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Abstract

An important aspect of the reproduction of European silver eels is the huge distance (ca 6000-km) they have to swim to reach their spawning grounds. So, obviously endurance swimming is a major requirement for successful reproduction. As migrating eels don’t feed, two major questions should be asked: 1) what are the energy costs to cross the Atlantic Ocean, 2) How much fat do eels need to cover the expenses. From swim tunnel experiments run for days till even 6 months we could determine the cost of transport (COT): 0.5 kJ km-1 kg-1. This amount is about 5 times lower than COT values of salmonids of similar size, also those low costs allow eels to spend only 60 g fat/kg for the whole journey. As eels have around 200g fat/kg, there is more than enough left after reaching the spawning site for gonad development and spawning behavior. Although the swim endurance of eels is impressive, large female eels (1-2 kg, 70-80 cm) can swim for prolonged periods not much faster than 1.6 BL/s, with their best performance around 0.8 BL/s. When leaving the European West coast, silver eels are still in a prepubertal condition, suggesting that somewhere during their journey sexual maturation needs to be activated. We hypothesized that swimming might be such a trigger. Indeed we observed significant changes in female eels already after several weeks of swimming. We observed a significant increase of eye size, gonad weight and oocyte diameter, the latter mainly due to the deposition of many lipid droplets. However, no further development of oocyte size was observed when the eels continued swimming. The mechanism behind the oocyte growth might be connected to stimulation of lipid mobilisation required for swimming. Lipid stores are not only important for long distance swimming, but also for gonad growth and gamete production. Based on the lipid content of eggs, we estimated that around 60g fat/kg eel is required for reproduction. Combined with the requirement for swimming, total lipid content of succesful spawners must be at least 12%, i.e. 120 g fat/kg.

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