Dictionary of Ichthyology  

otter board = 1) a large, heavy, flat piece of wood and metal on each side of the mouth of an otter trawl; the boards plane or shear through the water and keep the net mouth open. Also called otter door, trawl door, trawl board. The term is also used for any shearing device used to separate lines or drive a line deeper in the water. Various models and designs exist. 

Fisheries Museum of the Atlantic, Lunenburg, Nova Scotia. Photograph by Brian W. Coad.

Canadian Museum of Nature