Factors affecting whale detection from large ships in Alaska with implications for whale avoidance

In response to growing concern over lethal ship-whale collisions, a number of efforts have been developed intended to enhance the ability of ships to avoid whales. However, the effectiveness of avoidance by large ships depends upon the ships detecting whales at a distance sufficient to allow for an appropriate avoidance measure. Here we explore the issue of whale detection using over 3000 unique detections of humpback whales recorded by observers stationed aboard large cruise ships in Alaska, USA. We used point transect distance sampling methods to generate detection functions necessary to understand the probability of whale detection and how it varies with distance under different environmental and biological characteristics. Detection probability of surfacing whales decreased markedly with increasing distance from the ship. We found visibility and group size to be the most important variables influencing detection. The worst visibility conditions reduced detection probability to near 0 at 1000 m. Compared to detecting a single whale, a group of 2 or 3 whales almost doubled detection probability at 1000 m. Surface active behavior increased detection compared to spouting while showing no flukes. In southeastern Alaska, single whales that spouted during excellent visibility conditions were most commonly encountered and had a detection probability of 0.569 at 1000 m. Understanding the ability of mariners to detect whales at distances sufficient to invoke avoidance measures is a key component in the effectiveness of 'ships avoiding whales' and is germane to efforts to reduce lethal ship-whale collisions.