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

Individual dry weight and carbon content, ingestion rate, growth rate and p roduction rate were determined for different developmental stages of Artemi a 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 deg rees C and 34 ppt salinity. The individual body dry weight increased expone ntially 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. fr anciscana was strongly influenced by the food concentration. Minimum food c oncentration required for maximum growth was 10 mg C l(-1). At this food co ncentration the individual dry weigh increased from 2.3 mu g ind(-1) (newly hatched nauplii) to 195+/-7.03 mu g ind(-1) (mean+/-SE) when the animals h ad reached pre-adult stage (11 days growth). In the same period A. francisc ana grown at lower food concentrations, 7, 5 and 3 mg C l(-1), reached 134/-3.41, 88+/-3.53 and 29+/-3.09 mu g (mean+/-SE), respectively. At the lowe st 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 (mu, day(-1)) and the inges tion 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 fo od concentration and reached a maximum level at an algal concentration of 1 0 mg C l(-1). The specific growth rate and the ingestion rate were not infl uenced by an increase of the food concentration above 10 mg C l(-1). At the se food concentrations the specific growth rate showed a maximum value when the animals were in the range 6-51 mu g ind(-1) (mu(max) = 0.9-1.15 day(-1 )). The weight specific ingestion rate (I-w, h(-1)) also increased with inc reasing food concentration. A. franciscana in the range of 7-20 mu g ind(-1 ), grown at food concentrations greater than or equal to 10 mg C l(-1), ing ested on carbon basis approximately 20-28% of the individual carbon content per hour (I-w = 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 ingest ion rate varied between 0.05 and 0.1 h(-1). The production rate (P-r, mu g C ind(-1) day(-1)) showed a distinct maximum for animals grown at the two h ighest food concentrations. When individual dry weight exceeded 6 mu g ind( -1) (post-metanauplius II) animal size affected both the production rate an d the relative production rate (%, day(-1)). Thereafter the food concentrat ion 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 concent rations (3, 5 and 7 mg C l(-1)) attained a yield of 0.50-0.65 at post-metan auplius stage III, and remained at this level until post-metanauplius stage VII (dry weight; 10-50 mu g ind(-1)). Animals at the same size but fed at the highest food concentrations (10 and 20 mg C l(-1)), showed a yield clos e to 0.15-0. 20 at post-metanauplius stage III increasing to 0.50 at post-metanauplius s tage 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. (C) 1999 Elsevier Sci ence B.V. All rights reserved.