Sea bass and sea bream aquaculture is an important source of income in many Mediterranean regions, especially in Greece, Turkey and Spain, which represent 80% of their entire production volume.
Over the last years, the total annual production of these species has levelled at 300,000 MT. Aquaculture in Greece is considered the 2nd most important key economic sector for the growth of Greek economy. Other species, such as meagre and sharpsnout sea bream, are new promising candidates for so-called multi-species cultivation in Mediterranean fish farms.
Grow-out farming in the largest producing countries takes place mainly in floating sea cages, while around 15% of the total volume is produced in land-based pond systems in other countries, e.g. in Portugal and Italy.
The importance of Dissolved
Sub-optimal dissolved oxygen (DO) concentration is a potential problem both in cages and ponds, and the risk of harmful deficit is generally highest in late summer – early autumn at water temperatures above 25 – 27°C. These species seem to cope rather well to low DO (hypoxia) compared to cold-water species, but long-term tests with sea bass has demonstrated reduced appetite and growth at DO saturation below 80% even at a temperature as low as 22°C. At increasing temperature in August - October, so-called early morning DO deficit, sometimes results in critical conditions (e.g. fish kill). A well-known contributing factor to such critical situations is the rising oxygen consumption in fish at increasing temperature.
Reported problems with sub-optimal and even critical DO in Mediterranean fish farms was the basis for a performed project applying aeration in order to better control DO.
A research project both incorporating research institutions and commercial partners in Greece (Universities of Patras & Thessaly, Zervas-Kyriazis fish farm), Portugal (IPMA, Atlantik Fish, F.Ribero Lda.), Italy (RefaMed) and in Norway (OxyVision, TI, IRIS) has been carried out over the last two years. The project was funded from the 7th Framework Programme of the European Union and conducted by OV – OxyVision Ltd. (project leader: Martin Gausen).
Among the main objectives are optimization of diffuser-based aeration, mapping of DO deficit problems in commercial farms, and comparative tests of cost – benefit employing aeration technology (OV) in ponds and cages. Additionally, the lacking knowledge of effects of DO deficit and fish density in sea bream culture has been compensated for by performed tests at IPMA.
Efficiency of diffusers
Numerous combinations of diffuser hoses with regard to structure/material, pre-treatment, puncture density, inner and outer hose combinations, etc. were tested in an experimental tank at OV’s lab (Fig. 1). The decisive parameter, Standard aeration efficiency (SAE), was determined in all tests and the most efficient diffuser hose was selected for further in-field tests at the facilities in Greece and Portugal.
The best in test diffuser hose attained a SAE-value of 2.7 kg O2 transferred from air to water per KWh of electricity consumed. Compared to reported efficiencies of other types of commonly applied aerators in aquaculture (paddle wheels, propeller-aspirator-pumps) the achieved SAE-value was most promising. Not least, the efficiency was clearly higher than reported from other conducted diffuser tests in the 1980’s.
Risk of DO deficit in sea cages
Aeration is common practice and considered vital in many land-based culture operations. Traditional sea cage culture, however, still relies on natural water exchange replacing oxygen consumed by the fish stock.
In spite of episodic fish kills due to critical DO drops, there are few reports available describing such episodes in Mediterranean cage farms. As a part of the project, monitoring of DO and water exchange has been performed at one farm site in Greece (Fig. 2). The oxygen conditions at this locality is primarily dependent on the rhythm of the tidal water, but the diurnal algal activity will also play an important role in the process. Especially at the turn of the tidal flow, respiration of the fish stock may cause serious DO deficits in the stagnant water column in the cages, as demonstrated at 10-11 AM on 16 November (Fig. 2). Improved DO control by aeration and thus stabilizing the concentration above 60% of saturation would be a significant attempt to optimize the conditions for the stock.
Diffuser based aeration tests in sea cages and earthen ponds
In the salmonid culture sector, oxygenation by injection of pure oxygen gas in fish cages is a common practice that has been proven successful, leading to desirable conditions and increased profitability in intensively run salmonids cage culture. A potentially cheaper alternative for more ‘robust’ species is to add oxygen by air injection (aeration). Thus, aeration is often preferred for oxygen control in semi-intensive land-based pond culture of marine warm water species, including shrimp culture, where air is supplied more or less continuously.
Surface aerators, such as paddle wheels and propeller-aspirator pumps, are commonly used aeration systems in earthen ponds. However, it is also possible to use diffuser-based systems that inject fine gas bubbles, using blowers that supply air at low pressure. In large earthen ponds and fish cages, where oxygen has to be distributed over a large area, usage of submerged diffusers is advantageous because of the large interface between injected air and the water body as the fine air bubbles are slowly rising to the surface.
