Open-Air vs. Closed Rotifer Bioreactors- A Comparative Overview and Analysis of Reliability, Nutritional Content & Cost

By: John Carberry, Matthew Carberry, Dr. Terry Snell & Tim Wilson

Sustainable Aquatics Inc (SA) hatchery has been raising fish using rotifers as a first feed for over 15 years.  In this article we compare the reliability, nutritional content, and cost of operating SA’s “Closed Rotifer System” vs. the “Open-Air Rotifer Systems” widely used throughout the aquaculture industry. Our conclusion is that SA’s closed rotifer production system outperforms open-air systems on all three accounts.

  • Reliability is the most important metric

    Our fish larvae are valuable and will perish within a day of not receiving an adequate number of live rotifers with high nutritional content. Any reduction in the number or quality of rotifers provided compromises the growth rate and overall health of the larval prey which are developing allometrically according to a schedule regulated by their DNA. If larvae fall behind that schedule, the maturing fish are compromised for life. Aquaculturists sometimes experience catastrophic crashes of rotifer mass cultures that completely compromise their ability to supply adequate numbers of rotifers to their fish larvae. This inconsistency in rotifer production is probably the biggest challenge in live food production in aquaculture.

  • Nutritional Content- Rotifer Feeds Matter

    Rotifers are indiscriminate filter-feeders eating an omnivore diet. Replicating their natural diet boosts the nutritional content of the rotifers in turn increasing growth rate and health of the predators as well and the growth rate of the rotifer culture itself. 

  • Closed Bioreactors deliver 1) and 2) above at about 50% of the cost of Open-Air Systems.

    Operating closed bioreactors using algae followed by omnivore feeds are much less expensive than you may think.

Reliability- SA’s Closed Rotifer System

Our rotifer production system is designed to produce about 500 million rotifers per day. Each day we inoculate a new 1000-liter bag with about a liter of live starter algae, either Tetraselmis or Isochrysis.  These algae are grown in clean systems, using RO water and sub-micron filtered air, and we also use sub-micron ceramic filters to remove about 98% of the water from the algae and replace that with clean sterile artificial seawater.

We add algae growth nutrients to the bag, illuminate it, and blow sub-micron filtered air through the bag from the bottom to create strong turbulence.

After about 7 days, we inoculate the bag with rotifers at about 0.1 per ml that are free of contamination from other rotifer species and ciliates.

Both the rotifers and algae amplify in this culture, but soon the rotifers overtake the algae and the bag color goes from deep green to nearly clear.

At this point we add our patented rotifer feed, Amplifeed Replete, a 8-micron feed containing a lipid rich, nano-emulsified blend of micronutrients including astaxanthin, selenium, DHA, vitamins, minerals, and metals such as iron, zinc, and magnesium and marine proteins and fats.   

Then on about day 14, we harvest the rotifers and at densities between 300 and 1000 rotifer per ml and feed them to fish larvae.  Notice that we are striving for reliability, not maximum density. We want to avoid at all costs conditions that could lead to rotifer crashes and the loss of fish larvae.   We typically operate our rotifer production systems crash-free for over 3 years, and limit crashes to single bags if one does occur.

Reliability- Open-Air Rotifer Systems

Open air rotifer production systems typically grow rotifers in large, uncovered tubs linked to protein skimmers and bacterial bioreactors.  These rotifer cultures are fed algae biomass from brands such as RotiGrow™ or RGComplete™[1].  Other common algae feeds are called “Nano Pastes”

These systems often operate at rotifer densities of 2000 to 4000 per ml.

Regular crashes can be expected unless the cultures are isolated in a clean room which is operated to scrupulously to eliminate contamination.

The following are typical types of contaminants found in rotifer cultures:


For instance, ciliates are common contaminants and are co-travelers with copepods, bacteria and other rotifers and are dispersed airborne.  They often out-compete rotifers for food and have higher population growth rates, leading to suppression of rotifer harvests. 


