PO Box 5, Hawthorne, CA 90250 USA

Tel: 310-973-5275 / Fax: 310-676-9387

email: inquiries@aquafauna.com

 

 

Use of Spray-dried Schizochytrium sp. as a 

Partial Algal Replacement for Juvenile Bivalves.

Phil Boeing

Pacific Seafarms International S.A. de C.V., Cerro Escondido #122, Mazatlan, Sinaloa, Mexico.

 

 

ABSTRACT

Two commercially important species of juvenile bivalves (Tapes semidecussata and Crassostrea gigas) were fed a spray-dried preparation of heterotrophically grown algae, Schizochytrium sp. in 20 L recirculating downwell systems. All systems were fed a weekly ration equal to 50% of their live weight as dry weight calculation for live algae or percentage Schizochytrium sp. substitute. Weekly growth rates of juveniles as live wet weight increase were measured to determine the effects of the different diets. Two experiments were performed in duplicate, the first using Tetraselmis suecica, a live algae control of moderate nutritional value, and the second using an equal mixture of Chaetocerous sp. and Tetraselmis suecica, a live algae control of high nutritional value. In the first experiment, 40% and 80% substitution for live algae Tetraselmis suecica were compared to controls fed 100% live Tetraselmis suecica. Significantly higher growth over the control was obtained with 40% substitution in Crassostrea gigas and 40% and 80% substitution in Tapes semidecussata. For the second experiment, 40% and 80% substitution for equal portions of live algae Chaetocerous sp. and T. suecica mixture were compared to controls fed 100% live algae mixture. Significantly lower growth rates were found for Crassostrea gigas at both 40% and 80% substitution. However no significant growth difference was found for Tapes semidecussata at 40% substitution, in contrast to a significantly lower growth over the control at 80% substitution. These results suggest that the routine use of Schizochytrium sp. as a partial replacement of live algae in bivalve culture to be economically viable, depending upon the unit production cost of the live algae for any given nursery facility.

Nursery culture of bivalve molluscs is a useful step for improving required growout time and survival in temperate climates (Anderson and Chew 1982; Bayes 1991). Algal systems for feeding bivalve nurseries represent a major production cost factor for shellfish enterprises (Coutteau and Sorgeloos 1992; Laing and Millican 1992; Meyers and Boisvert 1990; Walsh 1987).

Artificial and replacement diets for bivalve molluscs have been developed and extensively evaluated (Albentosa et al. 1989; Boeing 1986; 1992; Jones et al. 1974; 1993; Langdon et al. 1985; Langdon and Siegfried 1984; Langdon and Waldock 1981; Urban and Langdon 1984). Apparently none of these diets has shown sufficient cost-effectiveness to be commercially practical. The possibility of using heterotrophically grown spray-dried algae as a partial or total replacement for live algae has also seen considerable research, although with only a few algae species (Curtolo et al. 1993; Helm and Hancock 1990; Hidu and Ukeles 1962; Laing et al. 1990; Laing and Millican 1991; 1992; Laing and Verdugo 1991). Despite reported early successes with heterotrophically grown spray-dried algae species such as Tetraselmis suecica and Cyclotella cryptica, several investigators have pointed out the need for more nutritionally valuable species to be made available for the bivalve industry (Laing and Verdugo 1991; Jones et al. 1993). A recently commercialized heterotrophically grown spray-dried marine algae, Schizochytrium sp. (ALGAMAC-2000, Aquafauna Bio-Marine, Los Angeles, CA.) has shown very good acceptability and performance against controls when used at up to 70% algal replacement for penaeid shrimp (Boeing 1996; Rosenberry 1996). This product has nutritional qualities which would seem to make it an excellent candidate for algal substitution in other aquatic species such as bivalves and fish.

The present paper describes some preliminary results of substituting Schizochytrium sp. for live algae Tetraselmis suecica and Chaetoceros sp. in the nursery culture of juvenile Crassostrea gigas and Tapes semidecussata.

