Spiny lobster sea-cage grow-out records tell a useful story only when animal counts, stocking density, average body weight, mortality, cage changes and final harvest mass are kept together. A cage containing fewer animals can still produce more biomass if survivors grow larger, while a cage with rapid individual growth can lose production through mortality. To interpret performance, reconstruct each cohort from nursery transfer through final harvest rather than relying on a single survival percentage.
The basic calculations are simple but dependent on the correct denominator. Stocking density is animals divided by the defined cage floor area or water volume; standing biomass is living animals multiplied by representative mean mass; survival is the living cohort count divided by the number originally stocked after accounting for movements. Saleable harvest yield is not necessarily the same as standing biomass, because grading, discards, transfer and handling loss can reduce the marketed quantity.
The working summary: verify species and legal provenance; define the cage and usable area; record transfer date, starting count and starting weight; calculate areal and volumetric densities separately; record representative growth samples; reconcile escape, mortality and transfers; examine moulting and shelter; compare live biomass with harvest mass; document weather and cage damage; retain uncertainty; and avoid treating a profitable-looking output as proof of a causally superior husbandry method.
Grow-out starts after the nursery, not at fertilisation
Spiny lobster grow-out refers to raising already established young animals toward a specified market or transfer size. It is separate from the earlier puerulus collection and juvenile nursery stages. An enterprise may purchase nursery juveniles, transfer its own seed into sea cages or use licensed wild seed. The start of the grow-out cohort must be defined explicitly: how many animals were actually stocked, their species, mean mass, size distribution, health observations and the date they were accepted.
Different species can have different growth trajectories and market destinations. Panulirus ornatus and Panulirus homarus should not be merged into one species-neutral growth curve without evidence. In addition, a study of juveniles in flow-through tanks is not automatically transferable to sea cages: enclosure structure, water movement, shelter, diet and exposure differ. Scientific comparisons must preserve species and system boundaries, even when the arithmetic formulas look similar.
ACIAR-supported projects have studied spiny lobster grow-out and nursery systems in Vietnam and Indonesia. Research includes cage farming, nutrition and farmer practice, reflecting that biological output and practical implementation depend on more than just individual growth rate. A report about culture success should therefore explain the environment, sampling design and husbandry practice rather than present a single universal density or harvest schedule.
One cage, two kinds of density
An areal density divides the number of animals by a defined bottom area, with units such as animals per square metre. A volumetric density divides by water volume, with units animals per cubic metre. These are distinct quantities, and it is not accurate to use them interchangeably. A benthic or shelter-using animal may experience crowding in relation to floor and refuge space even when water volume appears generous.
Imagine a cage with four metres by four metres of horizontal footprint and three metres of water depth. Its footprint is sixteen square metres and its nominal volume is forty-eight cubic metres. If 160 lobsters are stocked, areal density is 160 divided by sixteen, or 10 animals per square metre. Nominal volumetric density is 160 divided by forty-eight, approximately 3.33 per cubic metre. The proper reporting unit depends on the intended ecological question and the reference experiment.
Nominal cage dimensions may not equal usable volume. Net deformation, uneven seabed clearance, unoccupied headspace and internal structures can reduce practical habitat. An operator should record the actual cage geometry and shelter configuration. A paper that reports stocking density without explaining which dimensions were measured creates a hidden ambiguity that prevents meaningful comparisons. Good translation reconstructs how the original authors defined their denominator.
Density trials provide context, not universal rules. A Philippines National Fisheries Research and Development Institute study compared P. ornatus nursery densities in floating cages, while other studies examined different animals and systems. The relevant evidence may show a density effect in one stage without proving the same outcome in larger grow-out cages. Confirm species, age, trial duration, density unit, replication and measured response before drawing an operating recommendation.
Count animals and measure biomass at the same time
Starting biomass equals the initial living animal count multiplied by representative mean body mass. If four hundred juveniles average fifty grams each, the starting standing biomass is twenty thousand grams, or twenty kilograms. If three hundred sixty animals remain and average two hundred fifty grams at a later checkpoint, their standing biomass is ninety kilograms. Survival is 360 divided by 400, or 90 per cent, while the apparent standing biomass increase is seventy kilograms.
