Endurance swimming of European eel

A long-term swim trial was performed with five female silver eels Anguilla anguilla of 0.8-1.0 kg (c. 80 cm total length, L-T) swimming at 0.5 body lengths (BL)s(-1), corresponding to the mean swimming speed during spawning migration. The design of the Blazka-type swim tunnel was significantly improved, and for the first time the flow pattern of a swim tunnel for fish was evaluated with the Laser-Doppler method. The velocity profile over three different cross-sections was determined. It was observed that 80% of the water velocity drop-off occurred over a boundary layer of 20 mm. Therefore, swim velocity errors were negligible as the eels always swam outside this layer. The fish were able to swim continuously day and night during a period of 3 months in the swim tunnel through which fresh water at 19degrees C was passed. The oxygen consumption rates remained stable at 36.9 +/- 2.9 mg O-2 kg(-1) h(-1) over the 3 months swimming period for all tested eels. The mean cost of transportation was 28.2 mg O-2 kg(-1) km(-1). From the total energy consumption the calculated decline in fat content was 30%. When extrapolating to 6000 km this would have been 60%, leaving only 40% of the total energy reserves for reproduction after arriving at the spawning site. Therefore low cost of transport combined with high fat content are crucial for the capacity of the eel to cross the Atlantic Ocean and reproduce. (C) 2004 The Fisheries Society of the British Isles.