Station-holding by Three Species of Benthic Fishes

Station-holding performance was determined on a smooth substratum and on a grid substratum for three species of benthic fishes differing in body shape, surface texture, density, friction coefficient and behavioural repertoire. The grid was made of wires parallel to the flow, which raised fish into the free stream. Limited observations were also made on the benthopelagic cod. Station-holding performance was evaluated at two speeds. The first was defined as the slip speed, above which activities such as swimming, fin-beating, body arching, body clamping and gripping the substratum were required to hold position on the substratum. The second was defined as the swim speed, when fish began swimming out of ground contact. Cod and lasher started swimming when they began slipping, so that slip and swim speeds were the same, averaging 6cms" for cod and 32cms" for lasher on the smooth surface. Body postures and fin-beating delayed swimming from a slip speed of about 20cms" to swim speeds of 47-58cms" for plaice and rays. The grid had relatively little effect on slip and swim speeds of plaice and rays. Lasher grasped the grid with their pectoral fins, increasing swim speeds to 55cms". Amputation of the posterior portion of the median fins of plaice reduced swim speeds on the smooth surface to 36cms". Amputation of the pectoral fins of lasher reduced the swim speed on the grid to 38cms".