Observations of fission/fusion processes in fish aggregations using a multibeam echosounder

Models of fish behavior within aggregations typically incorporate a fish's ability to sense near neighbors. Some newer models also include a cognitive functionality that allows the fish to understand not only the action of the near neighbors but also their intent. This cognitive function might be manifested, for example, in some stochastic estimate of the movement of a local population rather than at the individual level. Inherent in such a function are the temporal and spatial scales at which a fish's cognitive function operates. To help constrain these scales we have analyzed acoustic backscatter from walleye pollock collected with a multibeam echosounder as part of the NOAA Alaska Fisheries Science Center survey. During this survey, repeat passes that were approximately 1 nmi long were collected at approximately 15 mi intervals over an aggregation of fish. In at least one case, we have been able to acoustically observe an initial group of fish that undergoes both fission (splitting) and fusion (recombining) behaviors. In doing so, we are able to track the net movement, speed, and size of various parts of the group, thereby providing some ground truth for cognitive functionality models. [Research supported by the U.S. Office of Naval Research.]