West Sutherland Elver Survey

West Sutherland Elver Survey 2008
between the sea and fresh water exceeds 3-4 o C. If there is large spring tide more elvers are
transported up river in an apparent wave of upstream migration by actively ‘riding’ Spring
tide surges. All these factors generally result in a peak of Elvers entering rivers in Sutherland
between May and June. On entering rivers, during the next 5 years eels perform an initial
rapid dispersion upstream driven by density at the point source, with more 1-5 year old eels
found actively moving over weirs than older eels. During this first phase of this dispersion,
young yellow eels (often called bootlace, bolts or fingerlings) start a series of metabolic
changes and turn olive-brown or gray-brown on top with a silver or yellow-silver belly by
130mm, at which point they are considered to be “Yellow Eels” and fully adapted to life in
fresh water. These metabolic changes include the production of slime, an ability to breathe
through the skin and the production of a toxin to deter fish predators.
Figure 4: Simulated distribution of eel sizes up river over time (from Ibbotson et al. 2002)
SEDENTARY YELLOW EELS
After this initial phase, yellow eels follow a much slower dispersion rate, which is equivalent
to random diffusion of particles (Ibbotson et al 2002). Thus the traditionally perceived
movement of the eel population upstream by a wave-form migration process does not
occur, Instead colonisation of freshwaters by eels can be seen as a two-phase dispersion
process. During the second phase of dispersal, yellow eels >200mm display a more
sedentary lifestyle, particularly in small catchments (Laffaille 2005a), and slowly disperse to
areas of lower eel density (fig. 4) (Ibbotson et al 2002). At this point yellow eels spend most
of their time during the day lying flat on mud or with the upper part of their body upright, or
lying in holes and at night feeding on various animal foods. Yellow eels also display sexual
11 West Sutherland Fisheries Trust