That seventy-kilogram difference is not the same as total individual growth in every animal ever stocked. Some lobsters may have died after gaining mass, and partial harvesting or incoming transfers may have occurred. The two-point standing biomass difference describes the live inventory at two dates. To calculate feed conversion or cumulative production, an analyst may need mortality masses, harvest removals and other changes. Label the quantity correctly rather than calling any difference growth yield.
Average mass also requires a defined measurement method. Was every animal weighed, or only a sample? Were animals wet and recently fed, or drained under a standard procedure? Were very small animals excluded? A mean based on the easiest lobsters to capture may be biased toward active or larger individuals. If the sampled animals are not representative, multiplying the mean by the total count produces a deceptively precise biomass estimate.
The size distribution can matter more than the mean. A cage may contain 100 animals averaging 200 grams but with some well below a commercial size class. Another cage with the same mean may have a tighter distribution. A harvest planner needs grade counts and minimum-viable product specifications. An article that reports average weight alone cannot establish marketable yield when prices and eligibility depend on individual sizes.
Growth calculations without losing the unit
Absolute mean daily gain is the difference between two average individual weights divided by the elapsed days. If mean weight rises from fifty to two hundred grams over 120 days, the difference is 150 grams, and average daily gain is 1.25 grams per animal per day. This is an illustrative mathematical exercise, not a species-specific performance guarantee. A valid biological comparison also needs consistent sampling and consideration of animal turnover.
Specific growth rate based on body mass is often written as 100 times the natural logarithm of final mean weight divided by initial mean weight, all divided by elapsed time. For fifty to two hundred grams over 120 days, this gives 100 times natural logarithm of four divided by 120, about 1.155 per cent per day. That rate describes a particular growth metric, not a daily percentage added arithmetically to every animal and not the change in whole-cage biomass.
Do not confuse specific growth rate with the annual increase in money earned or the survival proportion. These have different numerators and denominators. A cage may have high specific growth among sampled survivors but poor economic yield if many animals die. A positive growth number needs to be read beside survival, stocking density and harvest output before it becomes evidence for a successful production strategy.
Moulting can make weight measurements episodic. Averages may shift after major moulting periods, handling can influence stress and different species have different growth histories. A short measurement window may not represent a longer cycle. For a defensible comparison, choose common checkpoints, preserve the original size range, and report whether the same cohort was followed or sampling relied on different individual animals each month.
Survival, escape and incomplete ledgers
A cage begins with 500 juveniles. At the end, 420 are counted alive, twenty are confirmed dead, thirty were deliberately transferred and ten are documented as having escaped after damage. The remaining twenty animals are unaccounted. The correct record distinguishes all categories. A superficial survival calculation of 420 divided by 500 gives 84 per cent present in the cage, but the living fate of transferred animals and the unresolved animals means this is not the same as biological survival of the initial cohort.
If the transferred thirty are known alive at dispatch, the number known to have survived to either remaining in the cage or transfer is 450 of 500, or 90 per cent to that defined checkpoint. Documented escape is not necessarily confirmed death. Missing animals may indicate mesh damage, counting problems or unobserved mortality. A conservation or biosecurity assessment may need to investigate escapes separately. Do not relabel discrepancies as confirmed predation simply to make a ledger balance.
Survival can be computed per interval and cumulatively, but transfer events complicate denominators. If a cohort is split into two cages, retain the original identifier and note the fractions transferred into each location. If a cage receives new animals, the population is open. A simple final divided by initial percentage may become meaningless without movement accounting. A cohort ledger can be maintained even when the animals are physically separated; it follows identity rather than only the container.
A high survival report also needs a clock. Ninety per cent survival over six weeks and ninety per cent over twelve months do not describe the same risk. The interval, starting stage, species, environment and handling events need to be stated. Avoid quoting a survival figure without its observation period or comparing unlike periods as if they were matched experiments.
