Keeping on the straight and narrow is significantly more complicated for fish than insects; coordinating as many as seven fins is a much more complex task than the two wings most insects have to bother about. How fish propel themselves through water has puzzled people for a long time. Only recently have scientists begun unravelling the complex fluid dynamics generated by each fin as fish scythe through the currents. More recently George Lauder at Harvard University, working with his colleague Elliot Drucker, have turned their attention to a fin whose role might be less obvious. They wondered how the dorsal fin contributes to a fish's comings and goings. Having studied the sunfish's dorsal fin several years before, they suspected that the trout's dorsal fin was also essential for high speed swimming. But the team were surprised when they found that trout use their dorsal fins less and less as they speed up, and when they looked closer, realised that instead of helping to propel the fish forward, the dorsal fin seemed to stabilise the fish in the water (p. 4479).

Lauder's technique of choice to investigate fish fluid dynamics is DPIV. Focusing a thin plane of laser light into a flow tank filled with microscopic reflective beads, the team encouraged the fish to swim at a constant speed so that their body or fins intersected the plane of light, while filming the swirls and vortices generated by their fins visible in the laser light. But this is where DPIV becomes less of a science and more of an art. Lauder admits that the main quality you need for success with this technique is limitless patience. He and Drucker remember spending hours cajoling the timid trout to perform in front of the cameras, but eventually the team had enough data to begin analysing the fish's dorsal fin.

Swimming the fish at a range of slow and high speeds, the team quickly realised that the fish used their dorsal fin most when swimming at slow speeds, with the fin hardly moving at all once the fish were cruising at a high speed of 2 total body lengths per second. And when they calculated the forces generated by the fin at different speeds, they realised that it didn't contribute to the fish's forward thrust at all. The fin seemed to be producing a large sideways force at low speeds that diminished as the fish cruised at higher speeds. Lauder and Drucker realised that the dorsal fin was stabilising the fish rather than helping to propel it forwards, which is in complete contrast to what they found when they looked at the sunfish's spiny dorsal fin; that seemed to contribute significantly to the fish's forward momentum. Lauder suspects that the support offered to the sunfish's dorsal fin by its spines help propel the fish forward.

Most surprisingly, Lauder and Drucker have identified a new `gait' which they call M-BCF and is distinct from median paired fin gait (MPF) and body caudal fin (BCF) gait. In this new gait, the fish combine dorsal fin oscillations with movements of the body and tail fin at low speeds. The team also suggest that this new gait may be part of a natural progression between the two previously identified and distinct gaits. But Lauder admits that future work on other fish species will be needed to test this idea.

Drucker, E. G. and Lauder, G. V. (
). Locomotor function of the dorsal fin in rainbow trout: kinematic patterns and hydrodynamic forces.
J. Exp. Biol.