erectus fry

Dear Roxie:

Yes, providing you can keep the system from crashing, a greenwater nursery can be very helpful in getting pelagic seahorse fry a head start and increasing their survivorship during the high-risk pelagic phase of their development. A greenwater nursery is very difficult to keep in balance for extended periods, but if you just want to keep the greenwater tank going for a week or two, then it’s certainly a viable option, as discussed in more detail below:

The Greenwater “Starter” Nursery.

Basically, this system involves giving small numbers of handpicked fry a head start by raising them in a tank with a well-established greenwater culture for the crucial first week or two of their lives. A tank of greenwater is set up in a well-lit area and once the microalgae culture has taken off, it is seeded with copepods or rotifers. The microalgae acts as the filtration, utilizing nitrogenous wastes for growth. The idea is to provide a balanced system in microcosm with a self-sustaining food chain: the phytoplankton (microalgae culture) utilizes sunlight and nitrogenous wastes for growth and helps maintain water quality, while zooplankton (copepods or rotifers) feed on the microalgae and larger predators (seahorse fry) keep the ‘pod population in check. Additional greenwater and/or copepods or rotifers may be added periodically as needed to keep the nursery going.

In actual practice, it is difficult to for the home hobbyist to maintain the proper balance for any length of time before the greenwater culture or the population of copepods (or both) crashes. As a result, greenwater nurseries have limited applications and are useful primarily for the short-term rearing of small numbers of pelagic fry.

However, they can be quite helpful in giving pelagic seahorse fry a badly needed headstart. For example, once the hobbyist has culled a large brood of Hippocampus reidi fry down to a manageable number of the hardiest newborns, they can be introduced to a greenwater nursery where they can fatten up on tiny copepods or rotifers. This can help get the newborns through the first week or two of their lives until they are able to accept first-instar Artemia nauplii and can be transferred to a conventional nursery for further rearing.

The turbidity provided by the greenwater helps keep the phototactic fry evenly dispersed throughout the water column and away from the surface. Jorge Gomezjurado has been very successful rearing Hippocampus reidi and H. ingens fry at the National Aquarium in Baltimore using kreisel nurseries with the proper density of microalgae (i.e., greenwater). Jorge has found that turbidity is an important factor in the juvenile rearing environment for these species and he achieves the proper level of murkiness for optimum results by using algae (Nannochloropsis and Isochrysis) at a concentration of about 100 cells per ml (Bull and Mitchell 2002).
Best of luck with your greenwater nursery tank, Roxie! If you give it a try, please keep us updated and let us know how well it works for you.

Happy Trails!
Pete Giwojna