Managing Temperature and Salinity Transitions in Mass Rotifer Cultures for Cold Water Marine Larviculture

By Dr. Terry Snell, Science Advisor,  Sustainable Nutrition Inc. 

Hatcheries targeting cold water species such as Cod and Ballan wrasse typically hatch larvae in water temperatures of 8-10°C and salinity of 35 ppt, mimicking the hatching temperatures and salinity they observe in nature. 

Both species suffer from very poor yields (as low as 5% hatch to harvest).  The majority of the failures are tied to poor larval survival rates while on live rotifer feeds.  After discussions with Cod and Ballan wrasse hatcheries, we have concluded rotifer cultures and practices are the likely culprit behind the poor yields.

Four Observations:

  1. There is little understanding of which rotifer species are actually being amplified in mass cultures.
  2. Rotifers are typically moved from their mass culture temperature and salinity into the larval tanks with quite different temperatures and salinities, causing the rotifers temperature and osmotic shock.  Little is known about what happens to rotifer mobility or their residence time in the larval tank prior to being eaten, hence how much nutrition is left in their gut.
  3. Most fish hatcheries are producing rotifers in open, indoor systems with common air flows with the main hatchery. This makes it impossible to control bacteria, protozoa and algae contaminants that reduce rotifer production and elicit periodic culture crashes.
  4. Rotifers are typically mass cultured using algae only diets, yet rotifers are indiscriminate filter feeders, consuming a variety of particles in the 5-10 µm range.


Species Selection

There are some marine Brachionus species that grow well at 8-10 °C, but little is known about their mobility and nutritional profile at low temperature, or their suitability for mass culture. 

We offer cysts of 15 brachionid species from fresh, brackish and salt-water sources from around the world.  Our rotifer cysts are free of contamination from other rotifer species and ciliates and are well characterized as adults for body size and population growth rate under standard conditions. This allows hatchery mangers to match the type of rotifer cultured to the mouth gape of their larval predator.  All of these rotifer species have been domesticated for aquaculture and have superior mass culture performance as compared to rotifers collected from wild populations.

Temperature and Salinity Transitions

Rotifer mass cultures are typically amplified at 25°C and salinity of 15-20 ppt.  Little is known about what happens to rotifers when they are transferred to larval rearing tanks at 8-10°C and 35 ppt salinity levels.  We know that rotifers typically survive this transfer, but not how long their mobility is suppressed or how these conditions affect rotifer gut evacuation rates. Rotifer mobility in larval tanks is critical because it affects larval predator-rotifer prey encounter rate and gut evacuation rate affects the nutritional quality of ingested rotifers.

We have observed that brachionid rotifers can be transferred to very low salinity water with minimal osmotic shock as long as the receiving water has high alkalinity (>300 PPM). Here we have seen that rotifers continue to swim vigorously for about two hours, by which time they are mostly consumed by larval predators.

We also find that we can ship and store rotifers chilled in water as low as 3°C, where they can survive for a few weeks. Further guidance on managing temperature and salinity transitions can be found at: Fielder DS, Purser GJ, Battaglene SC (2000) Effect of rapid changes in temperature and salinity on availability of the rotifers Brachionus rotundiformis and Brachionus plicatilis. Aquaculture 189:85–99. 

Toss out open rotifer mass culture systems

Operating open rotifer mass culture systems with common air flows with the main hatchery is a recipe for contamination. Best practice is to either isolate your rotifer mass cultures into a clean room or to use closed sterile bags in an open room.  We do the latter, growing algae and rotifers in sterile 1000L bags, prepared with RO water with commercial sea salts, passed through UV treatment, and aerated with blowers using Heppa filters. Either way, clean algae and rotifer innoculants are essential for producing reliable rotifer mass cultures. You can’t deliver consistent rotifer production if you are growing rotifers in contaminated cultures.

Shift from Algae only to Omnivore Diets

Since brachionids are indiscriminate filter feeders, they should be considered omnivores rather than herbivores. They are capable of ingesting particles of about 10 microns diameter. We patented and use a proprietary process to make a highly nutritious rotifer feed, Amplifeed Replete, which is less than 8 microns and very slow sinking. It includes lipid rich, nano-emulsified astaxanthin, DHA, folate, selenium, and several vitamins, as well as a variety of plant and animal proteins. Experiments have shown that Brachionus fed a combination of the green algae Tetraselmis plus Amplifeed Replete make more eggs, reproduce faster, and, when fed to clownfish larvae, produce 40% larger larvae at 42 days compared to rotifers fed without Amplifeed Replete enrichment.