15 DECEMBER 1999
The free-swimming rotifer of the genus Brachionus has been identified as pivotal for successful culture of a variety of marine and freshwater fish larvae. The rotifer is a popular live feed because of its small size (110-320 micro m), tolerance to a wide range of salinities, rapid reproduction rate, ability to be cultured at very high densities, slow swimming speed, and tendency to stay suspended in the water column. However, the nutritional value of the rotifer in its natural state has been found to be in most cases deficient for growth and survival of marine fish larvae. Until recently, the nutritional profile of the rotifer was largely determined by the food (e.g., phytoplankton) used to grow it. Phytoplankton species with a high Polyunsaturated Fatty Acid (PUFA) content resulted in a correspondingly more nutritious rotifer and thus the hatchery manager was limited to culturing a particular kind of phytoplankton. Hatchery operations were therefore dependent on continuous production of that particular phytoplankton. Approximately 60% of the space utilized by a hatchery can be accounted for the phytoplankton culture facilities alone. Development of other media which can boost the nutritional profile of rotifers now provides alternative strategies to how rotifers can be grown. A relatively new product is a patented organism, Schizochytrium sp., marketed by Aquafauna Biomarine, Inc. under the trade names AlgaMac-2000, 2010, and 3010 and under DOCOSA Gold by Sander's Brine Shrimp Company. In addition to boosting the fatty acid profiles of rotifers, it has an added benefit in that a higher density of rotifers can be enriched at one time in comparison to other enrichment media.
In a recently completed investigation, first feeding Pacific threadfin, Polydactylus sexfilis, larvae were fed unenriched rotifers or rotifers enriched with DOCOSA Gold and evaluated on the seventh day post-hatching. The evaluation consisted of suspending the larvae out of water for 15 seconds and then recording the survival one hour later. Because the larvae were so young, an "unstressed" control was also included in the testing to rule out effects of the netting process.
The results show a significantly lower ability to tolerate being physically handled in larvae that were fed rotifers without enrichment. The implications of this study is that the general health and well-being of the moi larvae have been significantly improved by the feeding of enriched rotifers as early as seven days post-hatching.
The impact of the development of methods to manipulate the nutritional profile of rotifers used in marine larviculture has led to a re-evaluation of hatchery facilities and rearing strategies. The hatchery manager can use whatever means is available for the culturing of rotifers as the nutritional constraint can now be circumvented and the hatchery manager can utilize a culture scenario more appropriate to his/her own situation and still achieve similar results. These findings place marine hatchery managers in an exciting era as there are now several options for culturing rotifers where once there were but one or two.
(article in University of Hawai'i Sea Grant College Program, Makai, Vol. 21, No. 9, Sept. 99, by C.S. Tamaru, H. Ako, V. Sato, S. Alexander)