Shelter, moulting and mortality hypotheses
Juvenile and adult spiny lobsters make use of shelter, and shelter design may influence interactions, vulnerability and husbandry access. The correct cage record includes refuge type, dimensions, inspection approach and any changes. A missing animal could be sheltered rather than dead, while an animal recently moulted may be vulnerable to stress or other lobsters. A casual description of aggressive behaviour should not substitute for measured mortality evidence.
An older study on P. ornatus grow-out in flow-through raceways examined density, survival and growth, and reported mortality largely associated with postmoult cannibalism under its tested conditions. Its result is relevant as a scientific hypothesis and caution. It does not prove that every unexplained cage loss is cannibalism or that a single shelter design will solve the problem everywhere. Sea-cage studies should record injuries, possible predators, escape opportunities, food availability and water incidents before attributing causes.
Moult shells should not be counted as dead lobsters. They are exuviae, the discarded exoskeletons left after moulting. A cage with abundant exuviae may have active moulting and still maintain a healthy stock. But a shell count alone also cannot establish how many distinct individuals moulted. The correct language is that shells were recovered, followed by careful biological interpretation rather than an unsupported count of living animals.
Sea-cage conditions and events
Cage position, water movement, depth, oxygen, temperature, salinity, fouling and storm exposure can affect farming conditions. A report should include the sensors, units and measurement times. A dissolved oxygen measurement without a time of day may miss a daily minimum. A temperature measured at the surface may not represent the animal’s cage depth. The operating record should preserve sensor placement and calibration, not merely the lowest or highest value.
Net fouling and cage maintenance can change water exchange or refuge conditions. Cleaning also creates disturbance and potential escape opportunity. These details matter when rates change around a maintenance period. A field note saying cleaned on Tuesday is stronger when linked to the cage ID, work undertaken, any mesh defects, animals handled and outcome checks. A manager can then investigate patterns without falsely attributing all subsequent losses to one visible intervention.
Storms create a separate class of risk. If a cage breaks, a missing animal inventory cannot automatically be entered as deaths; some may have escaped. The status of escaped farmed animals may carry ecological and legal consequences. Record gear loss and recovery, damaged net area, timing and the affected cohort. A survival analysis may need to distinguish husbandry mortality from catastrophic gear failure rather than averaging them invisibly into one number.
From standing biomass to harvested product
A harvest typically involves counting animals selected, weighing product, assigning grades and reconciling rejects, mortalities and postharvest transfers. An example cage may contain 360 live animals with estimated mean weight 250 grams, or ninety kilograms of standing biomass. If 320 animals are selected at an actual mean of 255 grams, their gross selected mass is 81.6 kilograms. If 8 animals are rejected at the same assumed mean, saleable count is 312 and saleable mass would be 79.56 kilograms before any subsequent loss, based on this simplified equal-weight assumption.
Real grading rarely has equal means among selected and rejected animals. Rejections may cluster among smaller or damaged animals. The correct saleable mass should be calculated from actual measured or grade-specific weights, not from the overall mean when its representativeness is doubtful. Record live weight, dead weight, reject weight and saleable weight with the same handling and measurement protocol. A price model in the companion finance article can then use the true saleable mass.
A statement that a cage yielded ninety kilograms may mean estimated live standing biomass, physical harvest weight or saleable graded output. These are different quantities. For a reader deciding whether the production system worked, the label matters as much as the number. Clear reporting enables comparisons among farms and prevents a strong biological result being confused with commercially delivered product.
FAQ
Does a larger cage always mean lower stocking density?
No. Density depends on both the number of animals and the defined area or volume. A larger cage stocked with proportionally more animals can have the same density, and shelter geometry may further affect usable space. State the density unit explicitly.
Is surviving animal growth enough to judge success?
No. Averages among survivors can rise even while much of the original cohort is lost. Report survival, cohort movements, representative individual weights and final harvest output together.
Is an empty moulted shell a dead lobster?
