Weaning, Nursery Phase, and Feed Transition

Weaning, Nursery Phase, and Feed Transition

Weaning is the process of transitioning larvae from live feed — such as rotifers, Artemia nauplii, or copepods — to a formulated dry diet. It is one of the highest-risk phases in hatchery production. Larvae that do not accept dry feed quickly lose condition, and survival can collapse within days. A structured, stepwise protocol reduces that risk by maintaining live feed as a safety net while gradually introducing the commercial microparticulate diet.

Why Weaning Is Difficult

Larvae respond to prey visually and by sensing water movement. A sinking pellet behaves differently from a swimming rotifer. Early in weaning, many larvae ignore pellets entirely or strike and reject them. Feed acceptance improves as the proportion of dry feed rises and larvae learn that pellets are food. The hatchery technician's job is to manage this learning period so that mortality and wasted feed stay low.

The Co-Feeding Schedule

A co-feeding schedule runs live feed and dry feed simultaneously, shifting the ratio over several steps. A typical five-day schedule for a warmwater species might look like this in concept: on days one and two, live feed supplies the majority of the ration and dry feed is introduced at a low percentage; by days three and four the ratio is roughly equal; by day five dry feed dominates. The exact percentages depend on the species, larval age, water temperature, and feed manufacturer recommendations. Always consult the species data sheet and your instructor's weaning template before calculating rations.

Rations are expressed as percent body weight per day (%BW/day). To convert to a mass or volume per tank, you need the estimated biomass in that tank. Biomass = (number of fish) × (mean individual weight). If individual weight is not known, it can be estimated from a length-weight relationship for the species.

Calculating Daily Rations

For each step in the schedule, calculate:

1. Live feed ration (g/tank/day) = Total biomass (g) × Live feed %BW fraction

2. Dry feed ration (g/tank/day) = Total biomass (g) × Dry feed %BW fraction

Deliver the total ration across the prescribed number of feedings per day. Divide evenly. Record every feeding in the production log with the time, amount offered, and a behavioral observation.

Feed Delivery and Behavioral Observation

Use the smallest appropriate particle size for the species and larval stage. Manufacturers publish particle size charts; choose the size that matches the gape width of the larvae. Deliver dry feed slowly across the surface so it disperses before sinking too quickly. Observe for:

• Schooling near the surface — indicates active searching and a positive response to feed addition.
• Individual strikes on pellets — direct evidence of acceptance.
• Uneaten feed on the tank bottom — excess feed harms water quality. Siphon waste from the bottom after the observation period at each feeding. Record the estimated amount removed.

Adjust ration downward at the next feeding if substantial waste is found; adjust upward if the tank clears quickly and fish appear to be competing aggressively.

Growth Sampling with MS-222

Every other day, measure a subsample of ten larvae to track growth. Anesthetization with MS-222 (tricaine methanesulfonate) is required to handle larvae safely and get accurate measurements.

Safety — MS-222 handling: MS-222 is a mild irritant. Wear nitrile gloves and safety glasses when preparing solutions. Always buffer MS-222 solutions with sodium bicarbonate (baking soda) at an equal or greater mass to bring the pH to a safe range for the fish; unbuffered MS-222 solution is acidic and stresses larvae. Prepare fresh solution each day. Dispose of used anesthetic solution according to your facility's chemical waste procedure. Never work with MS-222 alone — a second person must be present in case of accidental exposure or a fish emergency.

Post the working concentration on the container as instructed. Do not exceed the posted concentration — overdose is lethal to larvae.

Measure total length (tip of snout to end of tail) using calibrated digital calipers or a calibrated ocular micrometer under a stereo microscope. Record all ten individual measurements, then calculate the mean. Enter data in the production log.

Mean daily growth rate (mm/day) = (Mean length at sampling 2 − Mean length at sampling 1) ÷ Number of days between samplings

Survival Tracking

Count or estimate survivors at each sampling event. Survival rate (%) = (Current count ÷ Initial count) × 100. Declining survival during weaning may indicate feed refusal, water quality problems, or disease — investigate the cause promptly rather than attributing all losses to normal attrition.

Production Log Discipline

Every entry must be made at the time of the observation, not reconstructed later. Missing entries are a critical error in commercial hatchery work because they break the chain of evidence needed to diagnose problems. Date, time, tank ID, action taken, and observer initials are required on every line.

Safety Summary

• Wear nitrile gloves and safety glasses when handling MS-222 and sodium bicarbonate.
• Always buffer MS-222; always have a second person present.
• Use the posted working concentration — do not guess.
• Siphon and dispose of waste feed promptly to maintain water quality.
• Wash hands after contact with fish, feed, or tank water.