Rotifers are among the easiest of prey organisms to grow, yet they seem to be an endless puzzle and challenge to even the most experienced live feeds managers. For decades researchers have sought alternatives to rotifer production (and live feeds in general), and they have made advances. Still, quality rotifer production continues to be a key element for most marine fish hatcheries.
Who uses rotifers?
• Finfish hatcheries
• Ornamental fish breeders
• Shrimp hatcheries
• Medical and toxicology research laboratories using larval fish
Rotifers are used as a first feed. The traditional feed progression for larval fish rearing is rotifer, artemia, weaning pelletized feeds and finally grow-out feeds. Many fish require rotifers until 20-30 days post hatch (dph), some fish only require rotifers for 2-5 dph, though they often do better when fed on rotifers for a longer period of time. A number of fish species do not require rotifers at all and progress straight to artemia or pelletized feeds.
One recent trend is eliminating artemia from the traditional process by extending feeding with rotifers and starting earlier with weaning diets. The advantage of this trend is that it simplifies the production process and reduces larval stress by reducing the number of feed transitions.
Why rotifers are important
Rotifers are important because they work!
Specific reasons why this is so can be broken down into three aspects:
Nutrition is ultimately the reason why live feeds managers use rotifers and it is a HUGE subject. See the next section B–Nutrition for more detail.
Mass production and ease of use
Rotifers can be grown at very high densities and generate enormous biomass using little space or other resources. A continuous culture system growing Brachionus plicatilis rotifers at a density of 5,500 per ml (an easy number to achieve) with a dry weight biomass of 185 ng per rotifer will have a rotifer density in excess of 1 gram dry weight (or 6.5 g wet weight) per liter of culture. At a 40% harvest rate a 10 ton system can churn out 25 Kg of screened rotifers (4 Kg dry weight) per day.
Rotifers can consume feed equivalent to 10% of their biomass per hour, and when using high-quality microalgal feeds, convert the algal biomass to rotifer biomass at an efficiency exceeding 30%. No other live feed can compete with rotifer production on a biomass basis.
Rotifers swim at a steady speed, following a straight or sinuous path, rapidly deflecting or reversing course when an obstruction is encountered. These swimming motions attract the attention of larval fish and stimulate feeding and are a key advantage of this live prey. However, the movement of rotifers is not ideal; larval fish feed on copepods (which swim with a jerking movement) much more actively. But for most fish the movement of rotifers is sufficient. Many fish, in fact, will feed on preserved rotifers and other prey if stimulated by the presence of live, swimming rotifers.
There are other feed stimulation factors that are not so well understood. Fish larvae have been reported to respond to rotifer extract. This is not surprising but research remains limited.
Rotifers range from 90ng to more than 500ng per rotifer dry weight (90 to 500mg dry weight per million rotifers). A 185ng rotifer corresponds to a lorica length of about 165 microns; about the size of a small S-type rotifers or an exceptionally small L-type rotifer. This size is used as an example because it is the size of Reed Mariculture’s “Mini-L 160” rotifer. There is a very constant ratio of dry weight and wet weight biomass (screened and blotted). Dry weight is 15.5% of wet weight. Dry weight biomass calculation is the best way to assess to the density of a culture.