No. An exuvia is a shed exoskeleton. It must not enter a mortality count. A qualified animal-health assessment may be needed when a real carcass or disease signs are observed.
Can a single cage prove one density is better?
Not reliably. One cage per treatment risks confounding density with cage location, current, feeding or maintenance differences. Use appropriately replicated and comparable experimental units for a causal comparison.
What if the harvest weight differs from the inventory estimate?
Investigate counting, sampling representativeness, grading, weighing practices, hidden animals and handling loss. Do not assume either number is correct without a reconciliation.
Should legal size or export grades be copied from another country?
No. Limits and grading rules depend on species, market and jurisdiction. Treat examples as examples and consult relevant current local law and buyer specifications before commercial use.
Independent practice: diagnosing sea-cage records before drawing conclusions
Case 1: floor area and volume produce different density figures
A fictional grow-out cage has an internal footprint of five metres by four metres and two metres of water depth. Its nominal floor area is twenty square metres; its nominal water volume is forty cubic metres. If it receives 240 lobsters, the areal stocking density is 240 divided by twenty, or twelve animals per square metre. The nominal volumetric density is 240 divided by forty, or six per cubic metre. Both are mathematically correct, but they describe different measures of space. If a researcher reports twelve per square metre, the translator must not change the unit to twelve per cubic metre. Ask whether the study treats floor, cage volume or usable shelter area as its meaningful exposure variable. A density recommendation copied without its original denominator cannot be compared reliably with another trial.
Case 2: a deformed cage changes the denominator
During a strong current, a net cage partly collapses inward, reducing its usable horizontal space. The equipment sheet lists a nominal footprint of twenty square metres, but a survey estimates that only sixteen square metres remain accessible to sheltering animals. With 240 residents, nominal areal density is twelve animals per square metre; the estimated usable-footprint density is fifteen. The two numbers answer different questions, and the second is only as credible as the method used to estimate usable area. Record the inspection date, deformation, cage geometry and uncertainty. Do not quietly replace the nominal denominator with the observed one in half the time series. Changes in physical space can alter husbandry exposure independently of animal numbers, so a rising density may reflect net deformation rather than restocking.
Case 3: counting shelter surfaces instead of floor space
Suppose an enclosure has eight square metres of bottom and twenty removable shelter panels, each offering one-quarter of a square metre of surface on one face. A simple summation would give five square metres of one-sided panel area, but this is not automatically equivalent to five square metres of usable lobster refuge. Some surfaces may overlap, be inaccessible or be used differently by different size classes. For 160 lobsters, floor density is twenty animals per square metre, while a ratio to nominal one-sided panel area is thirty-two per square metre of panel surface. Report the latter only if the original study defines this metric and the surfaces are measured consistently. Do not turn structural dimensions into ecological capacity without evidence about animal use.
Case 4: biomass can rise despite fewer surviving animals
A cage begins with 400 juvenile lobsters averaging 50 grams, giving twenty kilograms of estimated starting biomass. Later it has 360 survivors averaging 250 grams, so the current standing biomass is ninety kilograms. Survival is 90 per cent; the increase in observed standing biomass is seventy kilograms. None of these figures tells us exactly how much biomass was produced and then lost by animals that died between checks. A claim of seventy kilograms of net standing-stock gain is supported by the two observations. A claim that every animal grew at the same rate, or that total production was precisely seventy kilograms, requires stronger tracking. Show the animal count and representative mean weight beside each estimate, not merely the end biomass.
Case 5: the sampled average weight is not representative
One technician catches the thirty largest active animals during feeding and finds their average weight is 300 grams. The cage actually contains many smaller individuals that remain hidden. Multiplying 300 grams by the full inventory of 200 animals creates an estimated sixty kilograms of biomass, but that estimate is biased upward if the measured animals were systematically larger. A defensible sampling method selects animals across size classes and cage zones according to a recorded design, or uses a complete count if feasible and appropriate. If size classes are known, estimate biomass from class-specific means and counts. Do not claim precision from a large number of weighed specimens when their selection was not representative of the population being estimated.
