It is also unlikely that both parents would have come from the same river. Elvers may penetrate rivers for many miles inland, depending upon the gradient of the river and the obstructions encountered, but a proportion remain in estuarine and coastal areas. Large female eels may exceed 1,000 mm in length and weigh over 1 kg, but males seldom exceed 600 mm in length. An abundance of small, yellow eels often dominates the lower reaches of rivers and estuaries with fewer, larger and mostly female eels predominant in the upper reaches. Some experts believe that the determination of sex may be influenced environmentally, with higher population densities resulting in a high proportion of males.
Eels apparently have relatively small home ranges within which they forage.
Changes of habitat tend to occur only during transitional periods such as the spring and autumn. The major movement occurs in autumn when silver eels migrate to sea and yellow eels travel to overwintering sites if their summer habitat is unsuitable. Yellow eels that moved downstream to the estuary in the spring now return to the river. Eels may return each summer to the same area within the river and eels transported up to 100 km away from their home waters have successfully found their way back. How this is done is not fully understood but may result from the eels' ability to orient to the earth's magnetic field and to detect river odors.