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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Aquaculture & Marine Science Tech › Hatchery Operations: Broodstock to Juvenile Production › Lesson 6
AGR-AQUA50-04 · Lesson 6 · 18 clock hours · lab

Larval Rearing: Tank Setup and Early-Stage Management

Objective. Students will set up a larval rearing tank to species-appropriate specifications—salinity, temperature, light intensity, and aeration level—and manage first-feeding and daily density assessments for a minimum five-day rearing period.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Larval Rearing: Tank Setup and Early-Stage Management

Successful larval rearing is one of the most demanding tasks in a hatchery. Larvae are fragile, sensitive to environmental change, and must receive the right food at exactly the right developmental stage. A mistake in tank setup or a missed feeding can wipe out an entire cohort within hours. This lesson covers how to commission a larval tank safely, set species-appropriate water quality parameters, manage first feeding, and track survival through daily density sampling.

Why Larval Conditions Are So Critical

Unlike juvenile or grow-out fish, larvae have almost no physiological buffer against environmental stress. Their gills, kidneys, and digestive systems are still developing. Even small swings in salinity, temperature, or oxygen can cause mass mortality or developmental abnormalities. Before a single larva enters the tank, every parameter must be stable and verified.

The four core parameters for most marine larvae are:

Tank Commissioning: Step by Step

Before filling the tank, the vessel itself must be chemically clean. Residues from previous batches — dead organisms, biofilm, feed particles — harbor pathogens that can infect larvae.

1. Disinfect the empty tank. Use the approved hatchery disinfectant at the exact dilution posted in your facility's standard operating procedure (SOP). Sodium hypochlorite (bleach) is common. Never guess the dilution — use the posted concentration only. Allow the disinfectant to remain in contact for the time specified in the SOP.

2. Rinse thoroughly. Drain the disinfectant and rinse multiple times with filtered seawater or freshwater as specified. Disinfectant residue is toxic to larvae. Use a chlorine test kit if available to confirm residue is below detectable levels before proceeding.

3. Fill to target volume. Record the actual volume filled so that density extrapolations are accurate.

4. Adjust and verify parameters. Use a calibrated multi-parameter meter to measure salinity, temperature, pH, and DO. Adjust as needed and allow the tank to stabilize before stocking.

5. Verify aeration. Aeration stones should produce a fine, even bubble curtain. Check that no single stone is creating violent turbulence near the tank bottom.

6. Record and sign off. The commissioning checklist must be signed by the instructor before larvae are introduced. This is a non-negotiable biosecurity step.

Safety Points

Daily Larval Density Sampling

Knowing how many larvae survive each day tells you whether conditions are adequate and when to intervene. The method is volumetric sub-sampling:

1. Gently mix the tank water to distribute larvae evenly (do this carefully to avoid injury).

2. Withdraw a measured 1-liter sample using a volumetric pipette or graduated cylinder.

3. Transfer the sample to a counting chamber. Under a stereo microscope, count all larvae visible in the sample.

4. Extrapolate: multiply your count by the number of liters in the tank to estimate total larvae.

5. Calculate percent daily survival: divide today's estimated total by the original stocking number and multiply by 100.

6. Record every figure in the production log immediately — do not rely on memory.

First Feeding

The timing of first feeding is species-specific and is determined by the larval developmental stage, not simply by age in days. Larvae must have absorbed their yolk sac and have a functional mouth and gut before live feed is added. Adding feed too early wastes feed, fouls the water, and does not benefit the larvae. Adding it too late causes starvation.

Live feeds commonly used in marine hatcheries include rotifers, Artemia (brine shrimp) nauplii, and cultured microalgae, depending on the species and stage. Feed is added at a density specified in the species protocol.

After feeding, observe larvae under a compound or stereo microscope and assign a gut-fullness score using the standardized scale posted in your facility. A score of 0 means an empty gut; higher scores indicate progressively fuller guts. Recording this score daily provides an objective measure of whether larvae are actively feeding.

