Swimming is an important physiological activity for fish because it is closely related to food capture, predator avoidance and reproductive behavior (Brett 1964, Blake 2004, Kieffer 2010, Cai et al. 2014a).
Fish may perform swimming activities while digesting their food, and the outcome of the competition be tween the processes of digestion and swimming has important ecological relevance because both activities are important fitness-determining physiological functions in fish species.
This competition might be increasingly severe in active foragers and in those fish species living in a highly demanding habitat (e.g. high water velocity or predation stress).
Recently, the competition between digestion and swimming under a postprandial locomotion status has drawn the attention of scientists (Alsop & Wood 1997, Thorarensen & Farrell 2006, Altimiras et al. 2008, Fu et al. 2009, JourdanPineau et al. 2010, Cai et al. 2014b).
These 2 physiological activities can be performed independentlthe so-called additive meta bolic mode found in fish species such as the common carp Cyprinus carpio (Pang et al. 2011) and the European sea bass Dicentrarchus labrax (Alti miras et al. 2008, Jourdan-Pineau et al. 2010).
Otherwise, the digestion process may defer to swimming (locomotion-priority mode), as in qingbo Spinibarbus sinensis (Pang et al. 2011), or viceversa (digestion-priority mode) as in goldfish Carassius auratus (Pang et al. 2011), rainbow trout Oncorhynchus mykiss (Alsop & Wood 1997) and Chinook salmon O. tsha wytscha (Thorarensen & Farrell 2006).
The physiological mechanisms, ecological causes and selective agents underlying the different metabolic modes among fish species are largely unknown. We previously suggested that the metabolic mode employed might be related to the ecological habitat of the individual species (Pang et al. 2011); i.e. high performance swimmers evolved under selective stress for high swimming demand, and therefore digestion should give priority to swimming when the metabolic supply is insufficient to meet the metabolic demands (and vice versa).
However, only 3 cyprinids have been investigated thus far with regards to metabolic mode. In this study, we investigated the metabolic mode in Chinese bream Parabramis pekinensis, a widely distributed cyprinid in China with high swimming performance (Yan et al. 2013). It has been suggested that the metabolic mode may vary with environmental conditions such as temperature and dissolved oxygen levels in some (but not all) fish species (Pang et al. 2011, Zhang et al. 2012).
The competition between digestion and swimming is resolved by their relative roles within a given species, and by the conservative capacities of the central cardio-respiratory system (oxygen supply) and the peripheral digestive and locomotory systems (metabolic demands). The change in metabolic mode results from a shift in the balance point between oxygen supply and the metabolic demands of digestion and locomotion in different environments.
Temperature has a profound effect on all physiological functions, including cardio-respiratory capa city, digestion and swimming performance (Fry 1971, Kieffer et al. 1994, Luo & Xie 2008, Zeng et al. 2009). Furthermore, oxygen availability also changes with temperature, which may profoundly affect metabolic competition in fish species (Pang et al. 2011).
Therefore, we selected 3 experimental temperatures to test whether the metabolic mode changed with temperature in Chinese bream. This species spends more than 30% of its time moving in either a fasting or digesting status, i.e. it is an active forager that frequently forages or swims while digesting food.
Furthermore, its habitat exhibits profound seasonal thermal changes and daily fluctuations. We selected 15, 20 and 25°C, which simulate the mean of summer, autumn and winter temperatures of local water bodies. Our aim was to test whether digestion had a significant effect on the swimming performance of this active forager, and if so, whether such effects varied with temperature.
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