Glenis Stevens of Christchurch asks :-

When whitebaiting we often catch smelt and small fish called cucumber fish? What is a cucumber fish and what is known about it? eg what does it grow into?

Gavin James, a zoologist with the National Institute for Water and Air, responded.

Whitebaiters often call the small silvery fish in their nets cucumber fish because of their distinctive cucumber smell. They are in fact smelt (family Retropinnidae) of which there are two very similar species in New Zealand. Common smelt (Retropinna retropinna) occurs throughout the country in lakes and in the coastal parts of most rivers, whereas Stokell’s smelt (Stokellia anisodon) is restricted to coastal Canterbury where it can occur in large numbers around river mouths. Both species spend a significant part of their life-cycles in the sea, although common smelt are also found as landlocked populations in lakes. Smelt can be distinguished from other fishes by their easily dislodged scales, adipose fin (a small fleshy fin on their back between the dorsal and tail fins), distinctly forked tail, and of course their cucumber smell. This distinctive smell is caused by the chemical “trans-2 -cis-6-nonadienal”, the purpose of which is unknown.

Specimens found in whitebait nets are juveniles or immature adults returning to freshwater to spawn after growing for up to a year or two in the sea. Spawning appears to occur over sandy or silty bottoms in slower flowing areas of the lower reaches of rivers. Smelt are pelagic, which means they swim in mid water rather than resting on or hiding in the substrate. Thus they can sometimes be seen out in the open in streams and lakes as they feed on drifting food organisms.

You can find out more about these and other New Zealand native freshwater fishes in “New Zealand Freshwater Fishes: A Natural History and Guide” by R. M McDowall.