Shapes of krill swarms and fish schools emerge as aggregation members avoid predators and access oxygen

Many types of animals exhibit aggregative behavior: birds flock, bees swarm, fish shoal, and ungulates herd [1]. Terrestrial and aerial aggregations can be observed directly, and photographic techniques have provided insights into the behaviors of animals in these environments [2] and data against which behavioral theory can be tested [3]. Underwater, however, limited visibility can hamper direct observation, and understanding of shoaling remains incomplete. We used multibeam sonar to observe three-dimensional structure of Antarctic krill shoals acoustically [4]. Shoal size and packing density varied greatly, but surface area:volume ratios (roughnesses) were distributed narrowly about ∼3.3 m−1 [5]. Shoals of clupeid fish (e.g., sardine, anchovy) from geographically and oceanographically diverse locations have very similar roughnesses [6,7,8]. This common emergent shape property suggests common driving forces across diverse ecosystems. Group behavior can be complex [9], but a simple tradeoff—that we model—in which individual fish and krill juggle only their access to oxygen-replete water and exposure to predation can explain the observed shoal shape. Decreasing oxygen availability in a warming world ocean [10] may impact shoal structure: because structure affects catchability by predators and fishers [11,12,13], understanding the response will be necessary for ecological and commercial reasons.