Critical population density triggers rapid formation of vast oceanic fish shoals.

Similarities in the behavior of diverse animal species that form large groups have motivated attempts to establish general principles governing animal group behavior. It has been difficult, however, to make quantitative measurements of the temporal and spatial behavior of extensive animal groups in the wild, such as bird flocks, fish shoals, and locust swarms. By quantifying the formation processes of vast oceanic fish shoals during spawning, we show that (i) a rapid transition from disordered to highly synchronized behavior occurs as population density reaches a critical value; (ii) organized group migration occurs after this transition; and (iii) small sets of leaders significantly influence the actions of much larger groups. Each of these findings confirms general theoretical predictions believed to apply in nature irrespective of animal species.

Fish population and behavior revealed by instantaneous continental shelf-scale imaging.

Until now, continental shelf environments have been monitored with highly localized line-transect methods from slow-moving research vessels. These methods significantly undersample fish populations in time and space, leaving an incomplete and ambiguous record of abundance and behavior. We show that fish populations in continental shelf environments can be instantaneously imaged over thousands of square kilometers and continuously monitored by a remote sensing technique in which the ocean acts as an acoustic waveguide. The technique has revealed the instantaneous horizontal structural characteristics and volatile short-term behavior of very large fish shoals, containing tens of millions of fish and stretching for many kilometers.

Changes in volume reverberation from deep to shallow water in the eastern Gulf of Mexico.

Scattering from fish is a primary cause of volume reverberation and, since fish populations change from deep to shallow water, the character of volume reverberation should also change. However, there are few data available to document expected changes. Therefore, an experiment was conducted in the eastern Gulf of Mexico to investigate possible changes in volume reverberation from deep to slope to shelf waters. Results showed that volume reverberation in outer shelf waters varied more rapidly with respect to both time and space than that in deeper waters. Day-time scattering was similar for deep, slope and shelf waters, total scattering strengths generally increased with frequency. Night-time scattering for the deep ocean and slope also increased with increasing frequency. Scattering modeling suggests that swimbladder-bearing fishes smaller than 10 cm were responsible for the observed volume reverberation. Night-time scattering at the outer shelf location was very different, with strong scattering peaks at low frequencies. Scattering modeling implicates 12-15 cm rough scad and round herring as potential causes of the low frequency peaks. Hence, experiment results confirmed that, as expected, volume reverberation over shelf waters is different and more variable than in deeper waters of the eastern Gulf of Mexico.