pH Control in Recirculating Aquaculture Systems for Pāua (Haliotis iris)
In high intensity recirculated aquaculture systems (RAS), metabolic carbon dioxide can accumulate quickly and have a significant impact on the pH of the culture water. A reduction in growth rate and increased shell deformation have been observed in farmed abalone that has been attributed to reduced pH levels that occur in RAS due to accumulation of CO2 in the culture water. The overall aim of this research programme was to assess two methods of pH control (physical vs. chemical) used in land-based aquaculture systems for the culture of the New Zealand abalone, pāua. In the first study the efficiency of physical carbon dioxide removal from seawater using a cascade column degassing unit was considered. Hydraulic loading, counter current air flow, packing media height, and water temperature were manipulated with the goal of identifying the most effective column configuration for degassing. Three influent water treatments were tested between a range of pH 7.4 to 7.8 (~3.2 to 1.2 mg L-1 CO2 respectively). For all influent CO2 concentrations the resulting pH change between influent and effluent water (immediately post column) were very low, the most effective configuration removed enough CO2 to produce a net gain of only 0.2 of a pH unit. Manipulating water flow, counter current air flow and packing media height in the cascade column had only minor effects on removal efficiency when working in the range of pH 7.4 – 7.8. A secondary study was undertaken to examine the effects on pāua growth of adding chemicals to increase alkalinity. Industrial grade calcium hydroxide (Ca(OH)2) is currently used to raise pH in commercial pāua RAS, however it is unknown if the addition of buffering chemicals affects pāua growth. Replicate pāua tanks were fed with seawater buffered with either sodium hydroxide, food grade Ca(OH)2 or industrial grade Ca(OH)2, with the aim of identifying the effects of buffered seawater on the growth of juvenile pāua (~30 mm shell length). Growth rate ([micrometre]/day) was not significantly affected by the addition of buffering chemicals into the culture water, and the continued use of industrial grade Ca(OH)2 is recommended for the commercial production of pāua in RAS. Shell dissolution is observed in cultured pāua reared in low pH conditions, however there is limited information surrounding the direct effect of lowered pH on the rate of biomineralisation and shell dissolution in abalone. A preliminary investigation was undertaken to examine shell mineralogy, the rate of biomineralisation and shell dissolution of pāua grown at pH 7.6 and 7.9 to determine their sensitivity to lowered pH. It was found that the upper prismatic layer of juvenile pāua shell (~40 mm) was composed almost exclusively of the relatively stable polymorph calcite, that suggests pāua are relatively tolerant to a low pH environment, compared to other abalone species that have proportionately more soluble aragonite in their prismatic layer. Regardless of shell composition, significant shell dissolution was observed in pāua reared in water of pH 7.6. Over the duration of the trial, the rate of mineralisation ([micrometre]/day) was significantly different between pāua reared in pH 7.6 and in pH 7.9 water. However, after a period of acclimation, low pH did not appear to effect rate of mineralisation in pāua.