Taiwanese scientists have described how a species of threatened hillstream loach migrates upstream by travelling out of the water over wet rocks.
Previous studies had suggested that Sinogastromyzon puliensis, a member of the balitorine loach subfamily, migrated upstream through very fast currents, but nobody knew how they did it, until now.
Biologists observed a large number of juveniles from the species climbing above the water surface in the wet and slippery splash zone to make their way upstream via the seemingly impenetrable Jiji Weir on Taiwan's Choshui River.
The fish were stuck to the wet rocks via their pelvic sucker disc, which is formed by modified pectoral fins, and is an adaptation for life in very fast currents, and were avoiding the fastest flowing water by swimming over the wet rocks above it.
Fast currentsThe water gushes out of the Jiji Weir at a rate of around 193 cubic metres per second " 193,000 lph/42,454 gph " but the loaches managed to find a smaller fishway within the weir with a lower flow rate.
However, the flow was still too much for the fish, so they moved up the wet rocks and out of the water to wriggle above the fiercest part of the current and get past the weir.
Massive numbers of the loaches were spotted in the splash zone above the water level attempting to migrate upstream.
MigrationOrdinarily, scientists who want to monitor fish movements tag the fish and then see if they can find any tagged specimens further upstream.
However, the puliensis were, at just 3cm long, much too small to allow this, so scientists instead had to monitor their movement upstream by draining pools and counting how many fish were found within them afterwards.
Over the course of the year, the peak migrations occurred between mid August and November and most specimens tackled the torrents between 3pm and 10pm, but migrations took place throughout the day and night.
Protected speciesThe species is the easternmost member of the genus and is found only in fast-flowing parts of the Kaoping, Dadu, Choshui and Tzengwen Rivers in southwestern Taiwan.
Populations have decreased dramatically in recent years due to habitat destruction caused by the extraction of sandstone from the rivers in which it occurs.
It has been a protected species since 1989 under Taiwan's Animal Protection Act.
Softly, softly catchy loachyThe discovery probably won't surprise many loach experts, who have seen the way in which balitorine loaches can move over wet rocks when they are removed from the water. Other fishkeepers have also witnessed the fishes climbing out of their tanks.
The loaches, which are legendarily tricky to catch in the aquarium, are most easily removed when the rock they are sitting upon is lifted out of the water with a net held beneath.
This YouTube video shows the technique the loaches use to move out of the water.
The fish are capable of wriggling over the top of the wet rock while remaining firmly stuck in place, but they eventually wriggle free and drop into the net.
For more information see the paper: Liao TY, Pan CC and CS Tzeng (2008) " Migration of Sinogastromyzon puliensis (Teleostei: Balitoridae) in the Choshui River, Taiwan. Ichthyological Exploration of Freshwaters, Vol. 19, No. 3, pp. 193-200.