A study into the locomotion of fishes

Fish species are present in many varying forms all over the world. The main difference between shapes of individual species would firstly concern the need for them to travel through a fluid with varying densities pressures and drags. With a combination of these problems and in turn making best use of there form inside their chosen environment each species has developed, sometimes radically, different body profiles.




Figure 1: Comparisons of flow around objects in viscous fluids. Source: Hosford (1997), p.16
  Species such as the rainbow trout (Oncorhyncus mykiss) have developed torpedo shaped bodies to counteract the drag confronted in a viscous fluid. Figure 1 shows the difference in streamline effects on two very different shaped objects in a fluid. The ideal situation to experience in a viscous fluid would involve laminar flow, this being smooth and thin with a creation of minimal drag. The circular shape in figure 1 is less streamlined in appearance and at the rear end turbulent flow is created. This is usually in the form of an eddy or wake and consists of a thicker layer, which increases the drag. For many fast swimming species the creation of a turbulent flow would result in the inefficient use of resources with respect to energy requirements. This may seem fairly straight forward but Blake (1983), suggests although the maintaining of a laminar boundary layer is a drag reduction system it happens to be less stable than a turbulent flow as it creates larger pressures upon separation. As this is the case the inducing of a turbulent boundary layer can also reduce drag. It has been suggested that some species such as the mackerel (with the use of added dorsal fins), and the rainbow trout (with the use of an adipose fin) tend to induce a change in boundaries.
Figure 2 shows that many years of development in aerofoil technology have created a design identical to the shape of a rainbow trout. The solid line is the aerofoil where as the dotted line is the trout. This concludes that the rainbow trout has formed into a highly streamlined structure.  
Figure 2: Comparison of a trout body form and a typical aerofoil. Source: Blake (1983), p.61
The position of the shoulder will determine the type of flow over the body of the species. The shoulder of the trout is somewhat further back than the likes of the common bream (Abramis brama). This seems to verify that the form compares to the habitat and lifestyle of the species. The final point on the subject of drag would involve the use of the mucus covering the skin of the fish. The use of mucus in filling irregularities on the body could improve the flow characteristics of the boundary layers.
In normal observations fish swimming can be split into steady speeds, where the fish moves in one direction at a constant velocity or more commonly un-steady swimming which is the continuous changing of speeds and directions (Hoar & Randell 1978). The critical speed of a species can only be measured from steady swimming but it is important to note that three other phrases, prolonged, burst and sustained swimming will take place in either of the firstly mentioned descriptions. It must be mentioned that potential growth response is effected by the cost of locomotion, which in tern contributes to the overall metabolic load (Ware, 1975), therefore efficient energy use is essential. Evidence has been found that juvenile fish do feed by moving at the appropriate speed to maximise their production rate (Ware, 1978).


Figure 3: Fatigue curves Source: Blake(1983), p.38


Figure 4: Body propulsion of subcarangifom mode. Source: Hoar & Randell(1978), p.10
  Figure 3 shows the fatigue curve of a rainbow trout (formally Salmo gairdneri). It clearly defines the three states of swimming used by all species of fish. Sustained swimming includes routine activities such as schooling and cruising and can be maintained in excess of 200 minutes. Prolonged swimming can continue between 200 minutes to 15 seconds and entails cruising with short bouts of vigorous movement. Burst swimming is the most inefficient form of activity with respect to energy use and can only be achieved for short periods of up to 15 seconds (Wilson & eggington, 1994).

