Humpback whale behavioral response to ships in and around major shipping lanes off San Francisco, CA

Citation

Szesciorka, A, J Calambokidis, J Goldbogen, and J Harvey. 2015. Humpback whale behavioral response to ships in and around major shipping lanes off San Francisco, CA. Abstract (Proceedings) 21st Biennial Conference on the Biology of Marine Mammals, San Francisco, California, December 14-18, 2015.

Abstract

When whales overlap with human activities in coastal habitats, whales are vulnerable to potentially lethal ship strikes. Despite changes to the major shipping lanes off San Francisco, ship strikes continue, and there is increasing concern about the sub-lethal effects of ship encounters and factors increasing encounters. From 2013 to 2014, we tagged 15 humpback whales in and around the shipping lanes off San Francisco to examine whale behavior within 3 km of a ship. We identified 9 encounters among 6 whales and 7 ships lasting 6 to 19 min, with the closest encounter being 26.5 m distance between whale and ship. Dive behavior was analyzed together with Mahalanobis distance as a proxy for overall response intensity and individually with randomized intervention analysis to investigate behavior before and after encounters. Overall response intensity decreased during ship encounters when whales made exploratory dives (non-foraging dives >50 m; p = 0.047), but increased during and after ship encounters when whales were making surface dives (dives between 10 and 50 m; p < 0.01). Individual response varied, however, whales generally descended faster, ascended slower, and had greater respiration rates after ship encounters (all p < 0.04). During the two closest encounters (<160 m) whales appeared to take action to increase the vertical distance between themselves and the ships. All but one encounter occurred north of the shipping lanes, indicating greater risk in Bodega Canyon. Most encounters involved a juvenile (n = 2) and an injured whale (n = 3), indicating increased risk for certain whales. Finally, most dives (70%) were made between 10 and 50 m, where 56% of the encounters occurred, indicating whales making shallow dives were at an increased risk of ship encounters. Many factors influence response intensity, including the ship, the whale, and the circumstances of the encounter. Whereas whales appeared to respond to ships based on visual observations, statistics did not always reflect those changes, and the relationship between dive behavior and ship distance became unclear at distances greater than 750 m. However, whereas ship encounters might not be lethal, if they disrupt normal foraging behavior, they can have sub-lethal effects, especially if chronic. Continual disruption could lead to population level effects even for a recovering endangered species