The effect of temperature and meal size on the aerobic scope and specific dynamic action of two temperate New Zealand finfish Chrysophrys auratus and Aldrichetta forsteri

Constraints of digestion on swimming performance and stress tolerance vary with habitat in freshwater fish species

Limited aerobic scope (AS) during digestion might be the main constraint on the performance of bodily functions in water-breathing animals. Thus, investigating the postprandial changes in various physiological functions and determining the existence of a shared common pattern because of possible dependence on residual AS during digestion in freshwater fish species are very important in conservation physiology. All species from slow-flow habitats showed impaired swimming speed while digesting, whereas all species from fast-flow habitats showed strong swimming performance, which was unchanged while digesting. Only two species from slow-flow habitats showed impaired heat tolerance during digestion, suggesting that whether oxygen limitation is involved in the heat tolerance process is species-specific. Three species from slow- or intermediate-flow habitats showed impaired hypoxia tolerance during digestion because feeding metabolism cannot cease completely under hypoxia. Overall, there was no common pattern in postprandial changes in different physiological functions because: (1) the digestion process was suppressed under oxygen-limiting conditions, (2) the residual AS decreased during digestion, and (3) performance was related to residual AS, while digestion was context-dependent and species-specific. However, digestion generally showed a stronger effect on bodily functions in species from slow-flow habitats, whereas it showed no impairment in fishes from fast-flow habitats. Nevertheless, the postprandial change in physiological functions varies with habitat, possibly due to divergent selective pressure on such functions. More importantly, the present study suggests that a precise prediction of how freshwater fish populations will respond to global climate change needs to incorporate data from postprandial fishes.

Atlantic salmon Salmo salar do not prioritize digestion when energetic budgets are constrained by warming and hypoxia

During summer, farmed Atlantic salmon (Salmo salar) can experience prolonged periods of warming and low aquatic oxygen levels due to climate change. This often results in a drop in feed intake; however, the physiological mechanism behind this behaviour is unclear. Digestion is a metabolically expensive process that can demand a high proportion of an animal's energy budget and might not be sustainable under future warming scenarios. We investigated the effects of elevated temperature and acute hypoxia on specific dynamic action (SDA; the energetic cost of digestion), and how much of the energy budget (i.e. aerobic scope, AS) was occupied by SDA in juvenile Atlantic salmon. AS was 9% lower in 21°C-acclimated fish compared to fish reared at their optimum temperature (15°C) and was reduced by ~50% by acute hypoxia (50% air saturation) at both temperatures. Furthermore, we observed an increase in peak oxygen uptake rate during digestion which occupied ~13% of the AS at 15°C and ~20% of AS at 21°C, and increased the total cost of digestion at 21°C. The minimum oxygen tolerance threshold in digesting fish was ~42% and ~53% at 15 and 21°C, respectively, and when digesting fish were exposed to acute hypoxia, gut transit was delayed. Thus, these stressors result in a greater proportion of the available energy budget being directed away from digestion. Moderate environmental hypoxia under both optimal and high temperatures severely impedes digestion and should be avoided to limit exacerbating temperature effects on fish growth.

SDA coefficient is temperature dependent in rainbow trout (Oncorhynchus mykiss, Walbaum 1792) in a practical approach using group respirometry

Rising global temperatures have raised the need for detailed knowledge of the effects of rising temperatures on the physiology of animals used in aquaculture. Here we used a multifactorial bioenergetic approach using groups of rainbow trout (Oncorhynchus mykiss) with an average single fish weight of 183.75 g ± 0.65 g to investigate the interactions of feeding and temperature with key metabolic variables. We used a recirculating aquaculture respirometry system (RARS) to test three ration sizes (0.65; 0.975; 1.3% of live body weight (BW)) over a range of three consecutive temperatures (14; 17; 20 °C). The fish were fed once per day for 6 days at each temperature and subsequently starved for 5 days to return to standard metabolic rate (SMR). This study aimed to answer the highly discussed topic of the temperature dependency of key metabolic specific dynamic action (SDA)-variables SDA_coef and SDA_dur. We were able to provide evidence, that in rainbow trout the SDA_coef is highly dependent on the environmental temperature in the first ever approach to assess these variables in a group respirometer with this species. We compared the results of this study with a sophisticated bioenergetic model by Elliot and Hurley (2002) and thereby provide evidence for the practicability of group respirometry as a method to assess bioenergetic data under culture like conditions.

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.

Investigating food limitations in wild fisheries: Estuarine fish form dynamic aggregations around a supplementary feeding station and increase localised secondary productivity

Whether wild fish populations are food limited in some inshore and estuarine marine ecosystems is an area of increasing research and focus. To investigate this phenomenon, the abundance and behaviours of fish in a temperate South Pacific estuary were observed in response to the provision of supplementary feed. Observations were conducted over 120-weeks, involving a 60-week period over which fish were actively fed followed by a 60 week without feeding. During active feeding, estuarine associated fish (primarily yellow-eyed mullet Aldrichetta forsteri) showed a highly predictable pattern of fish abundance, biomass and behavioural formation coinciding with the almost daily feeder operation. Tidal current velocities and turbidity appeared to have little influence on the attendance and formations of fish over this period, although season did influence some variables. Peaking at close to 9000 individual fish and 880 kg biomass, fish attendance during the operation of the feeding station was markedly higher than during the period when the feed station was no longer active. Whereby only 0-100 individuals were typically present, and fish no longer showed collective behavioural formations. A direct result of the large number of fish aggregating on this feeding station was an increase in the secondary productivity of the observable zone around the feeding station. Whereby secondary productivity increased by a factor of ∼30 above that observed when the feed station was not operating. Supplementary feeding effectively transformed the study site - a highly modified intertidal location - from an area of very low productivity to one comparable with highly productive temperate estuary environments described elsewhere. The behavioural drivers and ecological relevance of these observations are discussed.