Digestive and locomotor capacity show opposing responses to changing food availability in an ambush predatory fish

Temperature acclimation improves high temperature tolerance of rainbow trout (Oncorhynchus mykiss) by improving mitochondrial quality and inhibiting apoptosis in liver

Global warming is threatening the survival and growth of cold-water fish, and the methods to improve the high-temperature adaptability of cold-water fish need to be explored urgently. This study aims to explore the mechanism of improving high temperature tolerance of rainbow trout by temperature acclimation (TA). Rainbow trout were acclimated by two modes at 22 °C, including fluctuating TA (FA) and constant TA (CA), and thereafter subjected to heat stress (HS) at 25 °C. Results showed that TA markedly increased the critical temperature maximum (CTmax) of rainbow trout. Secondly, the levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in serum and malondialdehyde (MDA) in liver of CA + HS group significantly decreased compared to those in HS group without TA, indicating the reduction of liver injury by CA. Moreover, HS significantly induced ROS production and reduced mitochondrial membrane potential (MMP) in rainbow trout liver, but TA reduced the levels of ROS and increased the MMP in liver of rainbow trout after HS, indicating the reduced oxidative stress and mitochondrial damage. Furthermore, TA up-regulated the expression of genes related to mitochondrial autophagy, fusion, fission and biogenesis, as well as the expression of marker proteins of autophagy (LC3II) and mitophagy (Parkin) in the liver, so as to maintain mitochondrial homeostasis. Moreover, TA also inhibited the occurrence of apoptosis (decrease in bax/bcl-2), which may be owing to the reduced ROS and mitochondrial damage by TA. Interestingly, CA significantly up-regulated the genes expression of methyltransferase in the liver, which may inhibit the genes or transcription factors related to oxidative stress and apoptosis by DNA methylation. In conclusion, TA increased the upper limit of heat tolerance of rainbow trout by improving mitochondrial quality and inhibiting apoptosis in liver. This study will provide an effective solution to the risk of high temperature in cold-water fish culture.

The relationships between specific dynamic action, nutrient retention and feed conversion ratio in farmed freshwater Chinook salmon (Oncorhynchus tshawytscha)

Improving the feed conversion ratio (FCR; the amount of feed consumed relative to the amount of weight gain) can reduce both production costs and environmental impacts of farmed fish. The aim of this study was to investigate what drives FCR to understand how nutrients are retained, as well as the amount of oxygen consumed for digestion, absorption and assimilation (a metabolic process known as specific dynamic action, SDA). Feed-efficient and inefficient Chinook salmon (Oncorhynchus tshawytscha) in fresh water were identified using ballotini beads and X-radiography that tracked individual feed intake across three assessment periods under satiated feeding. This allowed a comparison of physiological traits and body composition between the two FCR phenotypes over two time points as Chinook salmon grew from 305 to 620 g. Fish with higher daily feed intake (DFI) had higher daily weight gain (DWG) as expected. Nonetheless, the relationship between FCR and DFI as well as FCR and DWG was variable between time points. FCR and DWG were not correlated at the first time point and were negatively correlated at the second time point. In contrast, FCR and DFI were positively correlated at the first time point but not the second. Despite this, efficient fish ate smaller meals and retained more protein, lipid and energy in their body tissues. There was no detectable difference in metabolism between the two FCR phenotypes with respect to minimal resting metabolic rate, maximum metabolic rate, aerobic scope, or SDA parameters. In conclusion, FCR is not consistently associated with growth and metabolic differences in freshwater Chinook salmon, but FCR-efficient fish retain more nutrients and consume smaller meals.

Aerobic scope in fishes with different lifestyles and across habitats: Trade-offs among hypoxia tolerance, swimming performance and digestion

Exercise and aerobic scope in fishes have attracted scientists' attention for several decades. While it has been suggested that aerobic scope may limit behavioral expression and tolerance to environmental stressors in fishes, the exact importance of aerobic scope in an ecological context remains poorly understood. In this review, we examine the ecological relevance of aerobic scope by reconsidering and reanalyzing the existing literature on Chinese freshwater fishes across a wide-range of habitats and lifestyles. The available evidence suggests that natural selection in fast-flowing aquatic habitats may favor species with a high aerobic scope and anaerobic capacity for locomotion, whereas in relatively slow-flowing habitats, hypoxia tolerance may be favored at the cost of reduced locomotor capacity. In addition, while physical activity can usually cause fishes from fast-flowing habitats to reach their aerobic metabolic ceiling (i.e., maximum metabolic rate), possibly due to selection pressure on locomotion, most species from slow-flowing habitats can only reach their metabolic ceiling during digestion, either alone or in combination with physical activity. Overall, we suggest that fish exhibit a continuum of metabolic types, from a 'visceral metabolic type' with a higher digestive performance to a 'locomotion metabolic type' which appears to have reduced capacity for digestion but enhanced locomotor performance. Generally, locomotor-type species can either satisfy the demands of their high swimming capacity with a high oxygen uptake capacity or sacrifice digestion while swimming. In contrast, most visceral-type species show a pronounced decrease in swimming performance while digesting, probably owing to conflicts within their aerobic scope. In conclusion, the ecological relevance of aerobic scope and the consequent effects on other physiological functions are closely related to habitat and the lifestyle of a given species. These results suggest that swimming performance, digestion and hypoxia tolerance might coevolve due to dependence on metabolic traits such as aerobic scope.

Hypoxia reduced upper thermal limits causing cellular and nuclear abnormalities of erythrocytes in Nile tilapia, Oreochromis niloticus

Global warming is a threat across the world that leads to estimates of the upper thermal limits of ectothermic species. Increased water temperature up-regulates oxygen consumption and metabolic rates, and alters the physiological processes. In this study, we identified the critical thermal maxima (CTmax) and physiological responses under normoxia and hypoxia in Nile tilapia, Oreochromis niloticus. CTmax was 41.25 °C under hypoxia and 44.50 °C under normoxia. Compared to normoxia, lower values of hemoglobin (Hb) and red blood cells (RBCs) were observed at the CTmax under hypoxia. In contrast, higher values of white blood cells (WBCs) and blood glucose (Glu) levels were observed at the CTmax under hypoxia. Consequently, higher frequencies of micronucleus, cellular and nuclear abnormalities of erythrocytes were observed at the CTmax under hypoxia. These results suggest that high temperature tolerance and subsequent physiology are significantly affected by the oxygen supply in Nile tilapia. As climate vulnerability is intensifying day by day, this data will be helpful in successful management practice for the aquatic environment having low oxygen content.