Case 6: growth in grams versus logarithmic specific growth
A hypothetical growth sample averages 80 grams on day zero and 200 grams on day 100. Absolute mean daily gain is (200 minus 80) divided by 100, or 1.2 grams per day. A logarithmic specific growth rate is 100 times ln(200 divided by 80) divided by 100, approximately 0.916 per cent per day. These are different descriptors of the same observed mass change. The logarithmic rate does not mean every lobster gained exactly 0.916 per cent of its mass each day. Sampling variation, moulting and changes in which individuals survived can complicate cohort interpretation. Include dates, units and the formula when translating reported growth rates.
Case 7: a misleading biomass gain after partial harvesting
An enclosure starts a month with 100 kilograms of live standing biomass. Halfway through, workers remove forty kilograms of live lobsters for sale, and the remaining animals are estimated at eighty kilograms at month end. Subtracting the starting stock from the final stock yields a twenty-kilogram decline, even though the enclosure also produced saleable output. A simplified accounting balance would include opening biomass, additions, growth, harvested removals, mortalities and closing biomass. With no other movement or loss assumed, the observed harvest plus closing biomass totals 120 kilograms, twenty above the opening amount. This illustrative balance does not independently prove twenty kilograms of biological growth if mortality masses or incoming transfers were omitted. Keep every stock movement in the ledger.
Case 8: surviving to transfer is different from surviving inside one cage
A starting cage has 500 lobsters. Workers transfer 100 living animals to a second enclosure, identify 20 confirmed mortalities and later count 380 living animals remaining in the original cage. The ledger balances: 500 minus 100 minus 20 equals 380. The proportion remaining in the first cage is 76 per cent, but that is not biological survival. If the transferred hundred were alive at dispatch, 480 of 500 were known alive at their respective checkpoints, or 96 per cent. Future survival of transferred animals is a separate question requiring follow-up. Reporting only the first cage’s inventory would misclassify deliberate production movements as mortality. Use a stable cohort ID across cages and record each transfer as an event with date, counts and destination.
Case 9: missing animals and documented escape
A storm leaves a cage with 280 lobsters where 320 had been recorded the previous week. Ten carcasses were recovered, and an inspection found a torn net, but there is no direct count of escaped animals. The forty-animal drop consists of ten confirmed deaths and thirty animals with unresolved fate. The evidence is compatible with escape, undercounting or additional mortality, but cannot distinguish them precisely. The repair is to record the damage event and missing category, inspect nearby equipment safely and use the incident in a biosecurity and ecological risk assessment when relevant. Calling all forty deaths would invent a diagnosis; calling all thirty missing animals escapes would also exceed the observations. An honest ledger can remain partly unresolved.
Case 10: a storm creates a break in time-series comparability
A grow-out experiment measures biomass every two weeks, but a cyclone damages cages, forces emergency handling and changes shelter arrangements halfway through. Measurements before and after the event are not from unchanged conditions. Retain the measured series but annotate the event, identify affected enclosures and report any changes in observer, gear, water exposure or animal movements. An increase in mortality after the storm is temporally associated with the incident, yet disease, crowding during emergency transfer and changed feed availability may also contribute. A comparison among undamaged and damaged cages can help explore mechanisms when the design permits. Do not hide a structural break in an otherwise smooth growth graph.
Case 11: mixed species cannot share a single assumed growth curve
A hypothetical cage contains 120 P. ornatus and 80 P. homarus juveniles. At the end of the interval the operator reports a mean of 260 grams across all survivors. That average is not enough to reconstruct each species’ growth, condition or size-grade yield. They may have different initial masses, moulting histories and mortality patterns. Record starting and ending numbers and weights separately by species, then present a total only when the aggregation serves a clearly defined question. If identification was uncertain at stocking, disclose that limitation. A single mixed-species daily gain can be a descriptive aggregate but should not be translated as a validated growth rate for either named species.
