A1 Swim Inefficiency

Topic

The fish's propulsive system is the key element of this model: its function is to generate the thrust needed to overcome the drag the water exerts on the moving body, while maintaining a normal metabolic cost. Inefficient swimming occurs when the propulsive system loses this capability—whether due to damaged propulsive structures, an injury or deformity altering body geometry, or the flow regime around the body becoming turbulent sooner than expected. Any of these three causes produces the same physical result: excess drag that the fish must overcome—beyond what a healthy individual would face at the same speed—which subsequently translates into a higher cost of transport and a lower critical sustained swimming speed.

Injuries and deformities that alter body geometry affect the first of the two mechanisms generating this excess drag. Altered body geometry creates zones of flow separation—regions where water ceases to follow the body's contour and breaks away from the surface—and generates regions of anomalous stagnation pressure, where water slows down and exerts abnormal pressure on the body. Both alterations increase the pressure drag coefficient: the more pronounced the flow separation and the more extensive the zones of anomalous stagnation pressure, the higher this coefficient becomes, and the greater the contribution of the damaged geometry to the excess drag.

The second mechanism driving excess drag originates at the body's surface rather than in its overall shape. Surface disturbances or damage induce excessive turbulence in the layer of water directly in contact with the fish: the boundary layer. On an intact body, this boundary layer remains laminar until the flow reaches a high relative speed; In contrast, for a body with a damaged surface, the transition from laminar to turbulent flow occurs at a much lower speed threshold than in an undamaged body. This premature transition increases the boundary-layer friction coefficient; thus, much like the altered geometry described in the previous paragraph, surface damage contributes its own share to the excess total drag the fish must overcome.

This excess drag—whether stemming from altered geometry, friction from the damaged boundary layer, or a combination of both—acts as a component added directly to the normal drag experienced by a healthy fish. Overcoming this excess drag requires the propulsive system to generate additional thrust, and such thrust can only be sustained through additional metabolic power: the greater the excess drag accumulated via these two mechanisms, the more extra metabolic power the fish must expend to overcome it, over and above the baseline metabolic cost mentioned earlier.

This additional metabolic power is measurably reflected in the critical swimming speed—the standard physiological indicator of a fish's aerobic locomotor capacity, determined using the Brett protocol, in which the fish swims in a flume at speeds that increase in stages until exhaustion. In a fish experiencing excess drag, the critical swimming speed is lower than that of a healthy individual; simultaneously, the slope of the curve relating oxygen consumption to swimming speed steepens, because each unit of distance traveled requires more energy. The cost of transport introduced in the first paragraph increases in the same way: roughly speaking, excess drag of nearly twenty percent above the baseline value results in an equivalent drop in critical swimming speed and a twenty-five to forty percent increase in the cost of transport measured at submaximal speeds.

These two measurable changes in the fish's propulsive system—a lower critical swimming speed and a higher cost of transport—have direct consequences for the animal's life. A fish whose propulsive system can no longer sustain the critical swimming speed required under healthy conditions faces compromised migration success, as it fails to maintain the pace demanded by migratory routes; furthermore, its ability to evade predators is diminished, because the very excess drag that raises its cost of transport prevents it from reaching or sustaining the escape speeds that a healthy individual can achieve. Thus, the inefficiency originating in the fish's propulsive system—whether due to injury, deformity, or premature turbulence—ultimately manifests as a tangible ecological disadvantage for that fish.

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