A theory of zooplankton and micronekton patch dynamics is developed that expressly includes animal behavior. This represents a departure from traditional models of patch dynamics, which generally treat animals as Lagrangian particles whose distributions are determined solely by processes of advection and diffusion. The “bio-continuum” theory is based on principles of statistical mechanics, and describes animal aggregations in terms of mean motion, random motion, random kinetic energy, distribution and abundance. The forces on an animal aggregation act both upon the aggregation as a whole (external forces) or between individuals (internal forces). We demonstrate here that the internal forces which serve to maintain autocoherence are, in essence, a force of biological attraction that can be quantified in Newtons. A coefficient of biological attraction is defined, and its magnitude evaluated in aggregations of Antarctic euphausiids (Euphausia superba). We hypothesize that the coefficient of biological attraction may be constant for all organisms in the sea. A method for measuring all key variables with acoustic Doppler technology is presented, with specific attention to application of the Acoustic Doppler Current Profiler (ADCP). We conclude that bio-continuum theory, coupled with acoustic Doppler observations, provides a practical approach for studying animal aggregation dynamics in the sea.