So it is a comparatively simple step from locating the sea bottom to locating a shoal of fish. With experience, and with improved apparatus, 

it is now possible not only to locate a shoal but to tell if it is herring, cod, or other well-known fish, by the pattern of its echo.

麻烦老师讲解一下惯用短语"by the pattern of sth"的含义和用法，查阅各种词典，没有找到解释和说明。

谢谢！