The standard method for growing the early stages of juvenile Pacific oysters is to hold them in systems called upwellers in land-based nurseries. Seawater is pumped through to provide the oysters with food particles. Growth rates of oysters cultured using this method were highly variable at Pipe Clay Lagoon, one of the major oyster nurseries sites in Tasmania. Growth rates during the 1996/97 production season were less than one-third of that seen in the five previous seasons, and were also significantly less than at another oyster nursery - Little Swanport.

We conducted 15 trials at Pipe Clay Lagoon to assess whether the oysters' growth rates could by improved by "supplementing" their natural diet with additional feed sources. These supplementary diets included cultured microalgae, dried or concentrated microalgae and a yeast-based artificial diet. The results were variable, depending on the diet, its concentration, and the season - though across all trials we found that supplementary feeding (on average) increased the oysters' growth rate by 60%. Diets that were most effective included the microalgae Isochrysis sp. (T.ISO), Chaetoceros calcitrans, Dunaliella tertiolecta, Rhodomonas salina and microalgal concentrates of Chaetoceros calcitrans and Skeletonema costatum. Two commercial "off-the-shelf' products - Microfeast® MB-30 and Algamac 2000 - were also effective, though not as good as microalgae. Nevertheless the cheaper cost of these commercial products (~AUS $80-100 kg1 dry weight) compared to microalgae (eg. AUS $ 375 kg-1) makes them viable alternatives to microalgae for supplementary feeding.

Supplementary feeding was most effective when natural levels of food (especially microalgae) in the inflowing seawater were low. For example, during one such 7 week period, supplementary feeding improved the growth rate of oysters by 3-fold.
Preliminary cost/benefit analysis of supplementary feeding was undertaken. Major factors include the cost of producing microalgae (dependent on scale and productions rates of the microalgae which vary from site to site, and seasonally) and the growth rates of non-supplementary fed oysters. The latter have a major influence on whether significant growth increases are possible through supplementary feeding, and were shown to vary significantly seasonally and from site-to-site. However, based on "average" microalgal production costs and the growth rates seen with supplementary feeding, we estimate that the direct additional feed costs would amount to $0.35 per thousand oysters to grow them from 0.5 to 3.0 mm. This compares to the total production cost of ~$15 per thousand oysters of 5 mm size. We believe the increased feeding costs for the nurseries would be more than offset by savings due to a reduced nursery time for the spat (less labour).

Supplementary feeding is probably restricted to use at sites like Pipe Clay Lagoon where oyster growth rates are reduced as a result of low or variable availability of food particles. There may be little benefit with supplementary feeding at Little Swanport, where the natural growth rates of oysters exceeded those of supplementary-fed oysters at Pipe Clay Lagoon. Nevertheless, we have demonstrated that supplementary feeding is an effective method for significantly enhancing growth rates of oysters at sites when natural productivity is otherwise low, providing the ability to have better control over juvenile oyster production. As a result, Shellfish Culture now plan to incorporate supplementary feeding as part of the routine production for juvenile oysters at Pipe Clay Lagoon.