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Metabolic rates vary widely within species, but little is known about how variation in the 'floor' [i.e. standard metabolic rate (SMR) in ectotherms] and 'ceiling' [maximum metabolic rate (MMR)] for an individual's aerobic scope (AS) are linked with digestive and locomotor function. Any links among metabolic traits and aspects of physiological performance may also be modulated by fluctuations in food availability. This study followed changes in SMR, MMR, and digestive and locomotor capacity in southern catfish (Silurus meridionalis) throughout 15 days of food deprivation and 15 days of refeeding. Individuals downregulated SMR during food deprivation and showed only a 10% body mass decrease during this time. Whereas critical swim speed (Ucrit) was robust to food deprivation, digestive function decreased after fasting with a reduced peak oxygen uptake during specific dynamic action (SDA) and prolonged SDA duration. During refeeding, individuals displayed rapid growth and digestive function recovered to pre-fasting levels. However, refed fish showed a lower Ucrit than would be expected for their increased body length and in comparison to measures at the start of the study. Reduced swimming ability may be a consequence of compensatory growth: growth rate was negatively correlated with changes in Ucrit during refeeding. Southern catfish downregulate digestive function to reduce energy expenditure during food deprivation, but regain digestive capacity during refeeding, potentially at the cost of decreased swimming performance. The plasticity of maintenance requirements suggests that SMR is a key fitness trait for in this ambush predator. Shifts in trait correlations with food availability suggest that the potential for correlated selection may depend on context. [ABSTRACT FROM AUTHOR]

The prevalence of extreme, short-term temperature spikes in coastal regions during summer months is predicted to increase with ongoing climate change. In tropical systems, these changes are predicted to increase the metabolic demand of coral reef fish larvae while also altering the plankton communities upon which the larvae feed during their pelagic phase. The consequences of these predictions remain speculative in the absence of empirical data on the interactive effects of warm temperatures on the metabolism, postprandial processes and growth responses of coral reef fish larvae. Here, we tested the effect of increased temperature on the metabolism, postprandial performance and fine-scale growth patterns of a coral reef fish (Amphiprion percula) in the latter half of its ~11-d larval phase. First, we measured the length and weight of fed versus fasted larvae (N = 340; mean body mass 4.1±0.05 mg) across fine temporal scales at a typical current summer temperature (28.5°C) and a temperature that is likely be encountered during warm summer periods later this century (31.5°C). Second, we measured routine metabolic rate (Mo2 routine) and the energetics of the postprandial processes (i.e., digestion, absorption and assimilation of a meal; termed specific dynamic action (SDA)) at both temperatures. Larvae fed voraciously when provided with food for a 12-hour period and displayed a temperature-independent increase in mass of 40.1% (28.5°C) and 42.6% (31.5°C), which was largely associated with the mass of prey in the gut. A subsequent 12-h fasting period revealed that the larvae had grown 21.2±4.8% (28.5°C) and 22.8±8.8% (31.5°C) in mass and 10.3±2.0% (28.5°C) and 7.8±2.6% (31.5°C) in length compared with pre-feeding values (no significant temperature effect). Mo2 routine was 55±16% higher at 31.5°C and peak Mo2 during the postprandial period was 28±11% higher at 31.5°C, yet elevated temperature had no significant effect on SDA (0.51±0.06 J at 28.5°C vs. 0.53±0.07 J at 31.5°C), SDA duration (6.0±0.6 h vs. 6.5±0.5 h), or the percent of total meal energy used for SDA (SDA coefficient: 10.1±1.3% vs. 13.0±1.7%). Our findings of higher Mo2 routine but similar SDA coefficient at high temperature provide the first empirical evidence that coral reef fish larvae may have to secure more food to attain similar growth rates during warm summer periods, and perhaps with chronically warmer conditions associated with climate change. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the effect of dissolved oxygen level ([O2]) on maintenance metabolism, feeding metabolism, aerobic swimming performance and their metabolic interaction in juvenile southern catfish (Silurus meridionalis Chen), we measured the following: (1) the resting oxygen consumption rate (MO2rest) over a range of water [O2] and from this we calculated the critical oxygen tension (P crit) of fasting fish; (2) the postprandial MO2 response (10% body mass meal size) at water [O2] of 1, 2, 4 and 8mgO2 L−1; and (3) the swimming performance of fasting and digesting fish at water [O2] of 1, 2, 4 and 8mgO2 L−1 at 25°C. The MO2rest remained constant over a broad range of water [O2] but then dropped markedly upon reaching the P crit (16.4% saturation). Hypoxic groups presented lower peak postprandial MO2 (MO2peak) (1mgO2 L−1 group), larger energy expenditure and longer digestive process (both 1 and 2mgO2 L−1) than those of normoxic groups. Both critical swimming speed (U crit) and the active metabolic rate (MO2active) of fasting fish remained unchanged over a decrease in water [O2] from 8 to 4mgO2 L−1 and then decreased significantly with further decreases in water [O2]. These parameters in fed fish showed a pronounced decrease as water [O2] decreased from 8 to 1mgO2 L−1. Feeding caused a significantly lower U crit in the 2mgO2 L−1 water [O2] group, a significantly higher MO2active in both the 2 and 8mgL−1 water [O2] groups and a significantly higher metabolic scope (MO2active − MO2rest) in both the 2 and 4mgO2 L−1 water [O2] groups compared to fasting fish. The MO2 increased greatly with swimming speed in the higher water [O2] groups, whereas it leveled off as swimming speeds approached the U crit in the lower water [O2] groups. Within all water [O2] groups, feeding caused a higher MO2 compared to fasting fish when fish swam at the same speeds, except in the 1mgO2 L−1 group. This finding showed that the critical water [O2] for maintenance of metabolism, digestion, swimming and postprandial swimming increased sequentially due to the increasing O2 demand, which challenges respiratory capacity. With the administration of the medium-sized meal that was used in this study, the metabolic mode of juvenile southern catfish changed from an additive mode during normoxia to a digestion-priority mode under moderate hypoxia and to locomotion-priority mode under hypoxia. [Copyright &y& Elsevier]

