A trammel net consists of two/three layers of netting with a slack small mesh inner netting between two layers of large mesh netting within which fish will entangle. These nets are strings of single, double or triple netting walls kept more or less vertical by floats on the headrope and mostly by weights on the groundrope. These are occasionally set in strings.
Trammel nets are most common as stationary gear, but they can also be used drifting. The fish entangle themselves in a pocket of small mesh webbing between the two layers and large meshed walls. Afterwards, the trammel nets are hauled back to the surface for extracting the entangling fish from the netting.
“Ghost” fishing of lost gears and capture of small sized organisms and non-target species, which are frequently discarded at sea. Such impact can be mitigated by using larger meshes. However, compared to gillnets the selectivity of trammel nets are lower and catches of small organisms and non-target species are common.
Source: FAO site, 2017