An experiment to verify the basic linearity of fisheries is described. Herring (Clupea harengus L. ) was the subject fish. Acoustic measurements consisted of the echo energy from aggregations of caged but otherwise free-swimming fish, and the target strength functions of similar, anesthetized specimens. Periodic photographic observation of the caged fish allowed characterization of their behavior ... [Show full abstract] through associated spatial and orientation distributions. The fish biology and hydrography were also measured. Computations of the echo energy from encaged aggregations agreed well with observation. This success was obtained for each of four independent echo sounders operating at frequencies from 38 to 120 kHz and at power levels from 35 w to nearly 1 kw.