Abstract: A 10-week feeding trial in seawater floating cages (1.5×1.5×2.0m) was conducted to investigate the effects of dietary canola meal (CM) levels on growth, survival, digestion and selected immune parameters of Japanese seabass (initial average weight 8.3±0.15g). Six isonitrogenous (crude protein 43%) and isoenergetic (20kJg− 1) practical diets were formulated by replacing 0 (the control), 10, 20, 30, 40, and 50% of protein from fish meal with CM. Each diet was randomly fed to triplicate groups of fish, and each cage was stocked with 20 fish. Fish were fed twice daily (06:30 and 16:30) to apparent satiation. The results showed that with increasing dietary CM levels, the survival, specific growth rate (SGR) and feed efficiency (FE) decreased. The survival was significantly lower compared to the control group (P <0.05) with a 40% substitution level. Fish fed the diet with 20% or more protein from CM had significantly lower SGR than the control group (P <0.05), but there was no significant difference in FE at this level (P >0.05). The activities of digestive enzymes and apparent digestibility coefficients of dry matter, protein, lipid and phosphorus significantly decreased with increasing dietary CM level. A similar trend was observed in the immune response where lysozyme activity significantly decreased compared with the control group when the substitution level was more than 20%. Results of the present study indicated that protein from CM could substitute less than 20% for fish meal protein without influencing the growth of Japanese seabass. The higher substitution levels of CM induced negative influences on growth, survival and serum lysozyme of Japanese seabass. [Copyright &y& Elsevier]

The digestion, absorption and post absorptive metabolism of a radiolabelled triacylglycerol (TAG; triolein) and a free fatty acid (FFA; oleic acid), delivered by tube feeding, was studied in herring Clupea harengus larvae, using metabolic chambers and video analysis. In general, a large amount of the delivered lipid was evacuated. Most of the evacuation occurred between 2 and 6 h after tube feeding although a group of larvae responded by rapidly evacuating the lipid (>50% before 2 h). The volume of the tube-fed lipid affected its utilization. A small volume of triolein (9·2 nl, representing c. 6% of gut filling capacity) resulted in a lower proportion of fast evacuating larvae and improved utilization (lower evacuation and higher absorption: body incorporation and catabolism) compared with 50·6 nl ( c. 17% of gut filling capacity). Increases in the volume of tube fed triolein enhanced only marginally label absorption and led to a steep rise in evacuation. At a comparable high volume (50·6 nl), oleic acid, which does not require digestion, was better absorbed and less evacuated than triolein. The video observation of the lipid digestive process revealed a considerable gut contractile activity that appeared effective in processing the tube fed lipid. Also, the gut wall seemed very sensitive to physical pressure. Signs of chemical degradation during lipid digestion were also noted. The metabolic studies, together with video image analysis, suggested that the limiting step for the utilization of high dietary lipid levels may have been the lipid absorption into the enterocytes and transport into the body, rather than lipid digestion. The results support the notion that the rate of lipid digestion and absorption in fishes is slower than that of mammals. [ABSTRACT FROM AUTHOR]