Several prototypes have been developed as a result of continual improvements throughout the project period. The first technical tests were performed in canvas lined raceway tanks (Sardinia, Italy) in spring 2013. These tests were succeeded by aeration of earthen ponds at IPMA’s research station (Algarve, Portugal) and of sea cages at a commercial farm (Greece) in summer and autumn 2013. The tests resulted in updated diffuser layouts for commercial-scale trials initiated during the summer 2014.
The earth ponds were stocked with both meagre and sea bream, whilst the sea cages were stocked with either sea bass, or sea bream. Parameters, such as DO and total gas pressure (TGP) in the water column, and pond sediment accumulation and characteristics, were routinely monitored. The results from small-scale tests of the AirX diffuser indicated oxygen transfer efficiency up to 20% at oxygen levels of 70 – 80% DO saturation in a seawater tank. Moreover, in waters where stratification occurs, oxygen levels can be even lower at the diffuser’s depth, which might increase the efficiency. Thus, it was estimated that the system would be able to control the running DO concentration above 70 – 80% of saturation assuming sufficient air supply.
However, when assessing the preliminary results from the ongoing pond trials conducted in late summer 2014, AirX diffusers and paddle wheels seemed to perform quite equally, with DO levels frequently fluctuating between 50 – 85% of saturation in the morning. This was clearly not a satisfying result and the setup was rearranged. After this modification, the AirX pond stabilized at 60 – 75% DO saturation, whilst the control pond applied paddle wheels still fluctuated between < 50% and 70% DO. This indicated that the AirX was able to maintain a more constant DO concentration in the pond compared to the paddle wheels, although still not able to transfer the intended amount of oxygen into the water.
There are still many identified aspects that should be adjusted to optimize the system for commercial conditions and these aspects will be further investigated. Firstly, an upgraded diffuser model will be introduced to increase the capacity of the system. Secondly, current creators will be applied in combination with the diffused aeration system to improve the oxygen transfer rate by moving water low in oxygen across the bubble curtains. Thirdly, the air injection should also proceed throughout the day also at low oxygen consumption of the fish stock, but only at a low rate to ensure that the diffusers are always pressured with air and not soaked with water.
In sea cages, it is possible to inject air at larger depths than in the earth ponds. This will increase the contact time between air and water, and thus increase the oxygen transfer. However, care must be taken when injecting pressurized air at large depths to avoid gas super-saturation which may result in potential harmful conditions for the fish. It is therefore important to select appropriate injection depth and practice. Moreover, it is highly recommended to monitor the total gas pressure (TGP) when aerating continuously at large depths (> 3 - 4 m).
The aeration system has been running at the Greek fish farm for several months, injecting air at 4 - 5 m depth without any indications of negative effects for the fish or signs of elevated TGP. The diffuser system managed to lift the mean DO in the cage by some 8% during August, even though the oxygen levels were very high (85 - 90 % of saturation).
Currently tests results are being analyzed to find out whether the aeration has resulted in improved performance of the fish stock, such as improved feed utilization, and to assess the costs of the aeration attempt (kg O2/kWh, kWh/kg fish produced).
The rather low fish density in the involved ponds and cages (earth ponds: 1 - 3 kg m-3, sea cages: 6 - 12 kg m-3) due to limited access to fish stock has turned out to be a challenge. Installation of aeration facilities is primarily a potential attempt for fish farmers who are planning to intensify their production. Thus, any interested party who wishes to cooperate in tests with higher fish densities, especially in ponds, are encouraged to make contact.
Small-scale tests with gilthead sea bream
Detailed studies were performed in indoor tanks at IPMA to improve the knowledge base about the required DO control in sea bream aquaculture. In brief, lower than 80% of DO saturation reduced growth rate and feed utilization, and also indicated higher haematocrit levels. Thus, DO should be kept above this limit throughout the production cycle in order to utilize the production potential of sea bream and maintain the fish’s welfare.
Within a fish density range of 5 - 20 kg/m-3, no significant differences were found with regard to performance of sea bream.
The outcome of these studies will be published in a scientific journal.
1: OxyVision, 2: TI, 3: IPMA, 4: Univ. of Patras, 5: Univ. of Thessaly, 6: NOAA, 7: IRIS
This work was supported by the 7th Framework Programme of the European Union. Project title: Oxygenation by efficient air diffusion system for aquaculture farms (AirX). Grant agreement no. 315412