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Nannochloropsis oculata is a 2-4 micron green flagellate. This is a fast growing species that is easy tomaintain. This phytoplankton is the one most commonly thought of when the term green-water is used. This is a dark green alga with a thick tough cell wall that interestingly is readily consumed by rotifers. N. Oculata is high in overall omega-3 HUFAs (ranging from 16-42%), and while most of the HUFAs are composed of EPA, there is little DHA present. A growth study performed by Okauchi et al [Okauchi 1990] determined that the highest level of EPA was attained at 7 days after batch cultures were inoculated. N. oculata has been shown to contain very high levels of vitamin B12, which is critical for larval fish survival, and it has also been suggested that vitamin B12 is important for developing diseases resistance in larval fish as well.
Isochrysis galbana is a 4-7 golden-brown flagellate. This species is commonly used in bivalve culture (clams, oysters, etc). While it has been occasionally used as a single rotifer food, it is usually mixed with other phytoplankton such as chlorella or N. ocultus. The EPA levels range from 2-3.5% and DHA is 3.5-4%. Different strains of this species have varying levels of HUFAs, and one isolate found off Tahiti (commonly known as T-Iso) contains high DHA (8-11%) and low EPA (0.2-0.7%).
Tetraselmis sp. is a 9-14 motile green flagellate, which has been successfully used in outdoor ponds because it is extremely temperature tolerant. There are several species of Tetraselmis sp. that are available and one such T. tetrathele is frequently used in aquaculture. Studies have shown that while EPA (~ 5%) and DHA (~7%) levels in this phyto are theoretically sufficient, several authors has suggested that rotifers feed diets exclusively on T. tetrahele were not capable of sustaining fish larvae [Fukusho 1985, Wilkerson 1998]. To combat this deficit, aqua-culturists have fed mixtures of T. tetrahele with other phytoplankton species and discovered that these combinations were significantly more nutritious than those cultured alone. Of interest to hobbyists, Tetraselmis sp, produces two antibiotic-like compounds which have been documented to increase survival in larval fish feed on prey items enriched with this phytoplankton.
According to commercial experts, rarely is the use of a single phytoplankton suitable for aquaculture of fish larvae. Nutritional deficiencies found in one phytoplankton species can be compensated for by adding another phytoplankton species superior in that missing HUFA. As an example; N. oculata which is high in EPA, but low in DHA can be paired with T-Iso, which is high in DHA. Some hobbyists even add a small portion of Tetraselmis to this co-culture just to add an antibiotic effect. Studies performed in commercial fisheries have shown that fish larvae fed prey items enriched on diets composed of multiple phytoplankton species have higher survival rates and quicker growth rates than those larvae fed food items enriched with a single type of phytoplankton.
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