Effects of increasing salinity on oxygen consumption and ammonia excretion in juvenile scallops Argopecten purpuratus at two rearing temperatures

GASPAR SORIA; GERMÁN MERINO; ELISABETH VON BRAND

Roanoke, Virginia, USA

Conferencia; 6th International Conference on Recirculating Aquaculture; 2006

Argopecten purpuratus has a high commercial value and its aquaculture has increased during the last three decades. Success reached in its reproductive conditioning and spat production in hatchery has supported that activity. At present, A. purpuratus could be cultivated using Recirculating Aquaculture Systems (RAS) as a profitable method to increase production. From this point of view, the knowledge of the physiological response of scallops under different salinity and temperature conditions is important to deal with the cultivation of this species. Rates of oxygen consumption and ammonia excretion were determined in small juveniles (H= 6-9 mm) and large juveniles (H=24-27 mm) at the end of trials. Scallops were exposed in triplicates to three salinities (34, 38 y 42 g/L), at 16 and 22 ºC in batch system with water’s renewal every 24-48 h. Daily feed ration was Isochrysis galvana and Chaetoceros calcitrans (ratio 1:1) equivalent to 10 % of the scallop’s dry weight. Scallops were acclimated to different experimental conditions. Salinity, temperature and DO were daily measured. TA-N, NH3-N, NH4+-N and pH were weekly measured. Mean cumulative mortality was calculated. In both experiment, DO was higher than 90 % and NH3-N ranged from 0.005 to 0.02 mg/L at 16 ºC (0.25-1 NH4+-N) and ranged from 0.05 to 0.08 mg/L at 22 ºC (1,2-1,5 NH4+-N). Both NH3-N and NH4+-N declined with increasing salinity. Survival rate has shown a tendency to decrease with increasing temperature in both scallop groups. At the end of trials, small juveniles have shown higher mortality rate (around 60 %) than larger scallops (up to 15 %). In small scallops an increase in salinity from 34 to 42 at 16 ºC was followed by an increased survival. However, this relationship was not evident at 22 ºC. By the other hand, salinity did not affect survival of large juveniles. Oxygen consumption rate in both size scallops was not affected by salinity at the end of the trials. Small scallops have shown similar oxygen consumption rates at 16 and 22 ºC (between 0.68 to 0.91 ml O2/h*g PS). However, higher values were registered at 22 ºC in large juveniles. The significant decrease (up to 35 %) in oxygen consumption at 16 ºC was evident in 24 h-starved scallops, which have reduced their oxygen demand in absence of algae. However, standard and routine oxygen consumption rates (0.06 ml O2/h*g PS) were not significantly different at 22 ºC, suggesting that oxygen demand is high regardless of feeding condition. NH4+-N excretion rate was inversely related to salinity, suggesting that scallops regulate its osmotic equilibrium by breaking down proteins to form intracellular osmolytes. Small juveniles have shown a higher NH4+-N excretion rate at higher temperatures, although this pattern was not clear in large juveniles. Our experimental design can be viewed as a chronic exposure to NH3-N and NH4+-N levels. In both experiments, NH3-N concentration reached values highlighted as toxic. An increase in salinity produces a reduction in ionized ammonia proportion and under a hypersaline conditions scallops tend to decrease ammonia excretion as a mean of osmoregulation. Thus can given explanation to our findings of higher survival rates at 38 g/L and 42 g/L salinity regimes, in opposition to 34 g/L. In conclusion salinity tolerance for A. purpuratus juveniles is around 38 g/L. At 22 ºC, survival rate decreases significantly and ammonia excretion is higher. NH3-N concentration should be lower than 0.02-0.05 mg /L and NH4+-N below 1.25 mg/L in order to avoid negative effects on juveniles.