Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Ostrich leather packing. Because it is expensive to manufacture all three processes, countries that produce ostrich skins on a smaller scale, export them at the "raw" and "crust" stages. South Africa is an important processor of finished skins for the main leather manufacturers in Japan. Other African countries engaged in ostrich skin processing are Zimbabwe, Namibia, Botswana. Botswana markets directly to South African tanneries. South African tanneries receives about 200,000 skins a year from the ostrich abattoirs of the region and from elsewhere in the world it receives around 15, 000 skins. South African tanneries export around 90% of its finished leather