Blue whale ecology and behavior in a human-impacted marine ecosystem: insights from acoustics and animal-borne tags
To avoid missing peak prey abundances, blue whales must detect available environmental cues and time migration by shifting arrival or departure dates to/from feeding grounds and balancing the time they spend foraging versus breeding. Blue whale feeding habitat overlaps with dense vessel traffic, making them vulnerable to vessel strikes — the leading cause of human mortality for blue whales off Southern California. Any changes to migration timing that increase residence time on the feeding grounds may increase vessel strike risk. The contextual factors influencing vessel strike risk are poorly understood and uncertainty remains about whale behavioral response to vessels. Understanding those interactions is important in preventing vessel strikes. Here, I investigated the timing and drivers of blue whale migration and blue whale-vessel interactions using seafloor- and animal-mounted acoustic devices. This allowed me to (a) examine the relationship among migration timing (inferred from blue whale “D” and “B” calls), environmental indices (e.g., sea surface temperature anomalies), and prey (spring krill biomass from annual net tow surveys) during a 10 year period (2008-2017) off Southern California and (b) assess vessel, environmental, and whale contextual variables associated with 216 close passages (<2 km) between 174 vessels and 35 tagged whales and look for differences and uniqueness in dive behavior resulting from close passages. Colder sea surface temperature anomalies the previous season were correlated with greater krill biomass the following year, and earlier arrival by blue whales, demonstrating a plastic response of whales to interannual variability and the importance of krill as a driving force behind migration timing. By the end of the 10-year period, whales were arriving at the feeding grounds more than one month earlier, suggesting climate change has led to blue whales extending their overall time in Southern California. None of the contextual variables showed any relationship with close passage distance with vessels. Whales did not leave the area, even when passages were chronic (>5/day), and we found no evidence of behavioral response. With no evidence of behavioral responses to close vessel passages, we need to continue managing vessel traffic under the assumption that whales do not avoid vessels.