Abstract
Summary The burbot (Lota lota) is the only cold-stenothermal gadoid inhabiting freshwaters with high temperature amplitudes. Summer temperatures up to 25°C have been reported as being far above the thermal preferendum of this species. Thus this study aimed to determine gastric evacuation, daily food consumption and energy uptake of burbot at high temperatures under in situ conditions. These data are prerequisites for bioenergetics modelling of the adaptive physiological behaviour of burbot in summer-warm freshwaters. The study was conducted in the lower Oder River, Germany between May and October 2003 covering a temperature range between 4.6 and 23.4°C. A total of 1683 burbot was caught in five successive 24-h fisheries. The mean index of stomach fullness showed a significant decrease with rising temperature. Highest stomach fullness values were reached at 4.6°C in October. A negative correlation was observed between gastric evacuation and temperature. In contrast to the data reported in the literature gastric evacuation was found to be high at 4.6°C, which may indicate an ongoing compensatory adaptation to the long period of high temperatures in July and August, when a significant reduction of the mean daily ration was observed. Balancing the energy flux probably resulted in a decrease in the hepatosomatic index at high temperatures and an increase during the cool period, when the liver energy was restored. The endogenous energy pool of the liver may determine the survival of burbot during summer months. The ability of burbot to actively forage during winter and to rapidly digest high amounts of food at cold temperatures was considered an adaptation to increase survival chances in an environment with high environmental, especially thermal, heterogeneity as is typical for a summer-warm lowland river. Limitations of the energy budget have to be expected with extended periods of high temperatures.
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