Production Log Discipline

Every measurement, every feed addition, every anomaly must be entered in the production log at the time it occurs. Accurate records protect the cohort (you can spot trends early), satisfy regulatory and certification requirements, and build the institutional knowledge your hatchery depends on. Incomplete logs are a significant compliance failure in commercial aquaculture.

Key terms8

Larval densityThe number of larvae per unit volume of rearing water, typically expressed as larvae per liter; used to calculate survival and adjust feeding rates.
Volumetric sub-samplingA counting method in which a precisely measured volume is withdrawn from a tank, the organisms in that volume are counted, and the count is scaled up to estimate the total population.
Gut-fullness scoreA standardized numerical rating assigned after microscopic observation to indicate how much food is visible in a larva's digestive tract; used to confirm active first feeding.
Tank commissioningThe process of preparing a rearing vessel for use: disinfecting, rinsing, filling, and verifying all water quality parameters before introducing organisms.
Yolk-sac absorptionThe developmental event in which a newly hatched larva fully consumes its internal yolk reserves and must transition to external feeding; marks the critical window for first feeding.
Dissolved oxygen (DO)The concentration of oxygen gas dissolved in water, typically measured in mg/L or percent saturation; a critical parameter maintained near saturation in larval tanks through careful aeration.
Sodium hypochloriteA chlorine-based oxidizing disinfectant commonly used to sterilize hatchery tanks and equipment; must be used at facility-specified dilutions and fully rinsed before organisms are introduced.
Percent daily survivalThe proportion of larvae alive on a given day expressed as a percentage of the original stocking number; a key production performance indicator calculated from daily density samples.

Worked examples4

1. A larval tank holds 500 liters of seawater. It was stocked with 50,000 larvae on Day 0. On Day 3, you take a 1-liter sample and count 82 larvae. What is the estimated total larvae in the tank on Day 3, and what is the percent survival?
  1. Step 1 – Extrapolate to total larvae: multiply the count per liter by total tank volume. 82 larvae/L × 500 L = 41,000 larvae estimated in tank.
  2. Step 2 – Calculate percent survival: divide estimated total by original stocking number and multiply by 100. (41,000 ÷ 50,000) × 100 = 82%.
Answer: Estimated total larvae: 41,000. Percent survival on Day 3: 82%.
2. On Day 5 from the same tank (still 500 L), you take a 1-liter sample and count 74 larvae. The Day 4 estimated total was 39,500. What is the Day 5 estimated total, the percent survival relative to stocking, and the percent change from Day 4 to Day 5?
  1. Step 1 – Extrapolate Day 5 total: 74 larvae/L × 500 L = 37,000 larvae.
  2. Step 2 – Percent survival relative to original stocking: (37,000 ÷ 50,000) × 100 = 74%.
  3. Step 3 – Percent change from Day 4 to Day 5: [(37,000 − 39,500) ÷ 39,500] × 100 = (−2,500 ÷ 39,500) × 100 ≈ −6.3%. Larvae declined by approximately 6.3% from the previous day.
Answer: Day 5 estimated total: 37,000 larvae. Percent survival from stocking: 74%. Day-over-day decline: approximately 6.3%.
3. You need to prepare a sodium hypochlorite solution for tank disinfection. Your facility SOP specifies a 200 ppm available chlorine solution. Your stock bleach is labeled at 5% available chlorine (50,000 ppm). How many milliliters of stock bleach do you add to make 10 liters of working solution?
  1. Step 1 – Use the dilution formula: C1 × V1 = C2 × V2, where C1 = 50,000 ppm, C2 = 200 ppm, V2 = 10,000 mL.
  2. Step 2 – Solve for V1: V1 = (C2 × V2) ÷ C1 = (200 × 10,000) ÷ 50,000 = 2,000,000 ÷ 50,000 = 40 mL.
  3. Step 3 – Verify: 40 mL of 50,000 ppm stock in a total of 10,000 mL → (40 × 50,000) ÷ 10,000 = 200 ppm. Confirmed.
Answer: Add 40 mL of 5% sodium hypochlorite stock to enough water to make 10 liters of working solution (i.e., add stock first, then bring to 10 L with water). Always add bleach to water, not water to bleach, and wear full PPE.
4. A student stocked a 200-liter larval tank with 30,000 larvae. On Day 2 a 1-liter sample contains 126 larvae. On Day 3 a 1-liter sample contains 119 larvae. Calculate estimated totals and percent survival for each day, and determine whether the Day 3 survival calculation falls within 5% of an instructor verification count of 23,400.
  1. Step 1 – Day 2 total: 126 × 200 = 25,200 larvae. Percent survival: (25,200 ÷ 30,000) × 100 = 84%.
  2. Step 2 – Day 3 total: 119 × 200 = 23,800 larvae. Percent survival: (23,800 ÷ 30,000) × 100 ≈ 79.3%.
  3. Step 3 – Check against instructor count of 23,400: difference = 23,800 − 23,400 = 400 larvae. Percent difference from instructor count: (400 ÷ 23,400) × 100 ≈ 1.7%. This is well within the 5% tolerance; the count passes.
Answer: Day 2: ~25,200 larvae, 84% survival. Day 3: ~23,800 larvae, ~79.3% survival. The student's Day 3 count is within 1.7% of the instructor verification count, passing the 5% accuracy requirement.

