It is often difficult to determine the depth at which a deepsea fish was caught.
Most trawl nets are open from the moment they enter the water to the moment they are brought back on board the vessel. A fish caught in the net could potentially have been collected anywhere from the surface to the bottom.
Special trawls (closing trawls) can open and close between a known range of depths. The capture depth of fishes caught in a closing trawl are known with more accuracy than those caught in a standard trawl.
Copyright © Australian Museum, 2006