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Energetics of fast-starts in northern pike, Esox lucius Frith, Harold Russ


Fast-starts are high powered events of short duration, used by fish for prey capture and escape from predation. Here, the energetic cost of fast-starts in escape and prey capture for a fast-start specialist, the northern pike, Esox lucius, are determined and physiological and behavioural constraints assessed. This is done by comparing costs with literature values for physiological limits set my muscle mechanics and biochemistry, and comparing costs with other components of the energy budget. The combination of high speed film analysis (200-250Hz) and hydrodynamic models are used to determine the mechanical costs, hydrodynamic efficiencies and power output of fast-starts in prey capture (S-starts) and escape behaviour (C-starts). Excess post-exercise oxygen consumption (EPOC) is used to estimate the metabolic cost of fast-starts. A comparison of model predictions with required (acceleration) force estimates shows results are within 22% and similar to previous findings at lower film speeds. The caudal region including the caudal, dorsal and anal fins contribute the most to thrust (>90%) and the dorsal and anal fins contribute 28%. Due to the necessity for deceleration of fin sections during each tail beat, kinematics are not always optimal as predicted by the Weihs model. Mechanical power output, hydrodynamic efficiency and kinematic parameters (maximum velocities and maximum angle of attack of the caudal fin) are determined for fast-starts during prey capture and escape. Hydrodynamic efficiency averages 0.37 (range: 0.34 to 0.39) for C-starts and 0.27 (range: 0.16 to 0.37) for S-starts. The acceleration of added mass contributes the most to power output at 39%. Power output and efficiency for S-starts are more variable than C-starts and hydromechanical efficiency increases with number of tail beats for S-starts. Maximum muscle power output and maximum muscle stress during fast-starts in comparison to literature values for muscle function shows muscle power output during fast-starts is at its physiological limit but muscle stress is not. Metabolic efficiency is higher at 0.094 for C-starts than S-starts at 0.047. However, muscle efficiency estimates are similar averaging 0.252 for both fast-start types. Mean energetic cost of fast-starts is determined to be 26.5 J/kg for C-starts and 18.6 J/kg for S-starts. Based on the observation that pike can repeatedly fast-start up to 170 times before becoming exhausted and on estimates of available energy reserves from literature values for ATP and CrP concentrations in white muscle, the duration of fast-starts is concluded to not be limited by muscle physiology. Average power output is found to be similar for C and S-starts at 406 to 412 W/kg. Only hydrolysis of ATP and CrP can supply energy at this rate. Therefore, based on fish white muscle biochemistry and mechanics, power output during fast-starts appears to be limited by muscle physiology. The cost of fast-starts represents 0.03 to 2% of maintenance costs for pike and therefore only 5 to 30 fast-starts per day would be required to increase the daily energy budget by 10%. In addition, the cost of fast-starts represents 0.52 to 27.4% of surplus energy available from assimilated prey. Therefore, the cost of fast-starts can be significant and reducing fast-start duration is a probable strategy for minimising activity costs and thus increasing the energy available for growth or reproduction.

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