Rotifers must be healthy and vigorous:
- The culture needs to be healthy and strong to remain stable.
- The rotifers must be healthy enough to tolerate aggressive enrichment. Enrichment is often a source of significant stress for the rotifers and can result in significant numbers of rotifers that are dead, dirty, atrophied or have low mobility.
- After enrichment, rotifers are often placed in storage for many hours. Only the most vigorous rotifers can withstand the stress of both enrichment and storage.
- After all these stresses, the rotifers need to be strong and motile so that they can be of value in the larval rearing tank.
- Further, although it is hard to document, it is plausible that strong, plump, healthy rotifers pass more nutrition, with less stress, to larval fish.
The qualities of healthy and strong rotifers are:
- A strong, fairly rapid swimming motion
- A healthy egg count (25-40% egg:rotifer ratio)
- Little of no lorica fouling
- Plump, round rotifers. The signs of rotifer wasting can be subtle, but when the lorica edges seem straight with angular transitions instead of being fully plump and smooth, this is a sign of rotifer stress.
- Full lorica cavity with minimal “clear lorica” area.