Case 12: size grades hide behind the same mean
Two cages each contain one hundred lobsters with an estimated mean mass of 300 grams. In the first, nearly every animal lies close to that mass. In the second, half are approximately 100 grams and half approximately 500 grams, giving the same average. If a hypothetical market accepts only animals at least 400 grams, the two cages have sharply different saleable counts even though their mean masses match. This threshold is merely an invented exercise, not a real legal or buyer rule. Record the grade specification, number and weight in each class, and the number not eligible. Never translate average mass as marketable yield without checking the underlying distribution and the applicable rules.
Case 13: gross harvest versus saleable harvest
A fictional harvest weighs 100 kilograms before sorting. Five kilograms are rejected because of damage, another three kilograms are outside a buyer’s hypothetical grade and two kilograms are lost during documented handling. Saleable mass is ninety kilograms. Gross-to-saleable yield is ninety divided by one hundred, or 90 per cent. This differs from biological survival and from the weight gain of the living animals before harvest. If the three-grade-exclusion kilograms remain alive for a later sale rather than being discarded, they must be recorded as retained inventory instead of loss. The physical destination of each category matters. A marketing statement should specify whether the mass denotes live harvest, accepted product or shipped final weight.
Case 14: price-weighted grade yield differs from mass yield
A cage sells fifty kilograms in a high-price grade at an invented price of 30 currency units per kilogram and another forty kilograms in a lower grade at 18 units per kilogram. The total saleable mass is ninety kilograms, but revenue is 50 times 30 plus 40 times 18, or 2,220 units. Average realised price is 2,220 divided by ninety, about 24.67 units per kilogram. It would be wrong to multiply the highest grade price by all ninety kilograms. This example illustrates a downstream economic calculation, not actual market pricing. Keep weight, grades, prices, currency and transaction date visible. Revenue is also not profit; feed, seed, labour and cage costs still need to be considered in a separate cost analysis.
Case 15: net cleaning coincides with mortality
Workers clean heavily fouled netting on Monday. By Friday, the animal count is lower and the report states that cleaning killed the lobsters. This is a causal claim not established by temporal order alone. Inspect water conditions, net damage, handling intensity, escape opportunities, feed changes, predation and evidence of actual death. If the programme compares alternative cleaning schedules, use matched cages and suitable replication rather than relying on one uncontrolled before-and-after event. The immediate reporting correction is to state that a decline was observed after the cleaning operation and that the causes require examination. That wording is cautious but operationally useful, because it points managers toward measurement rather than certainty unsupported by the ledger.
Case 16: oxygen sensors record different water conditions
One dissolved oxygen sensor sits above the cage in well-mixed water, while another measures inside a fouled shelter compartment. Their readings disagree. Without depth, position, calibration and timestamp, the operator may wrongly interpret this as a sensor malfunction or as a universal cage-wide oxygen condition. Describe both locations and check whether the sensors were calibrated and measured at comparable times. If daily cycles are expected, a noon reading is not necessarily representative of dawn or an overnight minimum. A mortality analysis should preserve the exposure history experienced by animals, not only whichever number is easiest to extract from a monitoring dashboard.
Case 17: the true replicate is a cage, not each lobster
A trial puts 400 lobsters into one cage receiving treatment A and 400 into one cage receiving treatment B. Final survival is 80 per cent in A and 70 per cent in B. The animal counts are large, but there is only one independently managed cage in each treatment. Location, cage construction and current exposure are perfectly confounded with the treatment assignment. A standard binomial comparison that treats each lobster as an independent treatment replicate can produce misleading confidence. Better experimental design would replicate treatment cages and allocate comparable groups appropriately, then account for variation at cage and individual levels. The observed contrast deserves reporting, but not a claim that treatment A is proven universally better.
Case 18: surviving animals create a selection effect
An initially mixed-size cohort contains many small animals and some large ones. After a mortality episode, surviving animals have a higher average mass. A report attributes all of that increase to rapid growth, although some of the smaller animals died. Compare repeated individual measurements or analyse cohort size-class distributions and mortality when possible. Mean weight among survivors can rise through selective survival even when individual growth is modest. Biomass accounting helps show whether the overall live stock actually gained mass, while individual tracking is needed to separate growth from composition changes more convincingly. This is an example of why a correct average can support an incorrect story when the group being averaged has changed.
