Humpback whale call-rate is synchronized with Atlantic herring shoal population density over a diurnal cycle.

(A) Mean humpback whale call rate (black line within gray standard deviation over 15 minute bins) over a diurnal cycle and mean herring shoal areal population density (blue line with standard deviation indicated by the blue error bars) from September 28 to October 3. When the areal population density of the diffuse daytime herring populations reaches a critical threshold of approximately 0.2 fish/m² (red dashed line) near sunset, the herring population density drastically increases at a rate of roughly 5 fish/m² per hour [18] to form evening shoals. (B) Diurnal humpback whale call rate follows a synchronous pattern with 0.82 correlation coefficient and 0–15 minute time lag between the two time series in (A). The period from roughly 2–6 EDT contains a data gap.