The rainbow trout can be classified into the group of subcarangiform fishes. This defines the type of body movement the species experiences when swimming. Most fish species swim with lateral body undulations running from head to tail. These waves run more slowly than the waves of muscle activation causing them, reflecting the effect of the interaction between the fish's body and reactive forces from the water (Wardle et al.1995). The undulations of the side to side movement in the body are slight in the anterior but there is a significant increase in the rear half to third of the body. The snout of the fish does not travel in a straight line but tends to oscillate at small amplitudes along the mean path of the fish (Hoar & Randell 1978). Figure 4 illustrates this type of movement and clearly shows that no part of the body travels in a straight line, but tends to follow a curving path through the water

Only at speeds of under 1 to 2 body lengths per second does the amplitude of the body undulations change, usually there is no change with swimming speeds. The frequency that the tail beats and the velocity at which waves are passed to the rear of the fish directly effect the speed of the fish. Speeds of up to 25 body lengths per second have been recorded for fish below 0.1 metre in length. Although this measurement is for fairly small fish, Webb (1975) discovered that maximum acceleration rates for rainbow trout (40-50cm/sec2) were in the same order for other fish from a wide range of sizes. This suggested that maximum acceleration rates may be relatively independent of size. Although acceleration and size may not be comparable the wavelength of the body undulations remain constant when relative to the body length within species. When the size of the fish increases the attainable maximum frequencies decrease. In relation to tail beats and swimming style, Webb (1991) found that over a period of 226 completed tail beats from trout no constant speed was registered. Figure 5 shows the results and clarifies that sustained swimming is never constant and will always involve acceleration, deceleration or turning.



Figure 5: Swimming styles compared to tail beats. Source: Webb (1991), p. 586
  The caudal fin is probably the most important attachment used for acceleration. During Webbs (1977) experiment involving fin amputation of rainbow trout he clarified that the large caudal fin is required for maximum acceleration performance and creates the majority of generated thrust during fast starts. The dorsal and anal fins are also important in generating thrust but they are not as nearly as significant. Some of the important aspects involving the movement of trout have been discussed, finally but just as significant is the production of energy needed to create these exercises. The thrust force of the fish is produced by contractions of the propulsive musculature. The velocity that the muscle contracts dictates the power that the fish can produce.
Muscle structures of fish can be split into white and red types. Red muscle consists of up to 20% of the total belonging to the trout (less for lower active species such as carp (Cyprinus carpio)). This muscle has a low rate of fatigue and is used for all of the low speed sustained cruising. The white muscle fatigues more rapidly but gives maximum power output, which is used for burst speeds. Due to the speed of fatigue the white muscle can be used only for short periods of time.
References

Blake, R.W. (1983). Fish locomotion. Cambridge university press, Cambridge.
Hoar, W.S., and Randall, D.J. (1978). Fish physiology, volume VII: Locomotion. Academic press, New York.
Hosford, M.B. (1997). Fluid dynamics of ship resistance and propulsion. Institute of marine studies, Plymouth.
Wardle, C.W., Videler, J.J., and Altringham, J.D. (1995). Tuning in to fish swimming waves: body form, swimming mode and muscle function. Journal of experimental Biology, Vol 198, p. 1629-1636.
Ware, D.M. (1975). Growth, metabolism and optimal swimming speed of pelagic fish. Journal of the fisheries research board of Canada, Vol. 32, p. 33-41.
Ware, D.M. (1978). Bioenergetics of pelagic fish: Theoretical change in swimming speed and ration with body size. Journal of the fisheries research board of Canada, Vol 35, p. 220-228.
Webb, P.W. (1975). Acceleration performance of rainbow trout (Oncorhyncus mykiss). Journal of experimental Biology, Vol 63, p. 451-465.
Webb, P.W. (1977). Effects of median-fin amputation on fast start performance of rainbow trout. Journal of experimental Biology, Vol 68, p. 123-135.
Webb, P.W. (1991). Composition and mechanics of routine swimming of rainbow trout, (Oncorhyncus mykiss). Canadian journal of fisheries and aquatic services, Vol 48, no. 4, p. 583-589.
Wilson, R.W., and Egginton, S. (1994). Assessment of maximum sustainable swimming performance in rainbow trout. The journal of experimental biology, Vol 192, no.1, p. 299-305.

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