the value of every kilogram.

Typical weighing tasks

The main tasks differ between large freezer longliners and small day boats.

Task	Vessel type	Typical load	Purpose
Individual weight of large fish	Fresh and freezer longliners	A few kg to well over 100 kg per fish	Grading by size class, price, labels
Weighing catch before processing	Freezer longliners	Fish or batches on a belt or platform	Catch accounting, yield control
Box or batch weights per species	All vessel types	Boxes up to tens of kilograms	Logbook estimates, sale
Weighing small quantities of bycatch	All vessel types	A few kilograms	Accurate logbook entries for minor species
Weights and lengths for labels or traceability	Vessels selling direct or to premium markets	Individual fish	Origin, gear and weight on each label

Weighing before processing is also a regulatory monitoring tool in some fisheries. In Alaska, freezer longliners with Pacific cod endorsements must choose between carrying two observers or carrying one observer and weighing Pacific cod on a motion-compensated scale before it is processed (NOAA Fisheries).

Rules that shape weighing on small vessels

In the EU, the amended control regulation extends electronic catch recording to vessels under 12 metres, so estimates of quantities on board are becoming a routine obligation for small boats too. Under Regulation (EC) No 1224/2009 as amended by Regulation (EU) 2023/2842:

masters of all Union catching vessels keep an electronic fishing logbook with estimated quantities per species in kilograms live weight;
masters of vessels under 12 metres submit this information after the last fishing operation and before landing starts, and may use simplified means;
these logbook provisions apply to vessels under 12 metres from 10 January 2028;
the margin of tolerance is 10 % per species, or 20 % for species retained on board in quantities not exceeding 100 kg live-weight equivalent.

Weighing small lots on board makes these estimates much easier to get right than judging by eye. For small-scale fisheries worldwide, the FAO SSF Guidelines also encourage reducing post-harvest losses and adding value through better handling, and accurate weights and labels are part of selling each fish for what it is worth. The Legal Metrology hub explains when weights must come from approved systems.

Recommended WPL set-up

For small boats, portability and battery power come first; for longliners, capacity and platform size for large fish matter most.

Situation	Suggested equipment	Notes
Small day boat, boxes and small catches	M2-S with 6 kg (1–2 g) or 15 kg (2–5 g), or M2-M with 15 kg or 30 kg (5–10 g)	Battery and carrying case options (carrying case for S, M and L only)
Longliner hauling or processing station, fish up to about 60–100 kg	M2-L or M2-XL with 60 kg (10–20 g) or 100 kg (20–50 g)	Wall mount standard, column mount optional
Very large individual fish	M2-XL (600 × 400 mm) or M2-XXL (800 × 600 mm) with 200 kg (50–100 g) or 400 kg (100–200 g)	Custom platforms can be discussed
Labels per fish or box	R50 Catch Label System	Records catch location, fishing method, weights and lengths; optional GPS and NFC sensors; data can be sent ashore during the trip
Wireless or cable-free data	M2 with Bluetooth LE or WeightControl IOT module	USB, RS232 and Ethernet also available

All M2 scales are motion-compensated, water-resistant and made of AISI 316 stainless steel with smooth, rounded edges for washdown.

Practical tips for small decks

Protect the load path. Position the scale where lines, hooks and gaffs cannot strike the platform, and where fish can slide on and off without lifting.
Record the product state. Note whether a weight is round, gutted or gutted and headed, because logbooks require live-weight equivalents.
Keep a charging routine. On battery-powered scales, charge between trips and carry a spare where possible.
Tare trays and cradles. Large fish are easier to weigh in a cradle or tray; store its tare in a preset.
Rinse after every trip. Salt and slime left on a scale are common causes of corrosion and sticking parts.

More on corrosion resistance and cleaning is in Hygiene and Materials, and on motion compensation in Weighing at Sea.

Frequently asked questions

Do vessels under 12 metres need to weigh catch at sea?

EU rules require an estimate of quantities per species in the electronic logbook, which applies to vessels under 12 metres from 10 January 2028. The regulation does not require a scale for that estimate, but weighing small lots is the most reliable way to stay within the margin of tolerance.

Can a bench scale weigh a fish that is longer than the platform?

Yes, if the fish is supported so that its whole weight rests on the platform, for example in a tray or cradle that is tared out. Parts of the fish resting on the deck or frame would reduce the reading, so the support must not touch anything else.

Is battery operation reliable enough for a full fishing day?

Endurance depends on the scale, battery option, temperature and how long the display is on. Ask for endurance figures for your configuration, charge between trips and plan for a spare or a mains connection on longer trips.