D.6.4 | Rotifer Production: Methods

Feeding Dose in Continuous Systems vs. Batch Systems

Because Continuous Systems and Batch Systems are simply two ways of looking at and managing the same rotifer population dynamic, you can use batch feeding protocols for a continuous systems. For example: If you are growing RMI “Mini-L 160” rotifers and you want the rotifer culture to grow at 67% per day, you will want to feed 0.063ml RotiGrow plus per hour per million rotifers. The next day you should harvest 40% of the culture which should bring the count back to where it was.

Metrics: Rotifer Biomass or Rotifer Count (Why rotifer counts don’t add up)

Microscopic observation and counting of rotifers is the traditional method of monitoring rotifer cultures and determining feed and enrichment requirements. For a specific strain of rotifers this can be an acceptable method.

However, rotifers have great variability in size and biomass, and therefore in feed and enrichment requirements per rotifer. Counts are very misleading and of little value for establishing protocols or comparing across strains. Even within strains different temperatures and reproduction rates can affect mean rotifer mass. As a result, feeding by rotifer count can lead to massive underfeeding or overfeeding of production feeds and enrichment feeds. This disconnect between rotifer feed and rotifer size (and biological requirements) is a significant source of culture stress and confusion for live feeds management and staff.


  1. A 120 µm S-type rotifer, by simple geometry has ¼ the volume, mass and feed and enrichment requirements of a 190 µm S-Type rotifer and 1/8th the mass and requirements of a 240 µm L-Type rotifer.
  2. If you feed a continuous rotifer system 1000 ml of RotiGrow Plus per day and harvest 35% per day, your system equilibrium biomass will be 135 g dry weight and your harvest will be 47 g dry weight per day. This will be true regardless of whether your system contains 730 million “Mini-L” 160 rotifers at 185 ng d.w./rotifer or 365 million larger rotifers at 370 ng d.w./rotifer.

    Note: If you now increase the harvest rate to 45% per day your final system equilibrium rotifer biomass will fall to105 g dry weight but your harvest will still be 47 g dry weight (the ratio of feed input to rotifer output remains constant). Feed biomass per rotifer biomass will have increased from 5.8% per hour.to 8.35% per hour. The yield in terms of rotifer biomass per volume of feed used remains unchanged.

Switching between Rotifer Count Metrics and Rotifer Biomass Metrics

To switch between management by rotifer count and management by rotifer mass requires knowledge of the relationship between the two. The following table is a useful first approximation of the relationship between rotifer length and rotifer biomass:

Table D.7


Note: These numbers are mathematical approximations the RMI “Mini-L 160” as the base data point. Actual numbers will vary depending on rotifer shape (thin vs. plump) and composition. Rotifer length is measured as the distance between the anterior tip if the lorica with the corona retracted and the base of the foot. Rotifers retract their corona when fixed with formalin.

This second point above and the attached note underscore one of the basic tenants of the Instant Algae method for growing rotifers – Don’t Worry About The Count. Calculate the biomass you want to feed to your fish. Then, based on the feed conversion rate and biomass yield for each feed, choose the correct amount of feed for your system and harvest at the rate that works best for you. You will get an equilibrium rotifer count that satisfies your larval needs. What that rotifer count is will depend on the size of your rotifer strain.

Now, once you have established a baseline equilibrium, rotifer counts are useful for monitoring your system.

Of course you will need a starting point based on the size of your rotifers and your history of how many rotifer you use. The following table provides a rough guide for converting lorica length to dry weigh and wet weight biomass:

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