Increased proximity of vessels reduces feeding opportunities of blue whales in the St. Lawrence Estuary, Canada

Blue whales Balaenoptera musculus occur seasonally in the St. Lawrence Estuary, Canada, where they spend most of their time foraging. Their recurrent presence has stimulated the development of a large whale-watching industry. Here, we examine the effect of vessel distance on blue whale foraging behaviour by measuring changes in surface and diving patterns. Vessels were within 2000 m of blue whales during 70% of 33 follows, and 59% of total observation time. At vessel distances 400 m, surface and dive times were on average 49 and 36% shorter, respectively, and the number of breaths taken by the whales was reduced by 51% compared to control observations without vessel presence within 2000 m of whales. The consequent reduction in foraging time was likely greater than 36%, given that transit time is incompressible and foraging depth is dictated by where krill densities are located. We showed that the relative proportion of lost foraging time from vessel exposure increased exponentially with prey depth. Whales were unable to compensate for lost feeding opportunities by increasing diving rate or swim speed, except when feeding within 10 to 15 m of the surface. Our results indicate that preventing vessels from entering within a 400 m radius around blue whales can help reduce the negative effects of marine recreational activities on blue whale foraging.