One of the mysteries of the animal kingdom is the longdistance migration (5000-6000·km) of the European eel Anguilla anguilla L. from the coasts of Europe to its spawning grounds in the Sargasso Sea. The only evidence for the location of the spawning site of the European eel in the Sargasso Sea is the discovery by Johannes Schmidt at the beginning of the previous century of the smallest eel larvae (leptocephali) near the Sargasso Sea. For years it has been questioned whether the fasting eels have sufficient energy reserves to cover this enormous distance. We have tested Schmidt's theory by placing eels in swim tunnels in the laboratory and allowing them to make a simulated migration of 5500·km. We find that eels swim 4-6 times more efficiently than non-eel-like fish. Our findings are an important advance in this field because they remove a central objection to Schmidt's theory by showing that their energy reserves are, in principle, sufficient for the migration. Conclusive proof of the Sargasso Sea theory is likely to come from satellite tracking technology.

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A long-term swim trial was performed with five female silver eels Anguilla anguilla of 0Á8-1Á0 kg (c. 80 cm total length, L T ) swimming at 0Á5 body lengths (BL) s À1 , 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 crosssections 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 19 C was passed. The oxygen consumption rates remained stable at 36Á9 AE 2Á9 mg O 2 kg À1 h À1 over the 3 months swimming period for all tested eels. The mean cost of transportation was 28Á2 mg O 2 kg À1 km À1 . 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.

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The effect of polychlorinated biphenyls (PCBs) on the energy consum ption of fasting silver European eel (Anguilla anguilla L.) was studied over a 27-day period during w hich the anim als w ere at rest or were sw im m ing 800 km in Blazka sw im tunnels. Three-year-old female hatchery eels (silver stage) betw een 73 and 80 cm long weighing around 1 kg w ere dosed intraperitoneally w ith PCBs at a nom inal dosage of 10x the consum ption standard as a m ixture representative for planar (7 ^gPCB126/kgeel), non-planar (5 m g PCB153/kg eel) and m etabolizable PCBs (50 ^g PCB77/kg eel) found in wild eel, or only w ith the vehicle (corn oil, 10 m l/kg eel). Four m ajor observations were m ade: (1) PCB-exposed anim als lose less w eight com pared to their unexposed controls; (2) PCB-concentrations on a lipid basis are 2.8-14 tim es higher in sw im m ing com pared to resting anim als; (3) the standard m etabolic rate is significantly lower in the PCB-exposed anim als than in unexposed controls. In addition, PCB-exposure significantly reduces oxygen consum ption during swim m ing, and starting at 4 0 0 k m (18 days) this effect increases w ith tim e; (4) the relative spleen and liver w eight significantly increased in the PCB-swim anim als but not in the PCB-rest animals. The sw im m ing anim als lost about 75% m ore w eight com pared to resting anim als and had about 50% lower plasm a fat content. Hematocrit, haem oglobin, plasm a pH, ion levels (sodium and potassium ), and plasm a lactate w ere not affected by PCB-exposure or swim m ing. Apparently, the current levels of PCBs and other dioxin-like com pounds m ay seriously im pair the reproduction of the European eel.

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