Response of small sharks to non-linear internal waves

Nekton may respond passively or actively to large-amplitude, non-linear internal waves (NLIW), with wavelengths on the order of 100’s of meters, and the NLIW can cause direct or indirect changes in distribution. Understanding the influence of NLIW on organism response and distribution is challenging, because of NLIW unpredictability, short temporal and spatial scales, and the difficulty in resolving the biological response. Measurements of currents, temperature, and shark acoustic traces in Massachusetts Bay were used to evaluate the short-term response of individuals of small sharks, Squalus acanthias. In two NLIW events, we detected 527 and 3,240 shark traces. Individuals moved up and down in response to the currents associated with the sinking and rising of the thermocline. However, mean distribution deepened during one of the events, suggesting that organisms did not merely move in concert with the thermocline oscillation, but that sharks instead may have responded actively. Measurements of vertical currents and shark’s depth-change during one of the NLIW events indicate that with downward currents (sinking of the thermocline) the sharks tend to react passively. However, in response to the fastest upward currents, sharks appeared to swim down, supporting an active response in the rising phase of the wave. NLIW can have a profound influence on the distribution of nekton, yet their ecological consequences remain largely unaccounted for