A view from the top of a blacktip reef shark. Applying a hidden Markov model to the observed overall dynamic body acceleration data, we discovered that the sharks use deeper water during the day when they were least active. (© Yuuki Y Watanabe)

A view from the top of a blacktip reef shark. Applying a hidden Markov model to the observed overall dynamic body acceleration data, we discovered that the sharks use deeper water during the day when they were least active. (© Yuuki Y Watanabe)