Most fish species are regularly subjected to periods of starvation during which a reduction of energy turnover might be favourable for the animal. This reduction of energy flux may be achieved by changes in thermal behaviour and/or swimming activity. We investigated such behavioural changes during starvation and subsequent refeeding in roach, Rutilus rutilus, with respect to energetic benefits and growth maximisation. Roach, acclimated to a wide range of temperatures (4, 12, 20, 24, 27 and 30 °C), were fed to excess, subjected to 3 weeks of starvation and subsequently refed in order to determine the temperature dependence of feeding rates, growth rates and conversion efficiency (K) under control conditions and during compensatory growth. When exposed to a thermal gradient, control animals preferentially selected a temperature of 26.8±0.9 °C, which is in the range of the optimal temperatures for feeding, growth and conversion efficiency. Starving fish showed a distinct circadian pattern of the mean selected temperature (MST). They migrated to cooler water in the dark (MST=22.8±1.1 °C) but returned to warmer water during daytime. This behaviour may be regarded as a trade-off between the potentially higher food density in warmer water areas and the energetic benefit of selecting cooler water patches. The circadian pattern of MST was gradually abandoned upon refeeding and control values were reached again after 3 weeks. Energetically more effective than behavioural hypothermia was the reduction of swimming activity. During starvation, activity peaks were slightly lower than under control conditions and mean daily activity decreased by about 50%. Swimming velocity, however, was not affected by feeding regime. After a period of starvation fish showed compensatory growth at all temperatures, even below 12 °C, where these animals normally do not grow. This suggests that after a period of starvation the critical temperature for growth shifts to lower values.

Seasonal acclimation versus permanent adaptation to low temperatures leads to a differential response in the expression of cytochrome- c oxidase (CCO) in temperate and Antarctic eelpouts. Although eurythermal eelpout from the North Sea ( Zoarces viviparus) displayed a cold-induced rise of CCO activity in white muscle, enzyme activity in the cold stenothermal Antarctic eelpout Pachycara brachycephalum failed to reflect such a compensatory increase. In Antarctic eelpout, CCO activity correlates with transcript levels of mitochondrial encoded subunits of CCO (CCO I and CCO II), whereas cold-acclimated eelpout from the North Sea show lower enzyme activities than expected on the basis of mitochondrial mRNA levels. In these animals, CCO expression at low temperatures may be limited either by nuclear CCO transcripts or by posttranscriptional processes. These may comprise translation of the subunits or assembly of the CCO holoenzyme. mRNA levels of CCO IV, one of the nuclear encoded subunits, increased strongly during cold acclimation, indicating that the expression of CCO is likely not message limited in cold-acclimated Z. viviparus. Our data suggest that seasonal cold acclimation of Z. viviparus results in a modification of the relationship between transcription and translation or posttranslational processes. In permanently cold-adapted P. brachycephalum, on the other hand, CCO expression shows similar characteristics as in the warm-acclimated confamilial species, e.g., low levels of enzyme activity correlated with low levels of mitochondrial message.

