Live Feed Culture: Rotifer and Artemia Production

Why Live Feeds Matter in Hatchery Production

Most marine and brackish-water larvae cannot eat dry or pelleted feeds at first feeding. Their mouths are too small and their digestive systems too immature to handle anything but living prey. Rotifers (Brachionus spp.) and Artemia nauplii are the two live-feed workhorses of commercial hatcheries worldwide. Rotifers, typically 100–350 µm long, serve larvae in the earliest stages. Artemia nauplii, roughly 400–500 µm at hatch, are offered as larvae grow. Your ability to culture both reliably — at the right density and the right nutritional quality — directly affects larval survival and the economics of the facility.

Rotifer Batch Culture

Rotifers are filter feeders that eat microalgae and commercial algae paste. In a batch system you start with a known volume of rotifers at a measurable density (rotifers per mL, abbreviated rot/mL), feed them daily, and harvest a portion each day while leaving enough to continue the culture.

Inoculation and feeding. Fill a clean five-liter vessel with aerated, temperature-controlled saltwater at the salinity appropriate for your target species (commonly 20–30 ppt for euryhaline rotifers). Add the starter culture, then feed microalgae or paste at the ration posted by your instructor. The ration is usually expressed as cells per mL or mL of paste per million rotifers. Overfeeding clouds the water, drives oxygen down, and crashes cultures; underfeeding starves rotifers and drops density.

Aeration and temperature. Continuous gentle aeration keeps rotifers in suspension, prevents algae from settling, and maintains dissolved oxygen. Rotifers grow fastest at roughly 25 °C but tolerate 20–28 °C depending on strain; hold your heater at the posted target and check it at each sampling.

Counting. Remove a 1 mL subsample with a volumetric pipette, place it in a counting dish, and count all rotifers visible under the stereo microscope. Repeat three times and average. Multiply by any dilution factor to get rot/mL in the bulk vessel. Target density, posted by your instructor, is the benchmark; your harvest-day count must be within 20 % of that number to pass.

Enrichment Protocol

Rotifers grown on standard algae diets are often low in highly unsaturated fatty acids (HUFAs), especially DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid). Larval fish require these lipids for neural and eye development. Enrichment is a short-duration step — commonly 6–24 hours depending on the product — during which rotifers are held in a concentrated enrichment emulsion so they ingest and store the lipids before being fed to larvae.

Procedure. On harvest day, transfer your rotifer sample into clean, aerated enrichment water. Add the commercial HUFA enrichment product at the exact manufacturer-stated dose and run for the manufacturer-stated duration. Do not over-enrich: extended exposure reduces rotifer survival and can cause the culture to deteriorate.

Verification. After enrichment, pipette a small sample onto a microscope slide. Enriched rotifers show a distinctive orange-yellow coloration in the gut from the carotenoid pigments in most commercial lipid products. Document your observation in your live-feed log with a written note or a sketch. An absence of color suggests the enrichment step failed or the dose was incorrect.

Artemia Hatching and Harvest

Artemia (brine shrimp) cysts are sold in sealed cans and remain viable for years if stored cool and dry. When hydrated in saltwater and illuminated, cysts hatch and release nauplii at the first instar (nauplius I/II). This is the correct harvest stage — nauplii are nutritious and the right size. If you wait too long, nauplii molt, consume their yolk reserves, and become less nutritious.

Hatching setup. Use a cone-shaped separator vessel. Measure your cyst dose (typically expressed in grams per liter of hatching water). Add full-strength saltwater (about 35 ppt), maintain temperature near 28 °C, and apply strong continuous aeration with bottom uplift so cysts circulate freely. Provide constant illumination to stimulate uniform hatching.

Harvest at 24 hours. Turn off aeration and allow a brief settling period. Empty shells float; unhatched cysts sink; live nauplii concentrate in the middle under the light. Drain nauplii through a fine-mesh plankton sieve (90–150 µm). Rinse the sieve with clean water to remove salt and metabolic waste. Resuspend nauplii in a small known volume and count a 1 mL subsample under the microscope three times, then average.

Hatch efficiency. Express results as: (nauplii harvested ÷ nauplii expected from cyst dose) × 100. Expected yield is based on the cyst manufacturer's stated hatch rate. Your calculated efficiency must be within 10 % of the instructor's reference value.

Safety in the Live-Feed Lab

• Microalgae paste and enrichment emulsions can harbor pathogens if contaminated; never introduce equipment from disease-positive systems without disinfection. Follow the facility's biosecurity protocol.
• Airstones and heater leads are submerged; inspect cords for damage before each use. Report frayed insulation immediately — do not operate damaged equipment near water.
• Enrichment products may irritate eyes and skin. Read the Safety Data Sheet (SDS) before use and wear nitrile gloves and safety glasses when measuring and pouring concentrated enrichment.
• Wet floors around culture vessels are a slip hazard; clean spills immediately and wear non-slip footwear.
• Record every addition (feed, enrichment, salt, water volume) in the live-feed log at the time it is done. After-the-fact entries introduce error and can compromise larval health decisions.