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Stocking density

Stocking density is a very important factor that has to be decided in advance when designing a RAS. Stocking density can be defined in different ways (Table 2), and it is important to be aware when and why different definitions are being used.

Table 2: Stocking density definitions

Density of individuals Biomass density per surface (#/m2) per volume (#/m3) per surface (kg/m2) per volume (kg/m3) Independent of tank depth. Relevant for bottom-dwelling fish Is often high for small fishes even though the biomass density is higher Independent of tank depth. Relevant for bottom-dwelling fish. It is often higher for bigger fish than for smaller species Relevant for free swimming species

Different fish species have different possible stocking densities. Density is a central factor in determining fish welfare, although all the biological aspects are not clear yet. There are fish species that have different behaviour at different densities. For example, tilapia adopts schooling behaviour at high densities, and territorial behaviour at low densities. In order to prevent fish harming each other, they therefore have to be farmed at a certain density. To use space efficiently, and to prevent cannibalism, a fish tank should contain fish of approximately the same size. This means (a) that an aquaculture facility should have several tanks to house fish of different size classes, and (b) that the fish population has to be graded according to size occasionally, and redistributed into the tanks. Low and high stocking densities in aquaculture systems have several consequences for the management of a RAS (Table 3).

Table 3: Characteristics of high and low stocking density systems

Influencing factors for systems with the same annual production High Density Low Density Change in water parameters Fast change Slow change Response time (for example to a pump failure) Is shorter. More stress for the fishes Is longer. The system operation is safer Capacity of the fish tanks for a given production volume Less capacity needed for the same production volume Higher capacity needed. This can be compensated partly by using deeper basins. However, these are more expensive and need a more expensive pipe and pumping system Necessary circulation/displacement rate for a given production volume [m3/h] Same Same. Due to the slowness of the system, there are softer peaks = smaller components = less expensive hardware for water reconditioning Displacement volume relative to tank volume High Low Tank dimensions Smaller tanks with a high density of individuals are, depending on the species, more prone to stress In larger tanks, easily scared fishes have a longer escape distance


Monitoring procedures should be defined according to the steps outlined in Figure 10. RAS or aquaponic systems are complicated, and consist of many parts. Many things can go wrong, so the operators have to remain permanently alert (Table 4, see also Chapter 9). The top priority of system mangement is the health of the fish and plants. Therefore, monitoring should be priorised according to the 'life support priorities’ (Table 5). Table 6 lists important items that should be monitored on a daily basis.


Figure 10: The logical steps of devising a monitoring procedure

Table 4: What can go wrong?

Type / System Causes Beyond your control Floods, tornadoes, hurricanes, wind, snow, ice, storms, electrical outages, vandalism/theft Staff errors Operator errors, overlooked maintenance causing failure of backup systems or systems components, alarms deactivated Tank water level Drain valve left open, standpipe fallen or removed, leak in system, broken drain line, overflowing tank Water flow Valve shut or opened too far, pump failure, loss of suction head, intake screen blocked, pipe blocked, return pipe ruptures/breaks/glue failure Water quality Low dissolved oxygen, high CO2, supersaturated water supply, high or low temperature, high ammonia, nitrite or nitrate, low alkalinity Filters Channeling/clogged filters, excessive head loss Aeration system Blower motor overheating because of excessive back pressure, drive belt loose or broken, diffusers blocked or disconnected, leaks in supply lines

Table 5: Priorisation of monitoring and response

Parameter Response time Priority High Electrical power Water level Dissolved oxygen Very fast (minutes) Alarm needed! Medium Temperature Carbon dioxide pH Moderate response time (hours) Low Nitrogen forms (ammonia, nitrite, nitrate) Total suspended solids (TSS) Slowly changing parameters (daily or weekly monitoring)

Table 6: Important items that should be monitored daily

Electrical power Single and three phase supply, individual systems on life-saving GFCI outlets Water level Culture tank (high/low), supply sumps to pumps (high/low), filters (high/low) Aeration system Air oxygen pressure (high/low) Water flow Pumps, culture tanks, submerged filters, in-line heaters Temperature Culture tanks (high/low), heating/cooling systems (high/low) Security High temperature/smoke sensors, intruder alarms

  1. #### Some advice for system design and safety
-   Choose sensors carefully, label everything, and include expansion capability in all components

-   Install the sensors and equipment where they are visible and easily accessible for servicing and calibration

-   Remember that water and electricity make for a fatal combination, so use low voltages (5 VDC, 12 VDC or 24 VDC or AC) to protect yourself and the fish

-   Clearly label the sensor's armed and unarmed modes, preferably with LEDs at each station to show sensor status.
  1. #### Some advice for system maintenance
-   Have a well prepared maintenance manual accessible for staff to read

-   Maintain a weekly/monthly/yearly maintenance scheduling plan and keep files of major service records and equipment manuals

-   Maintain daily/weekly/monthly instrument check lists

-   Perform regular (and some unannounced) system checks, including triggering each sensor and checking the operation of the automatic backup systems and phone dialer

-   Provide staff training on handling routine alarms

-   Ensure that staff are familiar with the complete operating system, including water supply, aeration, and emergency backup systems.
  1. #### When to monitor water quality?

Fish digest according to the time they are fed, and the amount of faeces depends on the amount of ingested feed. Thus, the highest levels of ammonium are to be expected after the last feed (in the evening) and the lowest value before the first feed (in the morning). Therefore, measurements of water quality have to be done at the end of the feed in order to catch ammonium peaks (Figure 11).

Automated monitoring and control systems

Automated monitoring is becoming increasingly affordable. There are several data acquisition and control systems commercially available for applications in RAS and/or aquaponics. A monitoring system includes (i) sensors to measure the desired variables, (ii) an interface to convert the electrical information into a form readable by a computer or microprocessor, (iii) a computer, (iv) software to run the system, and (v) displays. It is important to match the components, in order for the monitoring system to work.

One of the most important functions of a monitoring system is to provide alerts to the system operator in the event of malfunctions and problems. If critical variables are sensed to be outside of acceptable limits, alarms need to be sent out. It is important to design and test the monitoring and alarm system so that false alerts are not sent out too often. Too frequent false alarms make it less likely that the operator(s) will respond (Timmons et al. 1999). Alarms must be constructed and operated so that pertinent individuals are alerted. Visual and audible alarms can be placed in key areas within a facility to alert workers of problems. Outside normal working hours remote alarms (usually via SMS messages) need to be employed.


Figure 11: Daily time course of NH4-N concentrations in RAS water. Blue = before biofilter; grey = after biofilter; yellow = difference between blue and grey

Copyright © Partners of the [email protected] Project. [email protected] is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).

Please see the table of contents for more topics.

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