Mike Stevens of Amberley asks :-
Whilst whitebaiting during a heavy flood the dirty water was full of tiny eels. Where did they come from?
Don Jellyman, a zoologist with the National Institute of Water and Atmospheric Research, responded.
New Zealand has 2 main species of freshwater eel - the shortfin and the longfin. Although the marine spawning grounds of these eels are still unknown, they will be in the tropics. After hatching, the larvae (leptocephali - means "leaf-shaped") drift back to New Zealand on ocean currents. We are able to age glass eels (the “baby” eels that arrive back in freshwater) and know that shortfins spend about 8-9 months at sea, and longfins about 9-10 months. As leptocephali approach our continental shelf, they undergo a metamorphosis into glass eels, the stage that arrives in our river mouths
Glass eels are small (5.5-7.0 cm) versions of adult eels, and arrive in fresh water during spring, especially during September and October, although they may be present from July to December. Glass eels arrive at the mouths of rivers in pulses, awaiting suitable conditions to enter. During the early part of the season, they tend to migrate into rivers on the incoming (flood) tide, especially on the spring tides of new and full moons. At this stage they show more attraction to salt water than to fresh water and mainly move at night. Later in the season, glass eels show more attraction to fresh water - this may show as a shift towards migrating during the outgoing tide.
Once they are in fresh water, glass eels take several days to adjust before they migrate further upstream. During this phase, glass eels become attracted to fresh water, show a strong tendency to swim against currents, and will often migrate during the day as well as the night. They frequently form very large schools, and at this stage they can be captured in large quantities as they migrate to the limits of the tidal reaches, where they congregate before continuing their upstream migration over successive summers as elvers. Upstream movement often happens during floods, where especially as flows recede and this would be the basis of your correspondent catching glass eels in dirty freshwater.
Eels will then stay in rivers, lakes etc for many years until they mature as adults and go to sea to spawn. Females are always larger and older than males. Typically for South Island waters, female shortfins would range between 25-40 years of age, and longfin females 35-60 years. Like all species of freshwater eels, ours spawn once and then die.