Precision Weighing of Small Samples at Sea: Otoliths, Gonads and Stomach Contents

Why small samples are hard to weigh at sea

Small samples are hard to weigh at sea because the disturbing forces on a vessel are roughly constant in absolute terms, while the sample weight is tiny. A disturbance worth 0.3 g is irrelevant for a 3 kg fish but amounts to 10 % of a 3 g gonad.

Four effects dominate:

• Vertical acceleration. A scale measures force. When the vessel accelerates upwards, a sample presses harder on the load cell, and when it accelerates downwards the sample presses less, the apparent-weight effect familiar from a lift (OpenStax University Physics). An acceleration of 0.5 m/s² changes the reading by about 5 %, so a 20 g sample swings by roughly ±1 g.
• Air movement. Deck wind, doors and ventilation fans push on the platform and the sample.
• Vibration. Engines, winches and pumps transmit vibration through benches and bulkheads.
• Wet surfaces. Water on the sample, the platform or a weighing tray adds weight that changes as it drains or evaporates.

Laboratory weighing guidance treats air currents, lack of thermal equilibrium and electrostatic effects as main error sources even on stable ground (NIST IR 6969); on a vessel, motion and wind come on top of these.

Readability needed per sample type

The readability you need depends on the lightest sample you intend to record and on what the value is used for. The table gives typical mass ranges and our suggested readability, based on the 1 % rule explained in the next section; the last column indicates whether the weighing is realistic at sea.

Sample	Typical mass	Used for	Suggested readability	Where to weigh
Otoliths (dried)	Milligram range	Otolith mass–age studies	0.1 mg or finer	Laboratory ashore, analytical balance
Stomach contents, individual prey	Milligrams to a few grams	Diet composition by prey group	0.01 g or finer	Laboratory ashore (frozen samples)
Stomach contents, total per stomach	0 to tens of grams	Total prey weight, feeding intensity	0.1 g from about 10 g; finer below	At sea for heavier contents; otherwise ashore
Gonads of small or immature fish	Below 1 g to about 10 g	Gonadosomatic index, maturity	0.01–0.1 g	Mostly ashore; at sea only from about 10 g
Gonads of large mature fish	Tens of grams to over 1 kg	Gonadosomatic index, fecundity studies	0.1–1 g	At sea
Livers	Grams to hundreds of grams	Hepatosomatic index, condition	0.1–1 g	At sea
Small whole fish (sprat, sandeel, juveniles)	About 1–30 g	Length–weight relationships	0.1 g	At sea for most fish above about 10 g

Otolith mass is used in some ageing studies (reviewed by Pacheco et al., 2021), and such measurements are made in the laboratory on analytical balances, typically on cleaned and dried otoliths. That is a laboratory task: no motion-compensated scale on a vessel offers milligram resolution in realistic conditions, and otoliths are stored dry anyway. For stomachs, the ICES stomach sampling manual allows analysis on board at the minimum sampling level, while the extended level, which identifies invertebrate prey, freezes stomachs individually for laboratory analysis; in both cases total prey weight and the weight of each prey group are recorded.

The 1 % rule for choosing readability

A practical rule is to choose a readability no larger than 1 % of the lightest sample you will record. This is not a formal standard, but it keeps the rounding step small compared with the natural variation you want to measure.

Readability	Lightest sample at 1 %	Lightest sample at 5 %
0.1 g	10 g	2 g
0.2 g	20 g	4 g
0.5 g	50 g	10 g
1 g	100 g	20 g
2 g	200 g	40 g

Keep in mind that readability is not accuracy. The smallest display step says nothing about repeatability, linearity or residual motion noise, which is why a test weight of similar mass to the samples should be checked under the actual sea conditions.

Example: gonadosomatic index. The gonadosomatic index expresses gonad weight as a percentage of body weight and is widely used to support maturity classification (Flores et al., 2015). An 80 g fish with a 2.0 g gonad has an index of 2.5 %. With 0.1 g readability the gonad could read 1.9–2.1 g, giving an index between 2.38 % and 2.63 %, an uncertainty of about ±5 % in the index from rounding alone. For a 40 g gonad from a 1 kg fish, the same readability affects the index by only 0.25 %.

Wind covers: controlling air movement

A wind cover is a shield around the weighing platform that keeps moving air off the sample, and for small platforms at 0.1 g readability it is often the difference between a stable and a drifting reading. The physics is simple: the pressure of moving air grows with the square of its speed, so a draught twice as fast pushes about four times as hard (NASA Glenn Research Center).

Air speed	Dynamic pressure	Equivalent load on 120 × 120 mm, if fully vertical
0.5 m/s (gentle draught)	0.15 Pa	about 0.2 g
1 m/s	0.6 Pa	about 0.9 g
5 m/s (light breeze on deck)	15 Pa	about 22 g

Only part of this pressure acts vertically on a flat platform, so real errors are smaller, but even a few per cent of these values is comparable to a 0.1 g display step. In practice:

• weigh small samples inside the wet laboratory, never on open deck;
• keep the scale away from doors, supply-air vents and fan heaters;
• keep the wind cover closed while the reading settles;
• avoid tall, light containers that catch air; use low weighing trays.

Motion effects and how compensation helps

Motion compensation lets a marine scale correct for vessel motion, tilt and gravity, so that the displayed value settles on the true weight rather than following the vessel's movement. It narrows the error, but it does not make a vessel as quiet as a laboratory bench, and its performance is best judged by checking a test weight at realistic accelerations and tilt.

Vessel motion is described by six components: heave, sway and surge (translations) and roll, pitch and yaw (rotations). For weighing, heave and the vertical components of pitch and roll matter most. The further a scale is from the vessel's centre of motion, the larger the vertical accelerations it experiences, so position strongly affects results. More background is available in the Weighing at Sea hub.

Every compensated scale balances speed against steadiness: a steadier value generally takes longer to settle. For small samples, accept a slightly longer settling time and rely on the stability indication rather than reading the display by eye.

Practical protocol for small samples at sea

1. Position. Install the scale on a rigid bench close to midships and as low in the vessel as practical, away from engine and pump vibration.
2. Shield. Fit the wind cover and close nearby doors and vents.
3. Prepare. Switch on well before sampling so the scale reaches operating temperature, then zero it.
4. Check. Weigh a test weight close to the typical sample mass at the start of each watch and record the reading.
5. Standardise moisture. Blot gonads, livers and prey in the same way for every sample, and weigh stomach contents in a pre-tared tray.
6. Wait for stability. Record only stable values, preferably directly into the data system to avoid transcription errors.
7. Record conditions. Note sea state or flag values taken in difficult conditions.
8. Divert what is too light. Freeze or preserve samples below your threshold and weigh them ashore.

How these samples fit into the overall survey, and why fine resolution also matters for small whole fish, is explained in Biological Sampling on Research Vessels and Length–Weight Relationships in Fish.

How WPL approaches this

For small samples, WPL's M3 Series Scientific Marine Scale on the 120 × 120 mm platform offers a 300 g capacity with 0.1–0.2 g readability and a 600 g capacity with 0.2–0.5 g readability. It is motion-compensated, built in 316L stainless steel and waterproof, and the 120 × 120 mm version can be fitted with an optional wind cover. Battery, USB, Bluetooth LE and the WeightControl IOT module are available as options. For choosing between ranges and platforms, see Choosing a Scientific Marine Scale.