To provide an alternative to the use of oxygen cylinders in live transport bins, Seafood Transportation Developments (STD) initiated development of an aeration unit which could be fitted to the live bins. The system provides oxygen to the live fish by air diffusion at a high flow rate through an air diffuser attached in the bottom of the bin. Initial export shipments of live fish using the STD system were very successful. The company had obtained air diffuser pipe which appeared equal to the demand of supplying sufficient oxygen into the water. Then however, high mortalities were reported for transport of coral trout.
Upon thorough investigation of all potential circumstances that may be responsible for the unsuccessful shipments, the only obvious difference seemed to be the reordered supply of air diffuser piping used in the bins. This research work assessed the aeration efficiencies of several air diffuser pipes with different properties and characteristics.
Airflow rates and bubble size appear to have correlation with type of diffuser piping rather than whether the specific diffuser had been previously used or not. Airflow rates were similar over short initial aeration use. The efficiency of the fine grade diffuser decreased significantly over a period equivalent to that for shipping live product to Hong Kong, for example. The coarse grade diffuser was not robust enough for the purpose of use.
The 'original' air diffuser piping supplied during development of the transport bins, even though previously used for live transport, retained acceptably high aeration rates. This efficiency decreased slightly following a continuous 96h run, however it is unlikely that this period would be demanded of a bin used in a commercial operation.
The new supply of air diffuser piping was shown to be less efficient than that of the 'original' supply. The efficiency of a previously used air diffuser made from the reordered supply was low, taking greater than double the time to cause 100% 02 saturation of the water. This air diffuser had been used for a shipment that resulted in very high mortalities of the fish. Additionally, an uneven distribution of air bubbles was observed throughout the water column during operation of this air diffuser. Unused air diffusers that were trialed also gave variable aeration efficiencies.
The answer to the question posed as basis for this research is: YES
The specific properties of piping used for air diffusion within live transport bins are crucial with respect to aeration efficiency. Of the types of diffuser piping trialed in this work only one, that supplied from England, demonstrated appropriate characteristics for multiple re-use within live transport systems. This information remains essential to the live fish export industry as aeration system transport bins are still being manufactured .
Unfortunately, this work was carried out retrospectively in reaction to huge revenue losses by industry. If investigations, similar to this one, were conducted during development of 'new' technologies, negative Australian industry image could be prevented.
