First dorsal fins from mako, thresher and hammerhead sharks are tall and slender from leading to trailing edge. In the image below, thresher and mako fins (top row) are slate to dark grey in color. Great hammerhead first dorsal fins (bottom row, left) have a distinctive curved shape and are a much lighter grey than mako or thresher fins. Scalloped and smooth hammerhead first dorsal fins (bottom row, right) are similar in shape to the dorsal fins of mako and thresher sharks, but they are much lighter in color and are usually light brown instead of grey.
FIN IS BROWN OR LIGHT IN COLOR
A "tall" brown or light-colored dorsal fin that is generally uniform in color belongs to a hammerhead shark, however there are other species of shark and guitarfish fins that also have "tall" fins. See the information below for additional details on how to distinguish hammerhead dorsals from other tall fins.