After a harsh winter with low water temperatures, the approach of spring may be accompanied by a rapid rise in temperature, especially in some areas such as northern Europe. This period can be challenging and risky for the fish, especially if the feeding strategy is not adapted to these circumstances.
During the first weeks of spring with rapidly rising temperatures, feeding of the fish must be very restrictive due to the fact that the fish have not yet developed sufficient red blood cell capacity for transporting oxygen to all vital parts of the body.
“Spring syndrome” in trout farming
The BioMar factory in Denmark serves trout farmers in many markets. When spring approaches, Anders Andreasen, member of the BioFarm team of technical customer advisers based in Denmark, is concerned about the occurrence of a special phenomenon, especially in trout farms, in some regions known as “spring syndrome”.
“Trout farmers turn to us in spring, wondering why, after feeding, their trout are hanging at the water surface, gasping for air. Farmers are worried. They are aware that this is a sign for stress which is challenging and can be risky for the fish,” Anders Andreasen reports.
“For trout, the most critical period is when water temperature rises very fast. The critical period depends on the location of the farm - in the very north of Europe this would for instance be when the ice on the water surfaces melts, or right after. Moreover, it varies from year to year depending on when and how fast springtime and temperature rises set in.“
Number of red blood cells is crucial for well-being
A key factor for fish well-being is to have a sufficient amount of oxygen in the muscles and other organs. The capability of getting enough oxygen depends on the number of red blood cells, i.e. the haemoglobin content, in the blood.
When the water is cold, the appetite of the fish drops, and the oxygen content of the water is often quite high. When the water contains high amounts of oxygen, there is no need for the fish to have many red blood cells at its disposal for transporting a sufficient amount of oxygen inside its body. After months with cold water and abundant oxygen, the fish have got used to producing a lower amount of red blood cells.
Steep increase of water temperature is challenging
If due to rapidly rising temperatures the appetite of fish all of a sudden increases, fish will by their nature feel hungry and take in as much feed as they can get. At high feed intake, the increase of feed conversion will, however, result in an increased demand for oxygen.The problem is that after the cold winter months the amount of red blood cells is low and thus the blood cannot transport sufficient oxygen to the body tissue. The fish will easily suffer from oxygen deficiency. The fish reacts by suffering from oxygen stress, resulting in enhanced blood circulation and gill ventilation.
Be observant to variations in temperature
Anders Andreasen adds: “When it comes to feeding the right amount of feed in spring, it is essential for the farmer to be observant to variations in temperature. Take a look back on temperature curves on the farm from previous years to foresee typical temperature development on the location.”
Adaptation of feeding regime is essential
It is vital for a fish farmer to understand that it takes time for the fish to develop a sufficient amount of red blood cells. Only by applying an adapted feeding regime the fish will get ready for being exposed to a high feeding intensity.
“When the fish have had decreased appetite during winter, we recommend strengthening the metabolic processes of the fish rather than to focus on growth. We advise our customers to apply a non-aggressive feeding strategy and to exercise the fish gradually. For this, the addition of an air diffuser could be beneficial,” Anders Andreasen says.
Typical feeding strategy
A typical strategy would be to keep the fish about two weeks behind in feeding compared to the feeding that is usually recommended at the given water temperature. It is advisable to start with very restrictive feeding.
“We recommend applying continuous feeding and adjust the amount of feed given according to the temperature expected for the following days. After 2-3 weeks of increasing the amount of red blood cells, the fish should be ready to metabolise the “normal” amount of feed as recommended in the feeding guides.”
“These steps contribute to avoiding waste of feed and are to the benefit of exploiting the huge growth potential of the fish,” Anders Andreasen concludes.
