North­eastern Uni­ver­sity and MIT researchers have observed—for the first time—the origin of a mass gath­ering and the sub­se­quent migra­tion of hun­dreds of mil­lions of ani­mals. Uti­lizing a new imaging tech­nology invented by the researchers, they were able to instan­ta­neously image and con­tin­u­ously mon­itor entire shoals of fish con­taining hun­dreds of mil­lions of indi­vid­uals stretching for tens of kilo­me­ters off Georges Bank near Boston.

They found that once large shoals of Atlantic her­ring reach a crit­ical pop­u­la­tion den­sity, a “chain reac­tion” trig­gers the syn­chro­nized move­ment of mil­lions of indi­vidual fish over a large area. The phe­nom­enon is akin to a human “wave” moving in a sports sta­dium. They also observed that the fish “com­mute” to the shal­lower waters of the bank, where they spawn in the dark­ness, then return to deeper water and dis­band the fol­lowing morning.

The find­ings, pub­lished in the latest issue of Sci­ence, con­firm gen­eral the­o­ries about the behavior of large groups of ani­mals that, until now, had not been ver­i­fied in nature. Pre­vi­ously, these the­o­ries for diverse animal groups, ranging from flocks of birds to swarms of locusts, had only been tested with com­puter sim­u­la­tions and lab­o­ra­tory experiments.

As far as we know, this is the first time we’ve quan­ti­fied this behavior in nature and over such a huge ecosystem,” said Nicholas C. Makris, pro­fessor of mechan­ical and ocean engi­neering at MIT, who co-​​led this project with North­eastern pro­fessor Purnima Ratilal.

As part of the project, two research ves­sels were equipped with Ocean Acoustic Wave­guide Remote Sensing (OAWRS) tech­nology, devel­oped by pro­fes­sors Makris and Ratilal. Both OAWRS and con­ven­tional sensing methods depend on acoustics to locate objects by bouncing sound waves off of them. OAWRS, how­ever, cap­tures images of a 100 kilo­meter diam­eter area every 75 sec­onds, pro­viding far more com­plete cov­erage of fish pop­u­la­tion and behavior than con­ven­tional methods. In addi­tion, OAWRS does so at a lower fre­quency than con­ven­tional methods, which allows the sound to travel much greater dis­tances with lower inten­sity and still pro­vide useful information.

After ana­lyzing the data care­fully during the ini­tial days at sea, I noticed what seemed to be a daily pat­tern of fish shoal for­ma­tion,” said Ratilal, assis­tant pro­fessor of elec­trical and com­puter engi­neering at North­eastern. “When I pre­dicted what would happen the fol­lowing day, and it turned out to be right, we knew we had dis­cov­ered some­thing really important.”

Makris and Ratilal see poten­tial in using OAWRS to better monitor—and conserve—fish pop­u­la­tions. Large oceanic fish shoals pro­vide vital links in the ocean and human food chain, they explained, but their sheer size makes it dif­fi­cult to col­lect infor­ma­tion using con­ven­tional methods.

North­eastern PhD. stu­dents Mark Andrews and Zheng Gong con­tributed to this research. Addi­tional col­lab­o­ra­tors include J. Michael Jech of the North­east Fish­eries Sci­ence Center, Olav Rune Godoe of the Insti­tute of Marine Research in Norway, as well as others from MIT, North­eastern and the South­east Fish­eries Sci­ence Center. The project was spon­sored by the National Oceano­graphic Part­ner­ship Pro­gram, the Office of Naval Research and the Alfred P. Sloan Foun­da­tion, and is a con­tri­bu­tion to the Census of Marine Life.