Case 19: harvest weight does not agree with sample-based biomass
A preharvest sample estimates 250 animals at 400 grams each, or 100 kilograms of standing biomass. On harvest, the full physical count is 230 animals weighing 87 kilograms. The discrepancy could arise from a biased mean weight, an inaccurate initial count, animals missed in the sample or physical inventory changes before harvesting. Do not declare the twenty missing animals dead solely from the difference between estimates. Reconcile sample design, elapsed time, harvest counts, grading and remaining live animals. The physical harvest measurement may be stronger evidence for saleable product, but even it needs tare, container and scale controls. Present predicted and measured values as distinct observations, and explain the scope of any gap.
Case 20: a planned transfer changes the meaning of yield
An operator stocks two hundred juveniles and later obtains 150 animals of marketable size, while twenty more healthy juveniles remain too small and are retained for another growth period. Thirty animals are confirmed dead. Survival through the checkpoint is 170 of 200, or 85 per cent; current market-ready count is 150 of 200, or 75 per cent. The thirty deaths and twenty retained animals should not be folded into one category of nonsaleable failure. A report concerned with current sales can count only 150 available for sale, while a biological performance report should acknowledge the twenty living animals that have not yet entered the market. Explicitly name the decision horizon to avoid confusing partial harvest with final production.
Transfer problems for the reader, with answers
A first transfer problem asks for total starting biomass when 180 living juveniles weigh an average of 75 grams. Multiply 180 by 75 to obtain 13,500 grams, or 13.5 kilograms. If the count is exact but the average comes from a sample, the biomass is still an estimate; do not present its decimal places as independent certainty. A second problem asks for nominal areal density in a four-metre by five-metre cage holding those animals. Divide 180 by twenty square metres to obtain nine animals per square metre. Without an actual depth and usable-volume measurement, a volumetric density cannot be calculated.
A third problem asks for survival when 200 animals were stocked, ten confirmed dead, twenty transferred alive and 170 counted alive in the original cage. The ledger balances, and 190 animals were known alive at transfer or in the cage, giving 95 per cent survival to those respective checkpoints. The original cage alone contains 85 per cent of the starting group, but this is a location-retention percentage rather than mortality-adjusted survival. A fourth problem asks for saleable yield when eighty kilograms are harvested and five kilograms rejected with no retained live stock. The accepted yield is seventy-five kilograms, or 93.75 per cent of gross harvest mass. These definitions must be stated beside the numbers.
The final review checks every transition in the chain: origin and stocked count, denominator of cage area, legitimate growth samples, mortality, escape and transfer categories, final physical inventory, actual graded harvest and reported saleable product. Every result should have units and a date, and the conclusion must not leap from a single cage comparison to a general causal law. The arithmetic becomes trustworthy when its biological and operational assumptions are visible.
Related eduKate explanations
Spiny lobster puerulus settlement indices explains the monitoring stage before seed is obtained. Spiny lobster puerulus transport and nursery survival explains how the starting grow-out cohort is established. Abalone grow-out cages and harvest yield is a useful contrast in mass and inventory accounting.
Sources and further reading
ACIAR, Research for development of lobster growout technology in Indonesia. Research programme addressing sea-cage culture and survival.
Philippines National Fisheries Research and Development Institute, growth and survival of ornate rock lobster at different marine cage densities. Stage-specific density research.
James Cook University, effect of density on growth and survival in Panulirus ornatus raceways. Experimental context and its limitations.
ACIAR, Spiny lobster aquaculture in the Asia-Pacific region. Research proceedings on grow-out systems and feeding.
World Aquaculture Society meeting abstract, Panulirus homarus grow-out density. Distinct-species density treatment and outcomes.
All calculations involving fictional cages and weights are teaching examples, not a report of observed farm performance.