Applications

Weighing in Aquaculture: Well Boats, Harvest and Sampling

Updated September 15, 20263 min readBy WPL Industries Engineering

Short answer

Aquaculture uses weighing to estimate biomass from sample weights, to document smolt deliveries and fish moved by well boats, and to grade and plan harvests. The equipment must weigh individual fish from tens of grams to several kilograms on moving decks, handle tubs of fish, keep handling time short and be easy to disinfect between sites.

Where weighing fits in aquaculture operations

In sea-cage aquaculture, weighing turns fish numbers into biomass: sample weights give the average weight in a pen, and number multiplied by average weight gives the biomass that feeding, treatment planning, transport and harvest depend on. At harvest, weights also determine what is sold.

Well boats connect these stages. They carry fish in wells through which seawater circulates, moving smolts to sea sites, moving fish between sites, handling fish during treatments, and transporting fish live to harvest. The Norwegian Veterinary Institute's 2015 report on wellboats gives an idea of scale: in 2012 the Norwegian wellboat fleet of 63 vessels transported approximately 350 million smolts and more than 1.4 million tonnes of harvested fish.

For Atlantic salmon, FAO describes smolts of 50–80 g being transferred to sea cages and reaching a harvest size of about 4 kg in 10–15 months (FAO Cultured Aquatic Species). Weighing equipment therefore has to cover individual fish from tens of grams to several kilograms, plus tubs and bins of fish.

Typical weighing tasks

Task	Where	Typical load	Why it matters
Sample weighing of pen populations	Service vessel, feed barge, pen walkway	Individual fish, tens of grams to about 6 kg	Average weight and size distribution for biomass and feeding
Smolt delivery sampling	Well boat, receiving site	Individual smolts, tens of grams upwards	Delivered biomass and fish size per delivery
Sampling during treatments or moves	Well boat, treatment vessel	Individual fish	Updated biomass and condition information
Harvest sampling	Well boat, harvest vessel, arrival at the processing plant	Individual fish and tubs	Average harvest weight, grading, planning of processing
Recording mortalities	Pen, service vessel	Bins of dead fish	Mortality records by weight as well as number

Individual sample weights combined with length also give a condition factor, which compares each fish's weight with the weight expected for its length and helps farms follow how well fish are growing; see Length–Weight Relationships in Fish.

Welfare and biosecurity when sampling

Every weighing involves handling live fish or moving equipment between sites, so welfare and biosecurity have to be designed into the procedure. Two points deserve attention:

Handling time. Crowding, netting and air exposure stress fish. A scale that settles quickly on a moving deck, with a tray shaped for the fish, keeps handling short. Sedation and handling procedures should follow national rules and veterinary advice; the Norwegian Veterinary Institute's work on animal welfare in aquaculture is a useful starting point.
Disease transfer. Transport of live fish by wellboat is considered to play a significant role in spreading disease between regions in Norway, and cleaning and disinfection of wellboats reduce but never eliminate that risk (Norwegian Veterinary Institute, 2015; Slette et al., 2024). Scales moved between sites should have smooth, closed surfaces that can be washed and disinfected completely.

Materials and cleaning considerations are covered in the Hygiene and Materials hub.

Recommended WPL set-up

Task	Suggested equipment	Notes
Smolt and small-fish sampling	M3 270 × 270 mm with 1,500 g (0.5–1 g) or 3,000 g (1–2 g)	Motion-compensated, 316L stainless steel, waterproof; battery and carrying case options
Grow-out and harvest-size fish	M2-M or M2-L with 15 kg (2–5 g) and a fish tray	AISI 316, rounded edges for washdown; battery option for work away from mains
Tubs, bins and mortalities	M5 or M6 with 600 kg (200–500 g)	M5 flips up for cleaning and deck space; M6 available with ramp
Data per pen or batch	WeightControl	Presets per pen or batch, statistics per period, CSV export and JSON API for farm management systems

WeightControl runs on the scale itself and is operated from a laptop or tablet browser through the scale's built-in access point, which is practical on service vessels without a fixed PC.

Practical tips

Sample representatively. Fish that are easiest to catch are not always typical of the pen; follow the farm's sampling procedure for selecting fish.
Use one preset per pen. Averages and size distributions are then available per pen without manual sorting of records.
Tare the tray wet. A wet tray weighs more than a dry one; re-tare after rinsing.
Clean between sites. Wash and disinfect scales and trays before moving to another farm.
Check with a test weight. Verify the scale at the start of each sampling day, especially after transport.

Weighing at Sea hub – motion compensation on vessels and pens
Data Integration hub – connecting weights to management software
Precision Weighing of Small Samples at Sea
Weighing Equipment for Fisheries Research Vessels

Frequently asked questions

How is pen biomass calculated from sample weights?

Biomass is the estimated number of fish in the pen multiplied by their average weight from a representative sample. Errors in either term carry straight into the result, so both reliable counts and well-chosen, accurately weighed samples are needed. Many farms combine manual samples with other monitoring methods.

Why does a scale on a well boat need motion compensation?

A well boat moves even in sheltered water, and vertical accelerations change the force on the load cell. Without compensation, readings fluctuate and handling time increases while operators wait for a steady value, which is bad for both data quality and fish welfare.

Can the same scale be used at different farm sites?

Yes, provided it is cleaned and disinfected between sites according to the company's biosecurity procedures. Smooth stainless steel surfaces, sealed construction and removable trays make this easier. Record where the scale has been used if equipment moves between farms.