Energy Requirements of European Eel for Trans Atlantic Spawning Migration

Energy Requirements of European Eel for Trans Atlantic Spawning Migration Guido van den Thillart, Arjan Palstra, and Vincent van Ginneken An important aspect of the reproduction of European silver eels is the huge distance they have to swim to reach their spawning grounds. After leaving the West European coast they still have to swim 5,000–6,000 km to the Sargasso Sea, the assumed spawning area. So, obviously long term swimming capacity is a major requirement for successful reproduction. Migrating eels don't feed; therefore they rely for their energy completely on fat stores (Tesch 2003), which can be as much as 30% of their body weight. Silver eels must swim across the Atlantic Ocean within 5–6 months, as this is the difference between the time they leave and the time the first larvae are observed in the Sargasso Sea. From the time needed to cross the ocean the minimal swimming speed of 0.4 m s⁻¹ can be calculated. The long distance migration suggests two major questions: (1) Do they have enough energy reserves? (2) Are they built to swim long distances? To know whether they have enough energy left over for successful reproduction after arrival at the spawning site, it is important to know the energy consumption during long term swimming as well as the amount of the initial fat stores.

Long term swimming experiments were, to our knowledge, never carried out before with fishes. This requires the construction of special equipment, suitable to run continuously for at least several months; such as available at the Institute of Biology Leiden. Long term swimming may be a much heavier burden to animals than short term swimming, since under those conditions the experimental animals do not have the opportunity to recover. This may be a constant stress making them sensitive to otherwise harmless viral and bacterial infections. Thus far nothing was known about the swimming and endurance capacity of eels. Swimming speeds, endurance capacity, and oxygen consumption rates have to be measured to answer the above questions. European eels migrate great distances to reach their spawning sites. As silver eels they leave the European west coast in the fall and are supposed to reach the Sargasso Sea after about 6 months in the spring (Tesch 2003). Although they leave for the spawning site to reproduce, they are still immature at that time. So, the gonads have to develop during or after their migration. Eels have much fat as energy stores, which are reserves for gonad development as well as for migration. For their long-distance migration to the Sargasso Sea the energy reserves may easily become critical particularly since the fat percentage varies largely (Svedäng and Wickström 1997a, b). An estimation of the energy required to cover 6,000-km was presented recently. Based on the oxygen consumption rates during a 10-day swim trial, the equivalent fat consumption extrapolated to 6,000-km was 120 g per kg or 40% of the initial fat reserve (Van Ginneken and Van den Thillart 2000). More extensive data were obtained from intermediate (1,000-km) to even long term (5,500-km) swim trials (Van den Thillart et al. 2004; Van Ginneken et al. 2005a), that showed the high endurance and low cost of swimming of the European eel.