Some aspects of the productivity of Lake Grassmere, Malborough, New Zealand and its possible utilisation. (1974)
AuthorsKnight, Grantshow all
The growth of Artemia salina Branchiopoda, Anostraca in the concentrating ponds of a solar salt extraction plant was investigated during 1972/63. The aim of the study was to assess the possibility of harvesting the Artemia production of the ponds. A pronounced seasonal fluctuation in Artemia density in ponds with over 20,000/m³ at the highest had a spring maximum and a later summer minimum. The spring upsurge was found to come from overwintering adults. The proportion of females carrying eggs ranged from 98% in spring down to 2% in winter but the number of eggs produced per female showed a highly significant variation with salinity (over a 30 day period 231 eggs at 100 ppt and 89 eggs at 250 ppt). By counting the number of eggs present in mature females a sharp drop was shown to occur at salinities above 200 ppt. Hatching times and the percentage of eggs laid that do not hatch were shown to be greatly influenced by salinity and temperature. At 26ºC hatching times at 30 ppt salinity and 240 ppt were 2 and 5 days respectively. However, at 240 ppt less than 5% hatched. In contrast, at 7ºC hatching times at 30 ppt and 150 ppt (max. at which eggs hatched) were 8 and 22 days respectively. It was concluded that increasing temperature reduced the inhibitory effect of higher salinity. Laboratory experiments showed that longevity of Artemia was greatest at 17ºC and decreased at 7ºC and 26ºC but life and fertility tables constructed from the data showed a much higher fertility rate at 26ºC than 17ºC (rc = 0.305 and 0.207 respectively) and it was considered that the increased at 26ºC outweighed the shorter life span when considering production of Artemia. Feeding trials using the unicellular flagellate alga Dunaliella euchlora showed that in a suspension of 4000 cells/ml adult Artemia assimilated 57% of ingested algae but as the density of algae was increased to 10,000/ml the efficiency of assimilation fell to 42%. Similar trials with juveniles (2 – 4 mm) showed a lower efficiency (54%) at 4000 cells/ml but higher (47%) at 10,000 cells/ml. Lipid content of Artemia ranged from 18.3% during the spring algal peak down to 11% in autumn. Both swimming rate and feeding efficiency were found to be significantly affected by increasing viscosity as brine concentration increased. The swimming effort was found to increase by 33.7% between 60 ppt salinity and 300 ppt. A highly significant decrease in food uptake was found over the same salinities with consumption of algal cells in laboratory experiments falling from 26.2% of that available to 21.6%. The combination of increased effort and reduced food intake was considered to be partially responsible for the reduced vigour of Artemia populations at high salinities. Experimental laboratory populations of Artemia reacted strongly when the algal growth rate in the culture was stimulated by fertilising and the addition of dried yeast. Stable populations of the equivalent of 860,000/m³ were produced in the laboratory. These were easily maintained and showed no evidence of metabolite poisoning or lack of oxygen. Production of the Artemia in the ponds showed that the maximum reached in a pond with an annual range of salinity from 111 ppt to 188 ppt (winter and summer respectively) and decreased in other ponds of higher values. Mean daily production for the 14 month sampling period was 0.4111 (g/m³)/day with a range from 1.53 (g/m³)/day in spring to 0.006 (g/m³)/day in autumn. Equivalent values for the most aline pond in the series were 0.049 (g/m³)/day, 0.24 (g/m³)/day and 0.003 (g/m³)/day respectively. The very large range of salinity, caused by saltworks production methods, were considered to be very detrimental to efficient harvesting where a continuous production of Artemia may be needed. The study of the Artemia production was supported by the determination of water chemistry in the ponds and algal production over the study period. Nutrients selected for analyses were reactive nitrate, reactive silicate and orthophosphate. Nitrate levels ranged from 0.43 g/m³ to 0.07g/m³ with a winter peak followed by a sharp spring drop and a summer low point. A similar pattern was found for orthophosphate concentrations but with a much larger range from 0.619 g/m³ in winter to trace amounts during summer. Silicate levels varied directly with salinity and no evidence of high biological consumption was found. The sediment of ponds at salinities over 150 ppt was cemented by precipitated calcium sulphate preventing development of turbidity even with strong wave action. pH of the ponds varied between 8.0 and 8.5 but the onset of high spring algal growth was accompanied by a drop in pH that could not be readily explained. It may be due to strong bacterial growth. The algae were dominated by two species, Dunaliella euchlora and D. salina. D.salina was confined to brines that were almost saturated but D.euchlora grew throughout the series. Production of algae was estimated from laboratory incubator experiments. These suggested that primary production would be highest in salinities between 100 and 200 ppt. Growth trials of algae in artificial media showed that levels of reactive phosphate below 0.01 g/m³ imposed strong restrictions on growth. An annual budget of energy loss and gain was set up taking into account primary and secondary production and losses caused by Artemia respiration, faeces and egg production. Overall, for the most productive pond an ecological efficiency for conversion of algal energy into Artemia biomass of 19.1% was found. A brief review of species of commercially cultivated shrimp was given consistent with the long-term possibility of using the Artemia production as a food, or food supplement, for growing shrimp. Conclusions reached were that the crop of Artemia in the ponds, while they are being used to manufacture salt, is too sporadic to be easily harvested. The potential productivity in a carefully managed pond may be very high. Artemia can exist over a wide range of salinities but will perform well only within a restricted range.