Optimum prey capture techniques in fish

In this thesis hydrodynamic principles are used to quantify relations between form and function in the prey capture mechanism of actinopterygian fish. This work is closely related to the papers on the hydrodynamics of fish feeding by Muller et al. (1982) and Muller & Osse (in press). The effectiveness of different head forms and movements for prey uptake (in various habitats) is investigated by model simulations and verified by flow visualization and pressure measurements.Chapter 1 presents a technique to visualize the flow in 3-D around the mouth of the fish, sucking its prey. An expanding and compressing cylindrical or conical model of the fish's mouth cavity is used to quantify the relation between head movements and swimming. The opercular and branchiostegal valves are shown to function as control devices to obtain an optimal flow rate through the mouth aperture. The theoretical predictions were verified experimentally for the rainbow trout ( Salmo gairdneri ). Likewise, data from the literature appeared to agree with these hypotheses.Chapter 2 quantifies the contributions of the forward movement of the fish, the expansion of the mouth cavity and a possible protrusion of the jaws to the velocity of the prey. Optimum sucking techniques (i.e. techniques maximizing the chance of prey capture) in relation to swimming speed and habitat properties are derived by model simulations. Maximization of the initial prey distance by an exact adjustment of mouth expansion is rather useless for a fish. Much more is gained if the fish abducts its opercula at the maximal rate when the prey enters the mouth.Chapter 3 discusses recording techniques for pressures in prey-sucking fish. The dynamic properties of different measurement systems are investigated by Fourier analysis. Also, the frequency content of records of the fluctuating pressure inside the fish's mouth during feeding is shown. Prey capture events of different fish species are simulated using the hydrodynamical model of Muller et al. (1982). Measured and simulated pressure curves are compared and the effects of the use of different boundary conditions in the model are discussed. The literature on pressure measurements in prey-sucking fish is reviewed.