Effect of food concentration on the growth and production rate of Artemia franciscana feeding on algae (T. iso)

Abstract

Abstract Individual dry weight and carbon content, ingestion rate, growth rate and production rate were determined for different developmental stages of Artemia franciscana feeding on six different concentrations of Isochrysis galbana (Tahiti). The cultures were grown at nearly constant food concentrations, ranging from 0.2 to 20 mg C l−1 for 12 days at a temperature of 26–28°C and 34 ppt salinity. The individual body dry weight increased exponentially with age to a point of inflection before the growth rate gradually decreased when the animals reached the pre-adult stage. The growth of A. franciscana was strongly influenced by the food concentration. Minimum food concentration required for maximum growth was 10 mg C l−1. At this food concentration the individual dry weigh increased from 2.3 μg ind−1 (newly hatched nauplii) to 195±7.03 μg ind−1 (mean±SE) when the animals had reached pre-adult stage (11 days growth). In the same period A. franciscana grown at lower food concentrations, 7, 5 and 3 mg C l−1, reached 134±3.41, 88±3.53 and 29±3.09 μg (mean±SE), respectively. At the lowest food concentration tested (0.2 mg C l−1) the individual dry weight was reduced by 14–18% after 5 days of cultivation, indicating that the animals were starving. The carbon and nitrogen content (% of dry weight) of newly hatched nauplii was 45.5 and 10.1%, respectively. During the growth period the carbon content varied between 40.1 and 45.5%, and the nitrogen content between 9.1 and 10.5%. The specific growth rate (μ, day−1) and the ingestion rate (ng C ind−1 day−1) were strongly affected by the size of the animals and the food concentration. Both rates increased with increasing food concentration and reached a maximum level at an algal concentration of 10 mg C l−1. The specific growth rate and the ingestion rate were not influenced by an increase of the food concentration above 10 mg C l−1. At these food concentrations the specific growth rate showed a maximum value when the animals were in the range 6–51 μg ind−1 (μmax=0.9–1.15 day−1). The weight specific ingestion rate (IW, h−1) also increased with increasing food concentration. A. franciscana in the range of 7–20 μg ind−1, grown at food concentrations ≥10 mg C l−1, ingested on carbon basis approximately 20–28% of the individual carbon content per hour (IW=0.20–0.28 h−1). At the lower food concentrations (3 and 5 mg C l−1) the ingestion rate was reduced and the weight specific ingestion rate varied between 0.05 and 0.1 h−1. The production rate (Pr, μg C ind−1 day−1) showed a distinct maximum for animals grown at the two highest food concentrations. When individual dry weight exceeded 6 μg ind−1 (post-metanauplius II) animal size affected both the production rate and the relative production rate (%, day−1). Thereafter the food concentration had a more pronounced effect on the production rate of the animals. The conversion of ingested food into biomass (yield, day−1) was dependent on food concentration and animal size. Animals fed at the lowest food concentrations (3, 5 and 7 mg C l−1) attained a yield of 0.50–0.65 at post-metanauplius stage III, and remained at this level until post-metanauplius stage VII (dry weight; 10–50 μg ind−1). Animals at the same size but fed at the highest food concentrations (10 and 20 mg C l−1), showed a yield close to 0.15–0.20 at post-metanauplius stage III increasing to 0.50 at post-metanauplius stage VI–VII. In the older stages the yield decreased from a mean of 0.46 to 0.05–0.10 when the animals attained pre-adult stage.