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Kinki University, Faculty of Agriculture
Uncovering the True Nature of the Bluefin Tunas, “the Diamonds of the Sea”

The nature of the bluefin tunas (thunnus orientalis) – large, beautiful fish known as “the diamond of the sea” – is to swim freely in the ocean. This makes close examination difficult, which is why little of their biological characteristics has been explored. Professor Tsutomu Takagi from Kinki University, Faculty of Agriculture, has been applying fluid analysis to reveal the mysterious nature of the bluefin tunas.

Professor Tsutomu Takagi from Kinki University (Laboratory of Fisheries Production System,
Department of Fisheries, Faculty of Agriculture)

 The bluefin tunas are the most prized in the tuna family. In 2002, Kinki University succeeded in farm-raising the special fish and named the breed Kindai tunas, which attracted wide public attention.

 Professor Takagi from Kinki University (Laboratory of Fisheries Production System, Department of Fisheries, Faculty of Agriculture), explains that the biological data of the bluefin tunas has not yet been fully collected. Professor Takagi uses fluid analysis to investigate the true nature and unknown characteristics of fish in general, but has a particular focus on the bluefin tunas.

 Tunas are large, carnivorous fish that inhabit the sea. They are a member of the perciform fish, under the scombroidei suborder, in the scombridae family. Their biological classification is the same as mackerel and swordfish. Besides the bluefin tunas, eight other species exist, including the familiar yellowfin tunas and bigeye tunas. The bluefin tunas are the largest in the group. They can weigh 400kg (882lb) and be 3m (10ft) long. Known as “the diamonds of the sea,” they account for only 2% of the entire tuna catches, and are traded in the market at the highest rate. Professor Takagi and his team have been using fluid analysis to investigate the bluefin tunas’ swimming capability.

​ Tunas swim very fast. Some swim nearly 90km/h (56mph), though slower ones, such as the yellowfin tunas, swim at 75km/h (47mph). The Indo-Pacific sailfishes, which are known as the fastest marine creature, swim at 108km/h (68mph). In terms of the body length per second, the yellowfin tunas are at 20BL/s and the Indo-Pacific sailfishes are at 15BL/s, which indicates that tunas are one of the fastest, medium sized fish species.

Figure 1: A frozen tuna being scanned on a 3D scanner

(click to enlarge)

 ​Considering their speed and ability to swim long distances, the fluid drag force on the tunas’ bodies was thought to be small. But the unanswered question was: how can a tuna’s drag force be measured or calculated? To test a live tuna in an underwater environment, Professor Takagi would need a tank that was large enough for the fish to swim. The tank would also need the capability to change the fluid velocity. Attaching a resistance board to the fish would also be needed. Convinced that the results would be inaccurate even if the research team managed to conduct such a test, Professor Takagi explored the application of computational fluid analysis. Professor Takagi created a virtual model of a tuna by scanning a real fish using a 3D scanner. The Professor recalls that modeling the fish was a demanding task. It required preparing a frozen tuna to prevent decay, and painting the fish white to minimize diffuse reflections of the laser (see figure 1).

Figure2: Fluid analysis of a bluefin tuna with
the tail in motion (click to enlarge)

Simulating Lift and Thrust

 Using a fluid analysis tool, Professor Takagi calculated the drag on a gliding 34-cm (1.1-feet) long tuna to be 5gf (0.355pdl). This is the same drag as a 5-mm (0.2-inch) diameter, 15-cm (a half-foot) long cylinder. For a 100-cm tuna, the drag was 400gf (28.4pdl). This is equal to the drag of a 30-mm(1.2-inch) diameter, 15-cm(a half-foot) long cylinder. These analyses results demonstrated that the tuna’s drag is relatively low regardless of the body size.

 Professor Takagi conducted a simulation that accounted for the movement of the tail (figure 2). He created a smooth virtual representation of a moving tuna by recording a video of a real tuna and approximating the motion of each point on the body with periodic functions. This enabled him to investigate the motion of the bluefin tunas even further, such as identifying the tail thrust and analyzing the outward movement of the pectoral fins, which function as wings.

​ A pectoral fin is essential for the swimming capability of tunas, as it generates the lifting force. Although tunas possess swim bladders inside their bodies, the bladders do not fully sustain the upward force needed to keep them buoyant. This is one of the reasons why tunas must keep swimming. They use a similar mechanism as airplanes use to produce lift. Professor Takagi’s simulations showed that substantial lift can be generated by moving the tuna’s pectoral fins outward. In addition, tunas can streamline their bodies to minimize drag by tucking their pectoral fins into the indentions on the sides of their bodies. The caudal peduncles are that the bumps attached to the root of the tunas’ tails also function as minuscule wings. “For the larger migratory fish such as the Indo-Pacific sailfishes, two layers of caudal peduncles are attached just like a biplane. It’s fascinating that the reasons and purposes behind the designs of living organisms become evident when I look at it from the fluid-analysis perspective,” comments Professor Takagi.

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*Contents and specifications of products are as of January 1, 2014 and subject to change without notice. We shall not be held liable for any errors in figures and pictures, or any typographical errors.

Institute Details


Kinki University
Laboratory of Fisheries Production System, Department of Fisheries, Faculty of Agriculture
Founded 1949
Type of University  Private
Location of the Head Office Higashiosaka-shi, Osaka, Japan



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