The effect of 21 days of starvation, followed by a period of compensatory growth during refeeding, was studied in juvenile roach Rutilus rutilus during winter and summer, at 4, 20 and 27 C acclimation temperature and at a constant photoperiod (12L : 12D). Although light conditions were the same during summer and winter experiments and fish were acclimated to the same temperatures, there were significant differences in a range of variables between summer and winter. Generally winter fish were better prepared to face starvation than summer fish, especially when acclimated at a realistic cold season water temperature of 4 C. In winter, the cold acclimated fish had a two to three-fold larger relative liver size with an approximately double fractional lipid content, in comparison to summer animals at the same temperature. Their white muscle protein and glycogen concentration, but not their lipid content, were significantly higher. Season, independent of photoperiod or reproductive cycle, was therefore an important factor that determined the physiological status of the animal, and should generally be taken into account when fish are acclimated to different temperature regimes. There were no significant differences between seasons with respect to growth. Juvenile roach showed compensatory growth at all three acclimation temperatures with maximal rates of compensatory growth at 27 C. The replenishment of body energy stores, which were utilized during the starvation period, was responsible for the observed mass gain at 4 C. The contribution of the different energy resources (protein, glycogen and lipid) was dependent on acclimation temperature. In 20 and 27 C acclimated roach, the energetic needs during food deprivation were met by metabolizing white muscle energy stores. While the concentration of white muscle glycogen had decreased after the fasting period, the concentrations of white muscle lipid and protein remained more or less constant. The mobilization of protein and fat was revealed by the reduced size of the muscle after fasting, which was reflected in a decrease in condition factor. At 20 C, liver lipids and glycogen were mobilized, which caused a decrease both in the relative liver size and in the concentration of these substrates. Liver size was also decreased after fasting in the 4 C acclimated fish, but the substrate concentrations remained stable. This experimental group additionally utilized white muscle glycogen during food deprivation. Almost all measured variables were back at the control level within 7 days of refeeding. # 2005 The Fisheries Society of the British Isles

The oxygen consumption rates of two cyprinid fishes, carp (Cyprinus carpio L.) and roach (Rutilus rutilus (L.)), were analysed for a wide range of body mass and swimming speed by computerized intermittent-flow respirometry. Bioenergetic models were derived, based on fish mass (M) and swimming speed (U), to predict the minimal speed and mass-specific active metabolic rate (AMR) in these fishes (AMR=aMbUc). Mass and speed together explained more than 90% of the variance in total swimming costs in both cases. The derived models show that carp consume far more oxygen at a specific speed and body mass, thus being less efficient in energy use during swimming than roach. It was further found that in carp (AMR=0.02M0.8U0.95) the metabolic increment during swimming is more strongly effected by speed, whereas in roach (AMR=0.02M0.93U0.6) it is more strongly effected by body mass. The different swimming traits of carp and roach are suitable for their respective lifestyles and ecological demands.

The cold-stenothermal freshwater gadid Lota lota inhabiting the potamic regions of lowland rivers in central Europe, is exposed to summer temperatures up to 25 degrees C, which is far above the thermal preferendum of this species. Oxygen consumption rates, determined in field catches sampled at different times of the year, revealed that the basal metabolic rate is depressed during summer when water temperatures are high (152+/-16 micromol O2 100 g(-1) h(-1)at 22 degrees C in July compared to 250+/-33 micromol O2 100 g(-1) h(-1) at 6 degrees C in November). This observation led us to investigate whether the observed depression of the metabolic rate is caused by oxygen limitation due to thermal impairment of the ventilatory system, as has been observed in other species. Determination of anaerobic end products (lactate and succinate) in the liver tissue of fish caught at different sampling dates did not show an accumulation of anaerobic end products during the summer, indicating no oxygen limitation. Measurements of enzyme activities in the white musculature and liver suggest that enzymes involved in aerobic metabolism were down-regulated during summer, which may have contributed to the observed reduction of metabolic rate.

Earlier work on Notothenioids led to the hypothesis that a reduced glycolytic capacity is a general adaptation to low temperatures in Antarctic fish. In our study this hypothesis was reinvestigated by comparing changes in the metabolic status of the white musculature in two related zoarcid species, the stenothermal Antarctic eelpout Pachycara brachycephalum and the eurythermal Zoarces viviparus during exercise and subsequent recovery at 0°C. In both species, strenuous exercise caused a similar increase in white muscle lactate, a drop in intracellular pH (pHi) by about 0.5 pH units, and a 90% depletion of phosphocreatine. This is the first study on Antarctic fish that shows an increase in white muscle lactate concentrations. Thus the hypothesis that a reduced importance of the glycolytic pathway is characteristic for cold-adapted polar fish cannot hold. The recovery process, especially the clearance of white muscle lactate, is significantly faster in the Antarctic than in temperate eelpout. Based on metabolite data, we calculated that during the first hour of recovery aerobic metabolism is increased 6.6-fold compared with resting rates in P. brachycephalum vs. an only 2.9-fold increase in Z. viviparus. This strong stimulation of aerobic metabolism despite low temperatures may be caused by a pronounced increase of free ADP levels, in the context of higher levels of pHi and ATP, which is observed in the Antarctic species. Although basal metabolic rates are identical in both species, the comparison of metabolic rates during situations of high-energy turnover reveals that the stenothermal P. brachycephalum shows a higher degree of metabolic cold compensation than the eurythermal Z. viviparus. Muscular fatigue after escape swimming may be caused by a drop of the free energy change of ATP hydrolysis, which is shown to fall below critical levels for cellular ATPases in exhausted animals of both species.

