New Zealand smelt

Description

The New Zealand smelt (Retropinna retropinna), also known as the New Zealand common smelt or New Zealand cucumber fish, is a smelt of the family Retropinnidae, found only in New Zealand at shallow depths in estuaries and rivers. Their length is between 8 and 13 cm. Retropinna retropinna are elongated silver green fish with clear fins, just larger than palm size. Smelt can be identified by the adipose fin, a small fleshy nub, located halfway between the dorsal fin and the tail. Scales, a forked tail and a distinct “cucumber smell” are other ways of distinguishing Smelt from other species. There are two species of smelt in New Zealand, the common smelt and Stokell's smelt which can only be told apart by the size and number of scales. The length of a common smelt is between 8 and 13 cm. Common smelt are widespread in North Island waterways, but not in the South island (only Stokell's smelt are found there). Smelt are found in coastal seas and estuaries as well as in freshwater streams as far as they can migrate. While Smelt are not strong upstream swimmers, they have been found up to 300 kilometres inland at an altitude of 2000m. Smelt are sensitive to stress which means they have a narrow range of preferred conditions. Smelt live in waters between 15 and 17 degrees Celsius although they have been known to cope with temperatures up to 27 degrees. Smelt prefer waters with a pH of 8-9 and low levels of sedimentation. There are records of smelt being both diadromous (spending part of life cycle in freshwater and part in saltwater) as well as anadromous reports (fish that spend most of their life cycle in saltwater, returning to freshwater only to breed). The Department of Conservation (2015) reports that smelt live in large shoals in estuaries, spending most of their lives at sea, but returning to freshwater as adults in the summertime. They also report that smelt can be found in lakes and lowland rivers, which is confirmed by NIWA, which states that smelt are a widespread indigenous freshwater fish. To better understand the life cycle of the smelt the population must first be broken down into two groups: lacustrine/reservoir and diadromous populations. Lacustrine or reservoir referring to lake and artificially created water bodies and diadromous referring to fish that spend their life partially in freshwater, partially in saltwater. Diadromous smelt spend the first life stage in the sea, estuary or lower parts of rivers.