Water temperature in recirculation systems

Water tem­pe­ra­tu­re is an important para­me­ter to con­trol in recir­cu­la­ting sys­tems. It has various effects on the water qua­li­ty and the well-being of the ani­mals. Sin­ce fish are cold-bloo­ded ani­mals, their body tem­pe­ra­tu­re is not con­stant, but is deter­mi­ned by the envi­ron­ment. In the fol­lowing blog post we will exp­lain how water tem­pe­ra­tu­re affects the sys­tem in clo­sed aquacul­tu­re sys­tems and what mea­su­res can be taken to regu­la­te it.

Effects of water temperature on aquaculture systems

The most important rela­ti­ons­hip is bet­ween tem­pe­ra­tu­re and oxy­gen con­tent. Fish need a mini­mum con­cen­tra­ti­on of dis­sol­ved oxy­gen in the water to live. The dis­sol­ved oxy­gen is given as a per­cen­ta­ge. Depen­ding on the fish spe­ci­es, 50% can be suf­fi­ci­ent (e. g. Afri­can cat­fi­sh). For sea bass, howe­ver, the value should always be 80–100%. As the water tem­pe­ra­tu­re incre­a­ses, the solu­b­i­li­ty of oxy­gen in the water decre­a­ses. The­re­fo­re, at 0°C, 100% satu­ra­ti­on means about 15 mg O2/l (a. d.) dis­sol­ved oxy­gen. This means that the value at 0°C is almost twice as high as at 20°C, with about 9 mg O2/l (a. d.) dis­sol­ved at 100% satu­ra­ti­on. In addi­ti­on, in war­mer waters (e. g. tro­pi­cal seas) the­re is often a grea­ter bio­lo­gi­cal oxy­gen con­sump­ti­on by other aqua­tic orga­nisms than in cold waters (e. g. moun­tain streams). It beco­mes cri­ti­cal for fish at an oxy­gen satu­ra­ti­on of less than 4 mg O2/l.

In addi­ti­on, the growth of bac­te­ria, viru­ses and para­si­tes is also influ­en­ced by the water tem­pe­ra­tu­re. The Van ‚t Hoff equa­ti­on (later also known as Arrhe­ni­us equa­ti­on) sta­tes that reac­tions occur twice as fast for every 10°C incre­a­se in tem­pe­ra­tu­re. Col­der water inhi­bits the growth of bac­te­ria, whe­re­as warm water acce­le­ra­tes their meta­bo­lism, so that bac­te­ria and viru­ses mul­ti­ply par­ti­cu­lar­ly quick­ly here.

The RGT rule also app­lies to fish growth. The ani­mals grow fas­ter in warm water than in cold water becau­se their meta­bo­lism is then acce­le­ra­ted and they con­vert more ener­gy. This posi­ti­ve effect of warm water is the­re­fo­re in con­flict with the rapid growth of bacteria.

Factors influencing the water temperature

Basi­cal­ly, water is a very good ener­gy store. It has a spe­ci­fic heat capa­ci­ty four times that of air. The­re­fo­re, water takes much lon­ger to adapt to tem­pe­ra­tu­re than this. The lar­ger the basin volu­me, the more slow­ly the sys­tem reacts, i.e. the lon­ger it takes to warm up. The fol­lowing fac­tors have an influ­ence on the temperature:

  • In addi­ti­on to the ambi­ent air, the electri­cal units with their was­te heat and the ani­mals them­sel­ves also have an influ­ence on the water tem­pe­ra­tu­re in recir­cu­la­ti­on sys­tems. In our plant, fish and com­pon­ents con­tri­bu­te around 5 kW of heat to the system.
  • The housing of the plant is also rele­vant for the water tem­pe­ra­tu­re. Suf­fi­ci­ent insu­la­ti­on of the enclo­sure or buil­ding should be ensured.
  • With acti­ve ven­ti­la­ti­on of plants or pre­mi­ses, it can still be rea­li­zed that cold air remains out­side the plant in win­ter and warm air in sum­mer. (e. g. heat exchanger)
Control and regulation of the water temperature in the tank

The water tem­pe­ra­tu­re is moni­to­red with the help of a sen­sor that dis­plays the cur­rent tank tem­pe­ra­tu­re live on the con­trol panel. If pre­de­fi­ned limit values are excee­ded, alarm signals are given so that coun­ter­mea­su­res can be taken.

In the SEAWATER Cube, we use various mea­su­res to estab­lish an average annu­al water tem­pe­ra­tu­re that is com­pa­ti­ble with the spe­ci­es. On the one hand, the con­di­ti­on of the con­tai­ners with a 10 cm PU insu­la­ti­on of the walls is an important pre­re­qui­si­te to ensu­re con­stant tem­pe­ra­tures in the water. We can also actively influ­ence the air tem­pe­ra­tu­re in the sys­tem via an air-to-air heat exch­an­ger, depen­ding on the ambi­ent tem­pe­ra­tu­re. Over time, this also has an influ­ence on the water. For the fish spe­ci­es sea bass and dora­de, we rea­li­ze an average water tem­pe­ra­tu­re of 23 °C throughout the year. Fish wel­fa­re, fish growth and bac­te­ri­al growth are in the best balan­ce here. For exo­tic fish spe­ci­es that like to live in war­mer water, hig­her tem­pe­ra­tures are also practicable.

When water tem­pe­ra­tures are out­side the accep­ta­ble ran­ge of a fish spe­ci­es, fee­ding is first redu­ced. On the one hand, this is in respon­se to the fish’s loss of appe­ti­te and, on the other hand, the exces­si­ve growth of bio­film and unde­s­i­ra­ble bac­te­ria can be con­tai­ned. Ano­t­her mea­su­re to regu­la­te water tem­pe­ra­tu­re in extre­me wea­ther con­di­ti­ons is to use a heat exch­an­ger (e. g. tita­ni­um heat exch­an­ger pla­te) in the water at cer­tain points. In par­ti­cu­lar­ly hot are­as / sea­sons the water can be coo­led with the help of a coo­ling unit via the heat exch­an­ger or in cold are­as / sea­sons the water can be hea­ted via the heat exch­an­ger with the help of a hea­ter (or was­te heat from other pro­ces­ses). The selec­ti­ve use of hea­ting rods to heat the water would also be pos­si­ble, but is asso­cia­ted with a high expen­dit­u­re of electri­ci­ty and cos­ts and is less effi­ci­ent than the use of a heat exchanger.

Further information

Learn more about inno­va­ti­ve aquacul­tu­re in a recir­cu­la­ting system. 


  • Dia­gno­sen und Pro­gno­sen aus der Lang­zeit­for­schungS­e­en im Klimawandel.
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  • Pic­tu­re source: SEAWATER Cubes

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