Biomass Management

Standing biomass is the total live weight of fish held in a tank or system at a point in time. It is the product of the number of fish present and their mean weight, and both terms are estimates: the count carries the accumulated error of stocking records and mortality counting, and the mean weight carries sampling error.

• Feeding is set from biomass, so an overestimate causes systematic overfeeding and the water quality consequences that follow.
• Treatment load is proportional to feed, so a biomass error becomes a load error in the biofilter and oxygen system.
• Density limits are expressed against biomass, and exceeding them affects welfare, growth uniformity and disease risk.
• Harvest forecasting, contract commitments and cash flow all rest on the same estimate.
• Feed conversion cannot be calculated more accurately than the biomass figures used to derive it.

• Density in kilograms per cubic metre is a convenience figure, not a capacity limit. The real limits are oxygen delivery and ammonia removal per unit of flow.
• Sampling error falls with sample size, but only if the sample is representative — a net that preferentially catches slower fish produces a biased mean regardless of how many are weighed.
• Population error accumulates monotonically, because unrecorded mortality only ever inflates the count.
• Grading changes the distribution as well as the mean, so post-grading estimates must be rebuilt rather than adjusted.

• Provide sampling and grading infrastructure — crowding, netting, weighing and transfer — as designed facilities rather than improvised arrangements.
• Size tanks and treatment capacity for the biomass at the end of the cycle, when it is highest.
• Plan the production cycle so that peak biomass across all tanks does not coincide unnecessarily, if treatment is shared.
• Provide accurate weighing equipment appropriate to the sample size and fish size in use.
• Design record-keeping so that stocking, mortality, sampling and harvest are one continuous account per batch.

• Sample on a fixed schedule, with a defined method and a defined minimum sample size, so results are comparable across the cycle.
• Record every mortality, every day, by tank. This is the single highest-value record for biomass accuracy.
• Sample before feeding, consistently, so gut content does not vary between samplings.
• Treat sampling as a stress event: minimise duration, maintain oxygen during handling, and allow recovery before resuming full feeding.
• Reconcile estimate against actual at every harvest and grading, and carry the finding into the next cycle's method.

• Calibrate and verify weighing equipment on a schedule.
• Inspect and maintain nets, crowders and graders, because damaged handling equipment injures fish and biases samples.
• Audit the batch record periodically against physical stock and against feed consumption.
• Review the sampling protocol after each cycle against the harvest reconciliation.

• Treating the biomass estimate as a measurement and never quantifying its error.
• Recording mortality inconsistently, or in a separate place from the stocking record.
• Sampling too few fish, then acting on a mean that is mostly noise.
• Using a catching method that preferentially samples one part of the size distribution.
• Recording only the mean weight and discarding the distribution.
• Absorbing the harvest discrepancy into the accounts without investigating what produced it.
• Using density in kilograms per cubic metre as if it were a system capacity limit.

Optical and acoustic biomass estimation aims to remove handling from the measurement entirely, inferring size distribution from imaging in the tank. The attraction is obvious — continuous estimates with no stress event — and the practical accuracy depends on turbidity, density, species behaviour and calibration in the specific system rather than on any general specification.

Production planning across multiple tanks and batches is where biomass management becomes an operations research problem: stocking intervals, grading points and harvest windows determine whether treatment capacity is used evenly or in peaks, and a plan that is optimal for the market may be poor for the plant.

Standing biomass is an estimate that every operational and financial figure inherits. Its accuracy comes from complete mortality recording and disciplined, representative sampling — not from the sophistication of the calculation. Reconcile against harvest every cycle, keep the size distribution rather than only the mean, and remember that density per unit volume is a convenience, while oxygen and ammonia removal are the real limits.