The All-important Role Of Temperature

Cold Fish?  The All-important Role Of Temperature

 By Dr David Huchzermeyer


Fish, like birds and mammals, are vertebrates.  One striking difference in the way its body functions sets fish apart from birds and mammals.  Like reptiles and amphibians, fish are cold-blooded or poikilothermic. Unlike birds and mammals, they can’t regulate their body temperature through their own metabolism or body functions. This explains much of the behavior we observe in fish, particularly pertaining to the health and well-being of pet fish.

Tropical Fish Need Warmer Water

Fish that have evolved in the tropics are mostly adapted to warm water, so should the aquarium heater fail on a cold winter day, the body functions of tropical fish will shut down and the fish may die. Warm-blooded animals, on the other hand, will try to adapt and will only die from exposure if they are unable to shelter from the cold and have burnt up their energy reserves attempting to stay warm. Birds will fluff up their feathers to conserve heat on a cold winter’s day and some birds like mouse birds will huddle together to warm each other up.

Out In The Cold … That’s The Ticket!

Some fish have evolved in temperate climates and will not tolerate water above a certain temperature. Consider trout, a cold water species restricted to the cold mountain streams of our country. Moreover, the issue of temperature becomes more complex for fish due to the effects of temperature on physical parameters in the environment. Unlike air-breathing animals, most fish depend on oxygen that dissolves in the water in order to breathe. As water temperature increases, the solubility of oxygen and other gases decreases in the water. This has a significant impact on the health and behaviour of fish. A trout caught on a fly in summer will put up a poor fight because it is unable to extract enough oxygen from the water, whereas on a cold winter’s day with lots of oxygen in the water, the trout will delight the angler with a marvellous fight.

Feeling All Coy?!

The interaction with oxygen does not end here. The feeding response of fish and the ability to digest food are closely linked to temperature. Take for instance koi in a Highveld pond in mid- winter. The fish, although tolerant of the cold, will hardly bother to come to feed. The up-side of being cold-bloodedis that they do not need to burn energy to keep themselves warm. Fish can withstand long periods of time without food while remaining perfectly healthy. This fact we can put to good use when we go on holiday. My advice to people is to not have anyone feed their fish while on holiday. I have heard so many tragic tales where a well-meaning neighbour or relative was called in to feed the pet fish while the owners are on holiday. Unfortunately this often results in overzealous feeding, which causes the water to turn bad and the fish to die.

Summer Time … Disease Time

Certain diseases have narrow temperature preferences, hence the dreaded outbreaks of Koi Herpes Virus when water temperature is between 23 – 25 °C, during the summer months . Fortunately another  dreaded disease of Koi, Spring Viraemia of Carp, is largely precluded from occurring in South Africa as it is believed that this virus is unable to survive our warm summer temperatures.

Simply Irresistable

Resistance to disease in fish is also temperature dependant. As the water gets colder in autumn the resistance of the fish declines or becomes paralysed, so to speak. This is not a problem while the water remains cold as disease-causing organisms are similarly retarded by the cold. Come spring, however, the water gets warmer and harmful bacteria and parasites start to flourish. Fish’s immune systems need time to switch on after winter, which takes anything up to three weeks. If there is another cold snap in spring, the fish’s resistance will switch off again and require a further lag period to activate.  This is when  parasites and other bugs are quick to make the most of the situation. Herein lies yet another lesson for the fish hobbyist. Do not disturb pond fish, in particular goldfish and koi, in spring. Leave the spring cleaning of the pond until water temperatures have stabilised for at least three weeks.

© Pet’s Health

Leave a reply