The math in this lesson2

Learners compute percent survival from estimated larvae over the original stocking number.
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Brush up in the math halls at newSchoolWorks.com:
In class: Tank commissioning checklist: students sterilize a larval tank with an approved disinfectant at posted dilution, rinse with filtered seawater, fill to target volume, adjust all parameters to species specification using the multi-parameter meter, and verify conditions before receiving larvae. · Larval density sampling: each day for five days, students take a 1-liter volumetric sample, count larvae under a stereo microscope, extrapolate to total tank density, record in the production log, and calculate daily percent survival relative to stocking density. · First-feeding observation and record: on the appropriate developmental day, students add live feed at the prescribed density, observe larvae feeding behavior under a light microscope, note gut-fullness score on a standardized scale, and record feed addition in the log.
Materials: Fiberglass or polyethylene larval tanks, approved hatchery disinfectant (e.g., sodium hypochlorite at posted concentration), multi-parameter meter, aeration stones, light meter (lux), stereo microscope, compound microscope, volumetric pipettes, plankton counting chambers, production log forms
Check: Tank commissioning checklist signed off by instructor before larvae are added; five-day density log complete with daily survival calculations within 5% of instructor verification counts; gut-fullness scores recorded on the correct scale — all three required.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Hatchery Operations / Biosecurity
1. Before larvae are introduced to a newly disinfected larval tank, the most important final verification step is to:
Hatchery Operations / Production Monitoring
2. A 1-liter sample taken from a 400-liter larval tank contains 55 larvae. What is the estimated total larvae in the tank?
Hatchery Operations / Production Monitoring
3. A hatchery stocked 40,000 larvae. On Day 4, the estimated total is 32,000. What is the percent survival?
Workplace Safety / Chemical Hazards
4. When handling sodium hypochlorite to disinfect a larval tank, which personal protective equipment (PPE) combination is most appropriate?
Hatchery Operations / Larval Biology
5. First feeding of marine larvae should be initiated based primarily on:
Hatchery Operations / Feeding Management
6. A gut-fullness score is recorded during first-feeding observations primarily to:
Workplace Safety / Electrical Hazards
7. To service a malfunctioning heater in a larval tank, a technician should first:
Hatchery Operations / Production Monitoring
8. A student's Day 3 estimated larval count is 18,600. The instructor's independent verification count is 19,500. What is the percent difference, and does the student's count meet the 5% accuracy requirement?
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