Top of Page

 

MATERIALS AND METHODS

Lots of 2-4 mm Tapes semidecussata and 4-6 mm Crassostrea gigas from the same cohort were used in the experiments. Two independent trials for both species were set up and carried out in duplicate. For both experiments animals were dried on absorbent paper and lots of 1.5 g and 2.5 g were weighed out for Tapes semidecussata and Crassostrea gigas respectively for the first experiment and 2.5 g of each for the second experiment. Weighing was carried out on an electrobalance to an accuracy of 0.1 mg. Daily growth rate (DGR) of the bivalves was calculated as a function of increase in wet live weight per seven day sampling period from the equation:

DGR = (ln final weight- ln initial weight) / 7

Culture containers and methodology were identical for both experiments with the exception of the live diet species. The juvenile bivalves were grown in 20 L recirculating downwelling systems, juveniles were kept in 30 cm diameter circular plastic 750 µ m mesh screen containers suspended in 20 L buckets with exterior air lift circulators fitted. Flow rate to each container was maintained at 4 L/min. The temperature and salinity were maintained at 25 C and 34 parts per thousand (ppt) respectively. The seawater in the containers was completely changed daily with seawater filtered to 5 mm and maintained at 25 C. Live algae Tetraselmis suecica and Chaetoceros sp. were grown in semi-continuous cultures, in Guillard´s enriched f/2 medium with sterilized seawater filtered to 1 mm. Cultures were harvested every day in the exponential phase of growth in volumes dependent on the division rates of the algae.

Tests were run using a feed ration calculated at the beginning of each week from the live wet weight biomass of each container as a 50% dry weight of food to wet weight biomass of bivalves per week. The daily ration was one seventh of this value fed in two doses, one in the morning and one eight hours later. For the live algal diets the volume of culture fed was taken from the cell density of the algae culture calculated from hemacytometer counts. The dry weights of the algae were calculated by filtering a measured volume of algal culture of known cell density through Whatman GF/C 57 mm glass fibre filter discs, washing with 4% ammonium formate solution, then drying to a constant weight at 65 C. The dry cell weights were found to be 200 pg/cell for Tetraselmis suecica and 120 pg/cell for Chaetoceros sp. In the first experiment the live algae component used was Tetraselmis suecica, in the second experiment the live algae component used was an equal mixture by dry cells weights of Tetraselmis suecica and Chaetoceros sp. The required amount of the spray-dried algae for the substitution of the live diets was first weighed then mixed with filtered seawater for 30 sec. in a domestic blender.

Top of Page

RESULTS

The results of the first experiment show that Schizochytrium sp. is capable of sustaining significantly better growth than controls in juvenile T. semidecussata at up to 80% algae substitution when Tetraselmis suecica was the only live algae feed (Table 1). Significantly higher growth over T. suecica fed controls was also obtained with C. gigas juveniles at the 40% substitution (Table 2). Although T. suecica has been noted as a moderate to good food for juvenile bivalves (Laing and Verdugo 1991), our experiments show this species, cultured under our conditions, to be of poor nutritional quality as indicated by the slow growth of the control animals (Fig. 1). Other investigators have found T. suecica to have a low food value in bivalve feeding experiments possibly due to the presence of a rigid cell wall which may complicate digestion (Cordero and Voltolino 1994).

The growth rate of both C. gigas and T. semidecussata controls increased by a factor of 6.3 with the mixed T. suecica and Chaetocerous sp. diet in the second experiment over the T. suecica alone diet in the first experiment (Fig. 2). The growth rate of T. semidecussata at the 40% Schizochytrium sp. replacement was not significantly different than control, however, all other levels of replacement for both species were significantly lower than controls (Tables 3 & 4). Growth rates of both T. semidecussata and C. gigas at the 40% substitution were significantly better than with the 80% replacement. Growth of T. semidecussata at the 40%. replacement was 86% of the control while the growth rate of C. gigas was 73% of the mixed diet control.

Top of Page

Top of Page