ADC Report - Cultured copepods as food for West Australian Dhufish (Glaucosoma hebraicum)

An alternative strategy for using copepods to rear dhufish larvae is to add adult copepods directly to dhufish larviculture tanks. Algae is then added to these tanks to promote production of nauplii by these adult copepods. These nauplii are then predated by larval fish. This method of larviculture is similar to the standard greenwater larviculture techniques used at ADU with the main difference being the addition of adult copepods. This study aimed to develop techniques for incorporating copepods into greenwater larviculture tanks for successful rearing of dhufish larvae on a commercial scale. Six separate components of this project are summarised below.

The first trial examined the effect on copepod fecundity of two different species of algae. These algal species were considered to have potential for use in greenwater larviculture tanks that included copepods. Adult copepods were stocked at both high and low densities into greenwater systems comprising either Nannochloropsis oculata or Dunaliella tertiolecta. A small amount of the nutritious algae T-Iso was added to each tank daily. This alga is high in essential fatty acids and increases copepod fecundity. After eight days, the Dunaliella tank crashed, hence it was concluded that this alga was probably unsuitable for use in greenwater larviculture systems. In the tanks containing Nannochloropsis, a high density of nauplii was sustained. Nauplius density was greatest in those tanks stocked with a high density of adults. It was concluded that Nannochloropsis is suitable for use in copepod greenwater larviculture tanks and that nauplius production by adults stocked at a high density was sufficient to support high growth of dhufish larvae.

A second trial sought to compare growth and survival of dhufish larvae stocked into greenwater tanks (incorporating Nannochloropsis) with three different live food regimes. These regimes consisted of the daily addition of copepod nauplii and rotifers, adult copepod stocked once only at the start of the trial and adult copepods stocked at the start as well as daily addition of rotifers. Growth and survival were generally low in this trial and poor larval performance was attributed to inappropriate larval acclimation procedures, high initial light intensities, late stocking of adult copepods and low nauplius production from intensive cultures. Despite this, some survival of larvae that were placed into tanks in which only adult copepods had been stocked indicated the potential of copepod greenwater techniques.

Dhufish larvae were stocked into two large-scale greenwater systems. In the first system, relatively high densities of Nannochloropsis were maintained along with low supplementary feeding with a second algal species, T-Iso. The second system was maintained with low Nannochloropsis densities and high T-Iso supplementation. Adult copepods were stocked into both systems at the same time as larvae. In both systems, growth was moderate and survival was low. Examination of gut content showed that larvae had a clear preference for copepod nauplii at first feeding. Larger dhufish larvae appeared to predate rotifers only when few copepods were available. A substantial population of copepods developed in the second tank and dhufish larvae continued to predate these copepods as they grew. This high copepod population was attributed to the high level of T-Iso supplementation in this tank. Generally low survival rates were attributed to late stocking of adult copepods into the greenwater tanks. It was concluded that adult copepods must be stocked a number of days before larvae to enable a high density of nauplii to be available to larvae at first feeding. High copepod densities attained in the second tank and continued predation on these copepods by larvae further demonstrated the potential of copepod greenwater techniques.

Given poor success of the previous trials, it was decided to repeat a highly successful trial conducted last season (1999). The aim of this trail was to produce large numbers of dhufish juveniles for grow-out trials in the following year. Two larviculture tanks were established using similar protocols that were successful in the previous season. However, nauplius production from intensive cultures was low, hence rates of nauplius provision could not be replicated. Dhufish survival appeared high during the early part of the trial when nauplii were reasonably abundant. However, generally poor growth and survival corresponded to decreased nauplius supply. Low pH and DO were also recorded in this trial.

Poor nauplius production from intensive copepod cultures at ADU placed a major constraint on conducting dhufish trials during this project. Hence, this portion of the project examined methods for improving nauplius production. Poor copepod production at ADU was most often associated with either invasion by harpacticoid copepods from elsewhere in the facility or feeding with contaminated algae. To alleviate these problems, dedicated copepod and algal culture rooms were established. These rooms were supplied with filtered water and quarantined from the rest of the facility. In addition, the copepod culture regime was altered to include diluted seawater rather than full strength seawater. This effectively prevented invasion by harpacticoid copepods, as these copepods will only thrive in full strength seawater. The quarantined algal culture room enabled production of uncontaminated algae. Nauplius production from intensive cultures increased markedly with these improvements. A major contaminant of algal cultures at ADU was determined as a species of heterotrophic algae. In a controlled trial, copepod survival was reduced when fed this haptophyte compared to T-Iso.

At present, intensive copepod cultures at ADU are fed with the alga T-Iso. This alga does promote high nauplius production by adult copepods but it should not be used as the only food for long periods of time. Elsewhere in the world, other copepod species are being fed with cultured dinoflagellate species with great success. Thus a trial was conducted to determine if a dinoflagellate diet would increase nauplius production in copepod cultures at ADU. Small numbers of copepods were fed with the dinoflagellate Heterocapsa niei only, T-Iso only and a mixture of both. Copepod maturation time was fastest on a diet of Heterocapsa only, demonstrating that this dinoflagellate has the potential to improve productivity of copepod cultures. When Heterocapsa was first obtained, it required a growth media that was difficult and expensive to prepare. Over a gradual acclimation period lasting 10 months, a strain of Heterocapsa was produced that will grow on standard growth media. This makes culture of this alga more convenient and cheaper. Further work is required to determine the effect of this alga on copepod fecundity and nutritional content.

Unfortunately, this project did not fulfil its primary objective, which was to successfully incorporate copepods into commercial-scale larviculture practices for dhufish. This was an ambitious aim, considering that dhufish larvae were only available for approximately three months of this project. However, significant progress was made; a combination of algal species suitable for use in copepod greenwater tanks was identified, adult copepod stocking densities and the timing of stocking was determined and the preference for copepod nauplii by dhufish larvae was clearly established. Also, improved copepod culture techniques, including a new diet, were established at ADU with increased reliability and magnitude of nauplius production as a result. These achievements will greatly assist the establishment of copepod-greenwater systems for dhufish larviculture in the future.