Echo integration is the most important method in fisheries acoustics. The basis of the method is volume scattering theory, and the theory assumes that many fish are randomly and homogeneously distributed broader than a beam spread. In actual acoustic surveys, however, we encounter various distributions for which these assumptions do not hold. It is the echo integration method that hypothetically fulfills these assumptions. In this paper, we review the volume scattering theory and the echo

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... ation method historically and theoretically and confirm their bases. We then point out some misunderstandings affected by historical constraints such as the persistence of time integration. We also introduce and discuss some useful variants of the echo integration, especially calibration by standard-sphere echo integration.