How to Tuna Fish for Speed

HOW TO TUNA FISH FOR SPEED!Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics

How to Tuna fish for speed!

Game fishermen will tell you that the fastest fish are really fast – Mako Sharks, Bluefin Tuna, Marlin and Swordfish – all pursuit hunters that can chase and catch a lure trolled behind a fast-moving boat and strip line off a reel at astonishing rates. The height fish can jump suggests great speed, and swordfish have collided with ships at such speed that their swords have penetrated the hull – in the case of the Tinker of New York piercing the copper covering, a 4-inch birch plank, and 6 inches of the timbers, and in the South Seas whaler Fortune of Plymouth, Mass., the copper sheath, 18˙5 inches of hard wood, including 14˙5 inches of dense oak. Swordfish are big and heavy, and the sword is sharp, but still that must require impact at serious speed.

A striped marlin darting from the ocean off the East Cost of Australia - From Shutterstock
A striped marlin darting from the ocean off the East Cost of Australia – From Shutterstock

And yet, if you work out the drag of a fish, the power its muscles could generate, and the thrust its tail could produce – as Iosilevskii and Weihs did – you find that bigger fish are faster than smaller fish, but even the largest fish are limited to 20m/s (about 40mph) when swimming at depth, and 10m/s (about 20mph) close to the surface. At depth the limitation is just power vs drag. Even though a fish is mostly muscle (perhaps as much as 60% of a fish’s bodymass is muscle, a male human athlete might be 50% muscle), the maximum speed is limited by the power required to overcome the drag of the body. At depth, the water pressure is huge (it increases by about one atmosphere for every 10m depth), but near the surface the thrust generated by the tail is limited because the hydrodynamic pressure reduction that allows the tail to generate force can cause cavitation – vapourising the water to form bubbles. Cavitation is a destructive process (it is used in hypersonic cleaning baths), that damages the fish’s tail, generates noise as the cavitation bubbles collapse, and reduces the forces the tail can produce. So fish speed is limited by drag, and limited further close to the surface by cavitation, limited to speeds a cyclist would comfortably manage (although not underwater). How can that be true when fish can jump many metres into the air, and race to catch lures trolled behind speedboats?

The theory and the observations can’t both be wrong – something else must be going on. For Mako sharks it has long been known that their scales provide drag-reduction. That drag reduction was used to produce the riblet covered swimsuits now banned from the Olympics (they shaved half a second off the 50m front crawl). But swordfish don’t have riblets or Mako-like scales. Recent research from John Videler’s group suggests that Swordfish have a different set of adaptations for drag reduction. Swordfish have a beautifully streamlined shape, and the bill is roughened to act as a turbulator – tripping the boundary layer from laminar to turbulent to prevent  drag-inducing flow-separations, but Videler’s group have recently discovered that the reason the swords of the fish that hit the Tinker and the Fortune were left in the sides of the ships was because there is a weak-spot at the base of the sword where a massive oil-producing gland is situated. That gland secretes oil through pores onto the surface lubricating the boundary layer and providing a substantial drag reduction. Similar tricks have been tried in rowing and sailing for years, with tricks using egg-white, surfactants, washing up liquid and more complex polymers, the potential for pollution is obvious and being ecologicially-sound these sports have banned polymer use. The drag reduction, is nevertheless real, and swordfish probably don’t care too much about the pollution they cause by ‘sweating’ oily polymers onto their skin-surface – they are, after all, only recycling the squid they eat. Drag reduction may well be critical not just for speed, but also for efficiency because Swordfish are open-ocean fish, and that environment is resource-poor and patchy – a blue desert (that is why the water is so clear). Swordfish cruise long-distances to find patches of prey, and reducing drag may be crucial for efficient long-range cruising.

References

Iosilevskii G and Weihs D 2008 Speed limits on swimming of fishes and cetaceans. J. R. Soc. Interface.5329–338

http://doi.org/10.1098/rsif.2007.1073

Videler, J. J., Haydar, D., Snoek, R., Hoving, H.-J. T. and Szabo, B. G. (2016). Lubricating the swordfish head. J. Exp. Biol. 219, 1953-1956  https://doi.org/10.1242/jeb.139634.

Swordfish Attacking Vessels. Nature 146, 261 (1940). https://doi.org/10.1038/146261c0

Han, Wen Jiao. (2017). Role of Bio-Based Polymers on Improving Turbulent Flow Characteristics: Materials and Application. Polymers. 9. 209. 10.3390/polym9060209.

Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics

How to Tuna fish for speed!

Game fishermen will tell you that the fastest fish are really fast – Mako Sharks, Bluefin Tuna, Marlin and Swordfish – all pursuit hunters that can chase and catch a lure trolled behind a fast-moving boat and strip line off a reel at astonishing rates. The height fish can jump suggests great speed, and swordfish have collided with ships at such speed that their swords have penetrated the hull – in the case of the Tinker of New York piercing the copper covering, a 4-inch birch plank, and 6 inches of the timbers, and in the South Seas whaler Fortune of Plymouth, Mass., the copper sheath, 18˙5 inches of hard wood, including 14˙5 inches of dense oak. Swordfish are big and heavy, and the sword is sharp, but still that must require impact at serious speed.

A striped marlin darting from the ocean off the East Cost of Australia - From Shutterstock
A striped marlin darting from the ocean off the East Cost of Australia – From Shutterstock

And yet, if you work out the drag of a fish, the power its muscles could generate, and the thrust its tail could produce – as Iosilevskii and Weihs did – you find that bigger fish are faster than smaller fish, but even the largest fish are limited to 20m/s (about 40mph) when swimming at depth, and 10m/s (about 20mph) close to the surface. At depth the limitation is just power vs drag. Even though a fish is mostly muscle (perhaps as much as 60% of a fish’s bodymass is muscle, a male human athlete might be 50% muscle), the maximum speed is limited by the power required to overcome the drag of the body. At depth, the water pressure is huge (it increases by about one atmosphere for every 10m depth), but near the surface the thrust generated by the tail is limited because the hydrodynamic pressure reduction that allows the tail to generate force can cause cavitation – vapourising the water to form bubbles. Cavitation is a destructive process (it is used in hypersonic cleaning baths), that damages the fish’s tail, generates noise as the cavitation bubbles collapse, and reduces the forces the tail can produce. So fish speed is limited by drag, and limited further close to the surface by cavitation, limited to speeds a cyclist would comfortably manage (although not underwater). How can that be true when fish can jump many metres into the air, and race to catch lures trolled behind speedboats?

The theory and the observations can’t both be wrong – something else must be going on. For Mako sharks it has long been known that their scales provide drag-reduction. That drag reduction was used to produce the riblet covered swimsuits now banned from the Olympics (they shaved half a second off the 50m front crawl). But swordfish don’t have riblets or Mako-like scales. Recent research from John Videler’s group suggests that Swordfish have a different set of adaptations for drag reduction. Swordfish have a beautifully streamlined shape, and the bill is roughened to act as a turbulator – tripping the boundary layer from laminar to turbulent to prevent  drag-inducing flow-separations, but Videler’s group have recently discovered that the reason the swords of the fish that hit the Tinker and the Fortune were left in the sides of the ships was because there is a weak-spot at the base of the sword where a massive oil-producing gland is situated. That gland secretes oil through pores onto the surface lubricating the boundary layer and providing a substantial drag reduction. Similar tricks have been tried in rowing and sailing for years, with tricks using egg-white, surfactants, washing up liquid and more complex polymers, the potential for pollution is obvious and being ecologicially-sound these sports have banned polymer use. The drag reduction, is nevertheless real, and swordfish probably don’t care too much about the pollution they cause by ‘sweating’ oily polymers onto their skin-surface – they are, after all, only recycling the squid they eat. Drag reduction may well be critical not just for speed, but also for efficiency because Swordfish are open-ocean fish, and that environment is resource-poor and patchy – a blue desert (that is why the water is so clear). Swordfish cruise long-distances to find patches of prey, and reducing drag may be crucial for efficient long-range cruising.

References

Iosilevskii G and Weihs D 2008 Speed limits on swimming of fishes and cetaceans. J. R. Soc. Interface.5329–338

http://doi.org/10.1098/rsif.2007.1073

Videler, J. J., Haydar, D., Snoek, R., Hoving, H.-J. T. and Szabo, B. G. (2016). Lubricating the swordfish head. J. Exp. Biol. 219, 1953-1956  https://doi.org/10.1242/jeb.139634.

Swordfish Attacking Vessels. Nature 146, 261 (1940). https://doi.org/10.1038/146261c0

Han, Wen Jiao. (2017). Role of Bio-Based Polymers on Improving Turbulent Flow Characteristics: Materials and Application. Polymers. 9. 209. 10.3390/polym9060209.