Of course, bacteria are easily dispersed airborne and are between 200 nm and 2 microns. Some are pathogens, suppressing rotifer growth and infecting fish larvae.

Copepods and Rotifers:

Some copepods and rotifers produce eggs that survive desiccation. These eggs are about 30-50 microns in diameter and can be dispersed by air currents.

What are the culture conditions in the open vs closed approaches?

The SA approach uses closed sterile plastic bags that likely exceed a Class 1 clean room standards using submicron filtration of all air and water.

The open approach normally places rotifer mass cultures in an open room sharing common access and air flow with the entire hatchery.  The air is often rich in airborne by-products of the aquaculture environment and full of contaminants.
Conclusion: If reliability is the highest value in the management of producing rotifers in a hatchery, it is clear that the SA method is a prerequisite for long term, consistent success.

Nutritional Content: All rotifers are not equal

Most fish larvae developing in the wild consume a diet of rotifers and copepods, which provide marine proteins and fats plus a rich array of micronutrients, including xanthophyll carotenoids such as astaxanthin, DHA, folate, selenium, iron, magnesium, zinc, and vitamins.

We developed Amplifeed Replete to gut-load rotifers with the micronutrients that fish larvae typically consume in the wild, but are absent in rotifers raised on algae-only diets.

The nutritional content of all cultured rotifers is 100% reflective of their diet. In the wild brachionid rotifers are indiscriminate filter feeders, ingesting all particles from 2-15 µm. They are therefore not obligate algivores. Feeding rotifers to fish larvae lacking the full spectrum of what they normally consume in the wild will short-change the larvae and compromise their health, immune systems, and growth and maturation rates.

At SA we have performed experiments to explore the difference in larvae survival, growth, health and maturation rate based on supplementing rotifers with Amplifeed Replete.

In the SA approach, we add Amplifeed Replete 1-2 days prior to harvest.

This practice increases eggs per female, color, and nutritional value of the rotifers, as well as their density. The figure below illustrates what happens when larvae are fed these rotifers enriched with Amplifeed Replete compared to no Amplifeed Replete:  

In addition to its novel composition, Amplifeed Replete is a slow sinking feed, milled to sub-8 micron particles, so that they can be consumed by brachionid rotifers.   Amplifeed Replete remains in the water column for a few hours and is nearly 100% consumed, lowering feed cost, and improving water quality.

Cost Comparison

The following compares the cost of operating a closed vs open rotifer mass culture production system.  We have not considered the costs of periodic culture crashes and subsequent loss of fish larvae in this analysis, even though they are more common in open systems.

There is a cost in the SA method for cultivating algae, but this cost is far less than the cost of buying algae pastes at USD 90 per liter. Our analysis shows that the SA closed system of rotifer production is about 2.4 times less expensive to produce 1 billion rotifers than an open system using algae paste.


SA’s closed system approach to mass culturing rotifers is superior on each of these metrics:

  • SA’s approach is exceptionally reliable.

  • SA’s approach produces rotifers which are significantly more nutritious to fish larvae. Resulting fish larvae are bigger, healthier, grow faster and have stronger immune systems.

  • In addition to these advantages, SA’s closed system is less than half the cost of the open system method.

    1. Amplifeed Replete has a nearly 100% uptake because of its uniformly small size.  Up to 90% of Nano-pastes and other standard rotifer feeds are aggregates over 10 microns in diameter and not bioavailable to rotifers.  Moreover, the moisture content of Nanopastes are 80% water vs. Amplifeed Replete which has 12% water content.

    2. SA’s closed system is easily managed with a skill level common to aquaculture technicians vs. open-air systems that require complex buffering and fine control of water chemistry using products such as Cloram-X[2]. This complexity makes open systems more prone to crashes caused by operator error. 

Rotigrow™  is a trademark of Reed Mariculture, RGComplete™  is a trademark of ReefNutrition.

[2] Cloram-X™ is a trademark of Reed Mariculture