Swimming performance of three species of marine fish and some aspects of swimming in fishing gears
This thesis examines the swimming performance of mackerel (Scomber scombrus L.), herring (Clupea harengus L.), and saithe (Pollachius virens (L.)), and relates it with the mechanical and the electrical properties and the volume of the red and the white swimming muscle. Analysis of kinematics of swimming from cine films and video recordings allows development of a mathematical model of swimming movements and examination of thrust and power output during swimming. This output can be scaled for the time of the tail beat cycle and the size of the fish. Swimming of mackerel at low speeds is related to the lift required in this negatively buoyant species in order to maintain a constant swimming depth. Longitudinal tilting of the mackerel body at low speeds is observed and related to the echo sounder target strength in fisheries surveys. Swimming performance of mackerel, herring and saithe at intermediate speeds is measured in terms of speed and endurance. The maximum sustained swimming speed for each species is related to the maximum cross-section area of the red muscle. Maximum burst swimming speed up to 18 body lengths per second is measured in a 30.5 cm long mackerel. This maximum speed is found to be limited by the minimum contraction time of the white muscle which is measured in 60 samples of 20 individual mackerel. Recordings of the electromyograms of swimming mackerel and measurements of muscle contraction time and muscle cross-section area lead to a new fish swimming model demonstrating how the thickest part of the red muscle is used when the maximum thrust is output from the caudal fin during one tail beat cycle. The swimming performance of marine fish near fishing gears, especially otter trawls is summarised by analysing video tapes recorded at sea and compared with the measured swimming performance in laboratory conditions. Some models of fish swimming behaviour near fishing gears are developed.