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How do big blue-water game fish swim so rapidly?

September 20, 2018
BY DONNELL TATE , Lahaina News

For offshore fishing enthusiasts, understanding why fast game fish move so quickly and how they sustain their high-speed motion opens new vistas of appreciation for these highly evolved, finely tuned athletes that we challenge on hook and line.

Water is dense. How dense? To quantify water's density, it's more than 750 times thicker than air (salt water even more so than fresh). The stuff is nearly incompressible. That viscosity can be an advantage, helping to suspend long, limber-bodied creatures like billfish. But at the same time, anything intent on pushing through water needs some serious power behind it.

Fish "push through the stuff" by swimming. Well, not all fish - some fish hardly swim at all (they drift). This is particularly true of the tiniest critters, which simply lack the strength to displace the thick medium in which they live. Surface drag on their small-volume bodies proves much harder to overcome for them than for large fish. Size becomes a major advantage in attaining speed.

But "swimming" fails miserably at describing how fish move. Before considering specific approaches to swimming, I should point out that most fish, however they move, can swim either at a sustained pace or in driving, short burst. Burst speeds are, of course, the fastest speeds any species can attain.

Different types of fish have evolved remarkably different strategies for traveling from point A to B. Scientists have categorized those approaches to locomotion using characteristic family names.

One of the most widespread swimming modes, named after the jack family that typifies it, is the carangiform. In fact, the majority of quick-moving fish are carangiform swimmers, from jack and snappers to billfish.

The technique offers a most effective compromise, using the body but much less so than an eel does. These fish push ahead by bending only the back portion of their bodies into lateral waves while also relying much more on their tails for thrust. In terms of burst speed, this mode produces some of the fastest and most furious predators.

For tunas (and wahoo-like mackerels), scientists reserve a subcategory of the carangiform mode: thunniform swimmers. Highly specialized, in this mode the entire body remains stiff and unflexing (true of all tuna species); virtually all thrust is generated from the tail and the narrow stalk (peduncle) connecting it to the body.

Most offshore anglers know well the remarkable drumming a tuna makes once it's dropped onto a cockpit. Only its tail moves, and it moves with a powerful blur of blinding speed. While a tuna's burst speed falls a bit short of that of billfish, no group of fishes can boast higher sustained speeds than tunas.

Why? What makes tunniform swimming so efficient? In large part, it comes down to dealing with turbulence and drag.

Whether pushing through water or air, high-speed travel begets streamlining. Think the Concorde, or high-speed race cars, or submarines... and tuna - their bodies are models of streamlining, critical to moving efficiently through dense water.

Such shapes are known as fusiform (or torpedo-shaped), relatively long, narrow and pointed at both ends.

The science of hydrodynamics dictates that any fish must overcome the drag created by water in order to move through it.

Since drag equals velocity squared, higher speeds increase the challenge of overcoming the physics of water.

Clearly streamlining helps. In a most simple sense, the less a body disturbs the water around it, the less drag will hold the body back. That's why a tuna moves like a rigid missile with its tail as propeller.

When fish rely on lateral body movements to push through water, some of that motion is wasted, and it sets up vortex wakes that require that much more effort for the tail to overcome.

This also explains why tunas and billfish have evolved fin adaptations that allow them, for example, to fit their fins into grooves, leaving them flush with the body. Nothing unnecessary sticks out to create turbulence.



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