Hydrodynamic Characteristics of the Sailfish (Istiophorus platypterus) and Swordfish (Xiphias gladius) in Gliding Postures at Their Cruise Speeds

Abstract

The sailfish and swordfish are known as the fastest sea animals, reaching their maximum speeds of around 100 km/h. In the present study, we investigate the hydrodynamic characteristics of these fishes in their cruise speeds of about 1 body length per second. We install a taxidermy specimen of each fish in a wind tunnel, and measure the drag on its body and boundary-layer velocity above its body surface at the Reynolds number corresponding to its cruising condition. The drag coefficients of the sailfish and swordfish based on the free-stream velocity and their wetted areas are measured to be 0.0075 and 0.0091, respectively, at their cruising conditions. These drag coefficients are very low and comparable to those of tuna and pike and smaller than those of dogfish and small-size trout. On the other hand, the long bill is one of the most distinguished features of these fishes from other fishes, and we study its role on the ability of drag modification. The drag on the fish without the bill or with an artificially-made shorter one is slightly smaller than that with the original bill, indicating that the bill itself does not contribute to any drag reduction at its cruise speed. From the velocity measurement near the body surface, we find that at the cruise speed flow separation does not occur over the whole body even without the bill, and the boundary layer flow is affected only at the anterior part of the body by the bill.

Feeding in billfishes: inferring the role of the rostrum from a biomechanical standpoint

“…Surprisingly, the function of this structure is still controversial (Nakamura, 1983;Frazier et al, 1994). The rostrum has been hypothesized to improve hydrodynamics by reducing drag (Wisner, 1958;Ovchinnikov, 1970;Aleyev, 1977; but see Sagong et al, 2013), to be used for defense (as evidenced by rostral fragments found embedded in large predators; Fierstine, 1997;Fierstine et al, 1997) and to be used to strike, immobilize or dismember prey before ingestion, thereby facilitating feeding (Gudger, 1940;Talbot and Penrith, 1964;Scott and Tibbo, 1968). Although it is certainly possible that the elongated rostrum may have been selected for multiple roles, substantial evidence from stomach contents and recent field observations strongly support the latter feeding-related hypothesis (Scott and Tibbo, 1968;Stillwell and Kohler, 1985;Frazier et al, 1994;Shimose et al, 2007;Domenici et al, 2014).…”

Feeding Biomechanics in Billfishes: Investigating the Role of the Rostrum through Finite Element Analysis

“…Although the rostrum in billfishes is mechanically suited to function as a feeding weapon, it may serve other functions as well. For example, it is hypothesized that turbulence created by the rostrum could delay the point of separation reducing form drag on the billfish body (Wisner, ; Ovchinnikov, ; Aleyev, ), although a recent study found no support for overall drag reduction when modeling different rostrum lengths in two billfish species (Sagong et al, ). Even if the rostrum does not have an impact on overall body hydrodynamics, it could provide a hydrodynamic advantage during feeding.…”

Comparative morphology of the scales of roundscale spearfish Tetrapturus georgii and white marlin Kajikia albida

“…Structures that project from the epidermis into the environment provide a hydrodynamic function in various groups of fishes (Blake, 1983). One of the most important factors in determining hydrodynamic drag is the boundary layer flow surrounding a fish, due to its effects on friction drag and flow separation (Sagong et al, 2013). Scales, spiny projections and rough surfaces protruding from the epidermis of fishes may act to stabilize the boundary layer (Webb, 1975;Bushnell & Moore, 1991), thereby helping to reduce hydrodynamic drag during swimming by generating vortices around the bodies of fishes to influence flow (Bone, 1972;Fish, 1998).…”

“…In teleosts, it is thought that the cteni of ctenoid scales produce micro-turbulence as the fish moves through the water (Fish, 1998). Adult I. platypterus are described as having V-shaped protrusions in their skin that point posteriorly (Nakamura, 1985;Sagong et al, 2008Sagong et al, , 2013. These protrusions produce paired vortices that create low and high shear stresses which, as they interact with vortices induced from adjacent protrusions, affect the level of drag (Sagong et al, 2008).…”