Where Humpback Whales and vessel traffic coincide, a Colombian Pacific case study

Abstract

During the austral winter, G-stock humpback whales immigrate and occupy the Bahía Málaga area, Colombian Pacific, to breed and calve. However, due to fishing and whale-watching activities, and proximity to the major Buenaventura seaport, significant vessel traffic transits the Bahía Málaga. We counted the number of whales and boats present h-1 in a 14 km2 area, from a 20 m high shore platform. During 312 daytime hours of observations over 52 days in August-September 2008, we recorded 770 sightings of whale pods (many resampled), 0-15 humpback whales h-1 (mean 4.0 whales h-1) and 0-17 vessel trips h-1 (mean 4.5 trips h-1). Each additional day in the breeding season showed an 1.0% increase in the mean number of sighted whales, due to still arriving whales and new calves. The majority of trips (96%) consisted of small vessels, the rest were medium vessels. Most (71.1%) transited at fast speed (≥16 knots). To evaluate whether the vessels affected the density of whales per hour in the study area, we fitted a Generalized Linear Model with number of whales as dependent variable and vessel size and speed as factors. Number of vessel trips did not have a significant impact on the number of whales sighted h-1. Over the short term (hours), the whales remained in the same area despite the shipping. The evident risk of vessel-whale collisions and potential long-term impact are discussed. To mitigate risk of ship strikes, several measures are recommended, such as a vessel speed below 10 knots in the vicinity of whales.