In roach Rutilus rutilus growth ceases below a temperature threshold of 12° C. This cessation of growth is accompanied by a reduction in feeding. Do roach decrease feeding in the cold because of reduced energy demand, caused by the decelerating effect of low temperature on metabolism and growth, or is feeding directly limited by low temperatures, leading to reduced growth rates? It was found that at low temperatures the intake and digestion of food may be limited by reduced activities of digestive enzymes. Trypsin, amylase and γ‐glutamyl transferase showed a negative compensation with respect to temperature, resulting in very low activities at acclimation temperatures of ≤12° C. Trypsin activity, falling from 400·5 ± 131·2 U g−1 fresh mass of the gut at 27° C to 12·5 U g−1 fresh mass at 4° C, displayed the strongest linear correlation with growth rates, suggesting that trypsin activities may set a limit to growth in the low temperature range. If protein digestion is limiting at low temperatures, this should be reflected in reduced concentrations of amino acid in the white muscle. The size of the total amino acid pool was not affected by temperature acclimation and ranged between 19·2 ± 6·2 and 25·2 ± 3·6 µmol g−1 fresh mass of the white muscle. A decrease, however, was found of several amino acids, mainly of threonine and glutamine, in the low temperature range. Low concentrations of the essential amino acid threonine (0·14 ± 0·03 µmol g−1 fresh mass at 12° C and 0·12 ± 0·05 µmol g−1 fresh mass at 4° C) were probably due to nutritional or digestional limitations and may therefore have resulted from reduced trypsin activity in the cold. The non‐essential amino acid glutamine, however, can be endogenously synthesized and its low level observed at 4° C (0·16 ± 0·09 µmol g−1 fresh mass) was not necessarily a result of low trypsin activities. It is more likely that low temperatures impair glutamine synthesis. The possibility that glutamine concentrations may be down regulated under conditions when anabolic processes are not advantageous is discussed.

This study was designed to determine the mechanisms causing temperature-induced pH shifts in the white muscle of the marine teleost Zoarces viviparus. The white musculature undergoes an intracellular acidification with increasing body temperature at a slope of the pH-temperature relationship equal to -0.016 +/- 0.003 U/degree C. This is in good accordance with the overall relationship between the change in pK and the change in temperature of the intracellular proteins, which was determined to be -0.013 +/- 0.001 U/degree C. Thus the dissociation state of muscle proteins is kept fairly constant in white muscle of Zoarces viviparus. The passive component of the observed pH shift, which is due to the physicochemical response of the intracellular buffers to temperature change, accounts for only 35% of the pH transition. Ventilatory adjustment of intracellular PCO2 does not contribute to the temperature-induced shift of intracellular pH (pHi) in Zoarces viviparus. Therefore, the remaining 65% of pH adjustment must be ascribed to ion exchange mechanisms. The nonbicarbonate buffer value amounted to 34.4 +/- 2.3 meq.pH-1 kg cell water-1 at 12 degrees C and decreased slightly but not significantly with temperature. On the basis of our data we calculated that a removal of 0.52 mmol base equivalents.kg cell water-1.degree C-1 was necessary to shift pHi to its new steady state.